LATE SILURIAN MARINE SHELLY FAUNA OF CENTRAL AND NORTHERN VIETNAM

T h a n h TONG-DZUY, A r t h u r J . BOUCOT, J ia -y u RONG & Z o n g -jie FANG

TONG-DZUY T., BOUCOT A.J., RONG J.Y. & FANG Z.J. 2001. Late Silurian marine shelly fauna of Central and Northern Vietnam. [Une faune coquillière marine du Silurien supérieur du centre et du nord du Vietnam]. GEO­BIOS, 34, 3: 315-338. Villeurbanne, le 31.07.2001.

Manuscrit déposé le 20.07.1999; accepté définitivement le 10.11.2000.

ABSTRACT -The Late Silurian (late Ludlow-Pridoli) brachiopods and bivalves from North and Central Vietnam are described and illustrated from the Kien An (North Vietnam) and My Due (Central Vietnam) localities. The biostra- tigraphic relations and age of the fauna, the so-called Retziella fauna, are discussed. The My Due fauna is the same as that at Kien An, indicating that during the Late Silurian both North Vietnam and Central Vietnam, situated on the South China and Indochina Plates respectively were probably fairly close geographically to each other. The Late Silurian is globally a time of moderately high, although not highest, provincialism as regards marine benthic inver­tebrates. A new brachiopod species, Nikiforovaena vietnamensis B ouco t & Rong, and three new bivalve species Schizodus kienanensis Fang, S. ? myducensis Fang, and Modiomorpha paracrypta F ang are described. © 2001 Édi­tions scientifiques et médicales Elsevier SAS

KEYWORDS: BRACHIOPODS, BIVALVES, NEW TAXA, LATE SILURIAN, BIOGEOGRAPHY, VIETNAM.

RESUME - Les brachiopodes et les bivalves du Silurien supérieur (Ludlow terminal-Pridoli) des localités de Kien An (Nord Vietnam) et de My Duc (Centre Vietnam) sont décrits et figurés. L’âge de la faune à Retziella et les relations biostratigraphiques sont discutés. La faune de My Duc est la même que celle de Kien An, indiquent que pendant la fin du Silurien le Nord Vietnam et le Sud Vietnam, situés respective­ment sur les plaques Sud-Chine et Indochine, étaient probablement assez proches l’une de l’autre. La fin du Silurien est globalement une période de provincialisme marqué, bien que n’étan t pas le plus accentué, pour les invertébrés benthiques marins. Une nouvelle espèce de brachiopodes, Nikiforovaena vietnamensis B oucot & R o n g , et trois nouvelles espèces de bivalves Schizodus kienanensis F a n g , S. ? myducensis F a n g , et Modiomorpha paracrypta F ang sont décrites. © 2001 Editions scientifiques et médicales Elsevier SASMOTS-CLÉS: BRACHIOPODES, BIVALVES, NOUVEAUX TAXONS, SILURIEN SUPÉRIEUR, BIOGÉOGRAPHIE,

VIETNAM.

INTRODUCTION

The Retziella fauna has been known for some time from South China, North China, Central Asia, and Australia, based on specimens weathered out of cal­careous shales for the mostpart. The occurrences in northern Vietnam have not previously been illus­trated or described in detail. The Vietnamese bra­chiopods described here occur as casts and molds which substantially supplement the earlier des­criptions of the interiors based mostly on serial sec­tions. The paleoecological significance and overall biogeographic significance of the Retziella fauna were considered by Rong Jia-yu et al. (1995). The bivalves accompanying the Retziella fauna brachio­pods have not previously received much attention.

GEOLOGICAL SETTING

Silurian rocks with Retziella weberi have been known in North and Central Vietnam for some time. First, Duong Xuan Hao and'Nikiforova (1963) published on the My Due (Quang Binh Province,

Central Vietnam; 17°14'N, 106°41'E) occurrence of Retziella weberi, and later occurrences were found in the downstream region of the Da River, and at Kien An (20°48'N, 106°35'E) near Hai Phong in North Vietnam.

The Retziella weberi faunas described in this paper were collected by Tong-Dzuy Thanh and A.J. Boucot in 1994 at My Due (Quang Binh Province, Central Vietnam) and at Kien An near Hai Phong (North Vietnam) (Figs 1A-D).

KIEN AN AREA

Patte (1926, 1927) first described the Paleozoic fau- nal assemblage at the Tien Hoi, Xuan Son and Phu Lien Mountains of the Kien An area, some five kilo­meters, as the crow flies, from Hai Phong City, as Late Devonian with Spirifer cf. ziczac R o em er and Rhynchonella sp. A. I. Jamoida (in Dovjikov 1965) discovered brachiopods in the Xuan Son Mountain tha t Duong Xuan Hao and Nikiforova (1963) iden­tified as Retziella weberi, Eospirifer cf. lynxoides, and corals (Nipponophyllum sp., Xiphelasma sp.).

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F ig u r e 1 - A. Map showing loca­tions within tie Kien An area (Xuan Son, Phu Lien, Kho Lam and Cuu Vien Mountains). The fauna from the Xuan Son Mountain is described in this paper. B. Stratigraphie column for the Xuan Son Mountain locality, indicating where the Retziella weberi fauna was col­lected that is described in this paper. C. Map showing loca­tions within the My Due area, Central Vietnam, and the spot where the Retziella weberi fauna described in this paper was collected. D. Stratigraphie column for the My Due area, indicating where the Retziella weberi fauna was collected that is described in this paper. All figured specimens are from My Due. The specimens described in this paper are deposited at the Hanoi Geological Museum, Collection Number BT 178. All figured specimens of brachio- pods in Figs 2-5 are from My Due. The others are noted where appropriate in the figure legends. A. Carte des localités dans la région de Kien An (Montagnes de Xuan Son, Phu Lien, Kho Lam et Cuu Vien). La faune de la montagne de Xuan Son est décrite dans cet article. B. Colonne stratigraphique de la localité de Xuan Son mon­trant la position de la faune à Retziella weberi. C. Carte des affleurements de la région de My Duc, Vietnam central et localisation du gisement à Retziella weberi. D. Colonne stratigraphique de la région de My Duc avec le positionnement de la faune à Retziella weberi. Tous les spécimens figurés pro­viennent de My Duc. Les spéci­mens décrits dans cet article sont déposés au Musée Géolo­gique de Hanoï, sous le numéro de collection BT178. Tous les brachiopodes des figures 2-5 proviennent de My Duc. Les autres localités sont indiquées dans les légendes des figures.

B

- 1 0 0 m

-400 m

-120 m

Ash-gray limestone bearing FamennianForaminifers, Amphipora Ịaxepeđorata______________Marl, dark gray limestone lenses: /Uesotavositessp., Holmophyllum sp„ Mpponophyllumsp., Retziella weberi, Nikiforovaena ferganensis_________________________ _

Intercalation of quartz sandstone and mudstone: Howellella ett. bragensis

Calcareous mudstone, limestone lenses, thin beds of shale: Microplasma sp., Nikiforovaena fergana, (Retziella weberi in Phu Lien Ml)__________________

Cu Bai Formation

W4 Dark gray limestone. Marl at base of section, Stromatoporoids, Corals, BrachioDOdsSandstone, red or wine colored

Formation

Dai Giang

Formation

300 rn

400­450 m

mudstone with Unaula Sandtone, mudstone, marl and limestone at upper of section. Brachiopods. Tribbites, Corals, Vertebrates

450­500 m

Sandstone, mudstone Brachiopods (first Retziella weberi)

Long Dai Formation

Sandstone, shale Graptolites

Long Dai Formation (Upper Dai Giang Formation (Silurian) Tan Lam FormationOrdovician-Lower Silurian) intercalation of mudstone and (Lower Devonian)mudstone and datfc gray sandstone with mart tenses sandstone and red

and limestone at base or wine colored mudstone

U r o e r ' l £ S ) t a S ^ F°m'a‘ i°nlimestone, intercalation of ,.. . t(W1 ^ ri H (Permo-Carbomferous) limestone mudstone siliceous mutjSj0|le ’ ‘ 9fay limestone

. . . . . . . I ! TPliocene-Quaternary Basalt Quaternary and recent Fossil tocalitv

alluvium

Nguyen Quang Hap (1967) named these rocks the Kien An Suite (Formation), while Hoang Ngoc Ky et al., and Ngo Quang Toan et al. (Vu Khuc & Bui Phu My 1989) renam ed this unit the Xuan Son Formation. Priority rests with the term Kien An Formation.In the Xuan Son M ountain the tripartite rock sequence is as follows: Lower part consists of blue- gray marl, mudstone and yellowish-gray sandstone bearing Eospirifer cf. lynxoides and indeterminate rugose corals. The thickness is about 120 meters; following are gray sandstone and quartz sandstone with dark purplish-red mudstone interbeds having about a 320 meter thickness. These mudstones yiel­ded Retziella weberi, Nikiforovaena ferganensis and

Howellella sp. The upper part of the Section on the North slope of Xuan Son Mountain consists of thick bedded dark gray limestone with shale and marl interbeds in the uppermost layers, where we collec­ted an abundant Retziella weberi assemblage; the thickness of this unit us about 100 m. At the same level on the north slope of Tien Hoi Mountain the following brachiopods were noted: Retziella weberi, Eospirifer cf. lynxoides (= Nikiforovaena ferganen­sis), Howellella bragensis, Howellella sp. (Vu Khuc & Bui Phu My 1989). Additionally, corals are abun­dant, including Favosites admirabilis, Xiphelasma su., Nipponophyllum sp.; and recently Mesofavosi- tes was identified by Tong-Dzuy Thanh. Commonly the upper part of the Kien An Formation consists of

317

dark gray, thick-bedded limestones with some ash- gray shale and marl interbeds, reaching a thick­ness of about 320-400 meters.At Phu Lien Mountain the cross-section is shorter, with only two beds of the formation present, as fol­lows: brownish-yellow sandstone, siliceous shale and yellowish, purple-red, cross bedded mudstone at the top, with a thickness of about 200 meters, yielding poorly preserved brachiopods including Retziella weberi, Nikiforovaena ferganensis, Eospi- rifer ? sp., Howellella sp., and Camarotoechia sp. (­unidentified rhynchonellid). Intercalations of yello­wish sandstone and gray marl with calcareous mudstone having a thickness of about 50 meters, with poorly preserved fossils (Retziella weberi, Eospirifer ? sp., Howellella sp. and rhynchonellids) have been noted (Vu Khuc & Bui Phu My 1989).

The majority of the fossils cited above indicate a Late Silurian age, but some are close to the Lower Devonian, for example, Favosites admirabilis. No conodonts have been found. Based on the brachio­pods described here the Kien An area Retziella weberi bearing rocks are dated as Late Silurian, Late Ludlow or Pridoli.

THE DOWNSTREAM DA RIVER BASIN

In the downstream Da River Basin, western North Vietnam, the Retziella weberi assocation has been found in the Bo Hieng Formation. This formation consists of marl, calcareous shale and thin bedded, dark gray limestone with a thickness of about 500 to 900 meters. It lies between the Sinh Vinh Formation (Ordovician-Silurian) and the Song Mua Formation (Early Devonian). Like the fossil assem­blage in the Kien An Formation, the Retziella webe­ri association in the Bo Hieng Formation consists mainly of Silurian brachiopods (Retziella weberi, Tadschikia xuanbaoi, Lissatrypa sp. (probably an Atrypoidea), Fardenia ? sp.) and bivalves (Modio- morpha brevis and others). In addition, some corals ordinarily considered to be of Devonian age such as Favosites kunjakensis, Squameofavosites ex. gr. cechicus (S q f bohemicus), Tryplasma ex. gr. karce- vae and others are present, although their age here is Late Silurian. '

MY DUC AREA

In the My Due area (Quang Binh Province, Central Vietnam) the Retziella weberi association was col­lected from the upper part of the Dai Giang Formation. A.M. Mareishev and Tran Due Luong (Dovjikov ed. 1965) first described the Dai Giang Formation, and then Nguyen Xuan Duong et al. (1996) studied it in detail. The formation consists mainly of fine-grained sandstone, mudstone and shale, which crop out widely in Quang Tri and sou­thern Quang Binh Provinces, Central Vietnam (Fig. 1). In 1977 Nguyen Xuan Duong described the best section of this formation, along the Dai Giang River. Three parts of the formation are cited here from the eight members described by Nguyen Xuan Duong.

1) Lower part consisting of interbedded sandstone and mudstone which reach a thickness of 550 meters.2) A middle part consisting of dark gray, thin bed­ded marl and calcareous mudstone with some limestone interbeds, with a thickness of about 220­450 meters.3) An upper part consisting of mudstone with sand­stone interbeds with a thickness reaching 250 meters. The total thickness of the formation is more than 1,000 meters.Abundant fossil associations have been collected from different horizons a t different localities. Among them the most characteristic are graptolites (Monograptus sp., Bohemograptus bohemicus, Mo- noclimacis sp., Pristiograptus ludlovensis), brachio­pods (Retziella weberi, Nikiforovena ferganensis, Howellella nucula), trilobites (Metacalymene sp., Cromus beaumonti, Encrinurus sinicus) and corals (Multisolenia cf. formosa, Nipponophyllum anma- ense), indicating a Silurian, mostly Late Silurian age.In the My Due area, near An Ma Mountain and the Cam Ly Reservoir, west of the My Due railway sta­tion (Fig. 1C), the sequence consists of a lower member which is composed of fine-grained sandsto­ne and mudstone, th a t becomes coarser upwards, with a thickness of about 450-500 meters, and an upper member which is largely arenaceous, but becomes somewhat carbonaceous in its uppermost part, with a thickness of about 400-450 meters (Fig. ID). It is possible tha t these two members corres­pond to the above mentioned lower and middle parts of the formation. They were formerly dated as Middle Devonian (Fromaget 1927; Dovjikov ed. 1965). Later, thanks to the collection of Retziella weberi assemblage brachiopods, such as Retziella weberi, Nikiforovaena ferganensis, Howellella nucula, trilobites (Encrinurus cf. sinicus) and corals (Multisolenia cf. formosa, Nipponophyllum anmaense) these beds in the My Due area were assigned to the Late Silurian Dai Giang Formation (Tran Van Tri et al. 1977; Vu Khuc & Bui Phu My 1989; Nguyen Xuan Duong et al. 1996).The age of the Dai Giang Formation has been the subject of debate in the Geology of Central Viet Nam. Dovjikov ed. (1965) and Tran Van Tri et al. (1977) assigned it to the Silurian, whereas Nguyen Xuan Duong et al. (1996,1977, in lit.), Vu Khuc and Bui Phu My (1989) dated it as Late Silurian-Early Devonian. The Late Silurian-Early Devonian assi­gnment was supported by the ‘majority of fossils collected in the Dai Giang Formation which indica­te a Late Silurian age; but apart from these Late Silurian fossils, in some cross-sections Early Devonian fossils are also noted’ (Vu Khuc & Bui Phu My 1989). The authors did not provide a list of ‘Early Devonian fossils’, while all the fossils cited in their description, including graptolites, trilobites and brachiopods do not support an Early Devonian age for even the top of the formation. It is notable that in the top of some cross-sections the collected fossils indicate a Late Silurian age only including, for example, trilobites (Encrinurus cf. sinicus),

318

graptolites (Bohemograptus bohemicus, Pristio- graptus ludlovensis), and brachiopods (Retziella weberi and others). Thus, the Retziella weberi bea­ring rocks of Central Vietnam m ust be correlated with those in North Vietnam (Kien An area and the downstream part of the Da River Basin); all yield Late Silurian fossils.

AGE

Although no conodonts are found in the studied areas in Vietnam, based on regional stratigraphic correlation and the range of the brachiopod species, in particular, Retziella weberi, R. alaica ‘Howellel- la’ lynxoides, and Nikiforovaena, our conclusion for the age of this Vietnamese fauna is tha t it is of late Ludlow to Pridoli age.Additionally, the presence of earlier Ludlow grap­tolites below the Retziella fauna in the My Due area is consistent with a later Ludlow regional shallo­wing, i.e., upward replacement of the deeper water graptolitic facies by the BA 2-3 shelly facies in coar­ser clastics, followed in the earlier Devonian by nonmarine beds. In the Kien An area graptolitic beds, unfortunately, are unknown, but to the nor­theast in the offshore islands the upper parts of the Co To Formation have yielded earlier Ludlow grap­tolites, with all of this being consistent with the assignment of the Kien An and My Due shelly fau­nas to the la te r Ludlow-Pridoli. The Co To Formation, with citations to it's Ludlow graptolites, has been described by Nguyen Huy Mac and Pham The hien (1972) and Tran Van Tri et al. (1972).

No conodonts have yet been found from the Retziella-bearing stra ta in Central Asia. The most abundant species in the fauna in North Vietnam is Retziella weberi N ik if o r o v a which occurs in Ludlow to Pridoli rocks in Central Asia, although the type specimens came from the Isfara Horizon of Pridolian age.In the western part of South China Retziella-bea­ring stra ta are well developed, and their age was determined as late Ludlow to early Pridoli by Rong and Yang (1980, p. 264) chiefly based on the study of the brachiopods from the Miaokao Formation, Qujing, eastern Yunnan, Southwest China. It should be pointed out th a t there is conodont evi­dence; Ozarkodina crispa and others were found in the upper K uanti, Miaokao, and Yulungsuu Formations indicating late Ludlow to early Pridoli age (Wang 1980, 1981; W alliser & Wang 1989).

REMARKS

The occurrence of the Retziella weberi fauna at the three localities noted in this paper, two (Kien An area and downstream part of the Da River Basin) are north of the Song Ma Suture on the South China Plate, or Guangxi-Yunnan-North Vietnam Block (Wu 2000), and one (My Due area) south of the Song Ma suture on the Indochina Plate, argues

for these two areas having been reasonably close to each other during the Late Silurian. The situation for the Early Devonian is more difficult to inter­pret. North of the Song Ma Suture within North Vietnam there is a characteristic South China Region, Old World Realm fauna of marine benthos, as well as vertebrates similar to those present in South China. However, south of the Song Ma Suture there is no biogeographically useful infor­mation concerning marine invertebrates, although the information about the vertebrates does feature taxa distinct from those of the South China Region in both North Vietnam and South China. Tong- Dzuy Thanh has discussed the relation of the known Vietnamese Early Devonian faunas with those present in the Rhenish-Bohemian and Tasman regions (Tong-Dzuy Thanh et al. 1985, 1996, 1997).

PALEOGEOGRAPHY

Trying to work out the paleogeographic and litho- facies relations of the Chinese and Vietnamese Retziella fauna localities is difficult owing to lack of truly compelling data. However, we will attem pt to provide some insight into the varied possibilities. First, it needs to be recognized tha t lateral move­ments on the Song Ma Suture and the Red River Fault Zone may well have significantly displaced the My Due region, belonging to the Indochina Plate, relative to the Vietnamese and Chinese loca­lities to the north tha t all belong to the South China Plate. We will first discuss the gross lithological sequence at the localities.In the My Due region, as discussed above, the Retziella fauna is from the relatively shallow water, upper part of the Dai Giang Formation, which is underlain by Silurian graptolitic shales and overlain by uppermost siliciclastics th a t have yielded Silurian trilobites, which are overlain in tu rn by relatively unfossiliferous siliciclastics of Old Red Sandstone aspect (Tan Lam Formation) tha t probably represent the Early Devonian in lar­gest part, overlain themselves by younger Devo­nian, fossiliferous limestones.In the Kien An area, near Hai Phong, there are Silurian graptolitic shales in the small islands nor­theast of Hai Phong, and earlier Devonian nonma­rine beds to the south of the city. However, the com­plex structural relations in the Hai Phong region make the working out of a detailed lithostratigra- phic sequence difficult.In the Da River Basin, well to the west of Ha Noi the Bo Hieng Formation with its Retziella fauna, is overlain by typical South China Region, fully m ari­ne, probably Benthic Assemblage 3, fossiliferous, Early Devonian beds of the Song Mua Formation, and underlain by the relatively unfossiliferous Ordovician-Silurian Sinh Vinh Formation.In northern Vietnam, in the Lang Son region, non­marine Early Devonian strata , followed by marine Early Devonian, lie unconformably on Ordovician beds, w ith fossiliferous Silurian unrecognized.

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These relations are similar to those to the nor­theast in the nearby Liujing area, Hengxian Coun­ty, Guangxi where the marine Early Devonian, richly fossiliferous, Benthic Assemblage 1 through 3 Lianhuashan Formation lies unconformably on Cambrian strata . Silurian beds with marine fossils are unrecognized in this region.Far to the north in the Qujing area of eastern Yunnan, the Retziella fauna of the Miaokao Forma­tion is overlain by the Pridoli, marine Yulungsuu Formation, followed by paralic Early Devonian stra ta (Wang Nian-zhong 1997, concludes th a t the lower part of these beds may be of late Pridoli age), and underlain by the early-middle Ludlow Kuanti Formation of marine aspect, which also yields a very low diversity Retziella fauna.Trying to tie these very scattered bits of informa­tion together paleogeographically suggests the pre­sence of an Early Devonian land area from Qujing on the north, south to Liujing and nearby Lang Son, with an intervening region of marine environ­ments present further south in North Vietnam, while in the My Due area there is another land area during the Early Devonian. In Late Silurian, Retziella fauna time there was shallow sea everyw­here except in the Liujing and adjacent Lang Son areas where land may have been present. Aul of this is complicated, of course, in terms of how much lateral movement may have taken place on the Song Ma Suture and the Red River Fault Zone.The occurrences of the Retziella weberi fauna noted in this paper are located in three localities. Two (Kien An area and downstream part of the Da River Basin) are north of the Song Ma Suture, or in other words, are part of the South China Plate, whereas the third (My Due area) is located on the Indochina Plate. The reasonably endemic Late S ilurian Retziella weberi fauna occurs on both sides of the im portant Song Ma Suture. The shelly Early Devonian (including the highly endemic Dicoelo- strophia faunas of North Vietnam and adjacent Guangxi Province, so characteristic of the South China Region; Hou & Xian 1975; Wang & Rong 1986; Wang et al. 1987) and Eifelian, earlier Middle Devonian faunas north of the Song Ma Suture are of South China Region, Old World Realm aspect (Benthic Assemblage 5, deep water, Eifelian faunas were collected in 1997 at Na Ri in North Vietnam; they are similar to faunas of the same age in sou­thwestern Guangxi, near Nanning, and are now being studied). Until rich shelly Early Devonian and Eifelian marine faunas are discovered and col­lected south of the Song Ma Suture we will be uncertain of the earlier Devonian situation.

MATERIAL

The Late Silurian fossils described here all occur as casts and molds in heavily weathered, arenaceous mudstone. They came from collections made at two localities. The first includes 95 pedicle valves and 125 brachial valves of 4 species of brachiopods from the Xuan Son Formation in a quarry near the town

of Kien An, near Hai Phong, northern Vietnam. The second, a much larger collection, contains 1777 pedicle valves and 1391 brachial valves, belonging to 9 brachiopod species from the Dai Giang Forma­tion of the same age a t a locality near My Due, north-central Vietnam.679 specimens were collected near Kien An, with 221 brachiopods (32.7 % of the entire association). Among the brachiopods (including one specimen with conjoined valves), the commonest species is Retziella weberi N ikiforova (62.6 % of the brachio­pod total). The other brachiopods are ‘Howellella’ lynxoides (N ikiforova) (18.5 %), Nikiforovaena viet- namensis B oucot & R o n g , nov. sp. (9.5 %), and Retziella alaica N ikiforova (9.5 %). The brachio­pods are associated with many bivalves (total 389 specimens, 57.3 %), the commonest taxon in this association, a distinctive feature of this collection. Some gastropods (64 specimens, 9.4 %), 3 trilobites (2 pygidia and 1 free check, 0.044 %), and 1 ortho- ceratid (0.015 %) are also present. No other fossils were noted.In the My Due area, 3459 fossils were gathered. Among them there occur 3203 brachiopod valves (including 35 specimens with conjoined valves), 94.13 % of the entire association. Within the bra­chiopods, Retziella weberi is again the most abun­dant species, including 43.9 % of the entire bra­chiopod association, much less than in the Hai Phong area collection. The other two abundant taxa are Nikiforovaena vietnamensis B oucot & R on g , nov. sp. (23.8 %) and ‘Howellella’ lynxoides (N ik ifo ­rova) (21.0 %), higher percentages than present in the Hai Phong area. Retziella alaica N ikiforova (9.8 %) is fourth, and the percentage of this species is close to th a t in the Kien An area collection. Another five species are rare, each less than 30 valves with Atrypoidea (0.9 %), Morinorhynchus (0.1 %), Reticulatrypa, Spirinella ?, and gen. et sp. indet. (each 0.03 %). None of these 5 rare species has been found at the Kien An area locality. The My Due area brachiopods occur with various other marine benthic invertebrates, including bivalves (2.54 % of the whole association), gastropods (2.25 %), tabulates (0.03 %), rugose corals (0.12 %), trilo­bites (0.61 %), bryozoans and ostracodes (each 0.14 %), pelmatozoan debris, and vertebrates (0.52 %), and a few other undeterminable fossils (0.06 %). These last forms are relatively rare in number.The following fossils are recorded from the locality near Kien An, northern Vietnam.Taxa Pedicle Brachial Conjoined

valves valves valves

Brachiopoda 95 125 1Retziella weberi N i k i f o r o v a 51 86 1Retziella alaica N i k i f o r o v a 10 11‘Howellella' cf . lynxoides ( N i k i f o r o v a )

Nikiforovaena vietnamensis21 20

Boucot & Rong, nov. sp. 13 8

Bivalves 389; this count includes broken and incomplete specimens. The relatively complete specimens, those used in the taxonomic study numbe­red 81.

Actinopteria mansuyi Grabau 14Pterinea sp. aff. P. dianensis Guo 3Schizodus kienanensis Fang, nov. sp. 50

320

Sanguinolites sp. 1Sphenotus antecedens G r a b a u 1

Goniophora dianensis Guo 12Gastropods 64Trilobites 3Orthoceratid 1Total 679

The following fossils are recorded from the My Due area in central Vietnam.

Taxa Pedicle B rachial Conjoinedvalves valves valves

Brachiopods 1777 1391 35Morinorhynchus sp. 2 1Atrypoidea sp. 16 13R eticulatrvpa sp. 1Retziella weberi N i k i f o r o v a 688 663 35Retziella alaica N i k i f o r o v a 175 145‘Howellella’ cf. lynxoides 451 234Nikiforovaena vietnamensis

Boucot & Rong nov. sp. 441 335Spirinella ? sp. 1G en.and sp. indet. 2

Bivalves 88; this count includes broken and incomplete specimens. The relatively complete specimens, those used in the taxonomic study numbe­red 44.

Pterinea sp. aff. P. dianensis Guo Schizodus ? myducensis F a n g nov. sp.Sphenotus antecedens G r a b a u

Modiomorpha paracrypta F a n g nov. sp.Goniophora dianensis Guo Gastropods Trilobites Tabulates Rugose corals Ostracodes Bryozoans (?)Orthoceratids Vertebrates Indet. fossils Total

SYNECOLOGIC ANALYSIS

The Late Silurian brachiopods studied in this paper belong to the Retziella Fauna which is widely dis­tributed in east-central Asia and eastern Australia (Rong et al. 1995). It m ust be emphasized tha t at many localities in which R. weberi N ikiforova or R. uniplicata (G rabau) occur, each of these species is very abundant and dominant, and usually make up more than 30-70 %, or even more, of the total asso­ciation. There are two examples of this situation in which the last two species were recorded.1) R. weberi with very abundant individuals on the same bedding plane was recorded from the Late Silurian rocks of the northern slope of the Alay Mountains, southwest Kirghizistan, Central Asia (Nikiforova 1937, pi. 12, fig. 8; also see Rong et al. 1995, Appendix 1). More recently Kim and Sapel- nikov (in Sapelnikov et al. 1999) recorded Retziella weberi Community from the type locality (Havalbet Mountain, Severnyi N uratau Range, southern Tien- shan). This is a high dominance, low diversity unit consisting almost entirely of the eponymous taxon and a BA 2 or inner 3 position lias been suggested.2) Very abundant specimens of R. uniplicata were recorded with Striispirifer sinensis R ong & Y an g ,

and Atrypoidea foxi J on es in the lower part of the Kuanti Formation of the Qujing area, eastern Yunnan, Southwest China (see Rong & Yang 1980; Jones & Rong 1982), a low diversity, high domi­nance brachiopod association, previously named the Atrypoidea Community. The three species are all abundant, and the horizon in which they occur is not far from the base of the formation which is underlain disconformably by Middle Cambrian rocks. The local paleogeography and the paleoecolo- gy indicate a nearshore, normal, shallow, warm water, level-bottom BA 2 to inner 3 position for this association.

R. weberi from the My Due area locality, north-cen­tra l Vietnam, herein described, also is the most abundant taxon in the collection. The other bra­chiopod genera associated with R. weberi are not as numerous, being parts of a relatively low diversity fauna. This probably indicates a similar, nearsho­re, normal, shallow water environment for the widely distributed Retziella fauna in the Ludlow- Pridoli. This fauna may have inhabited a BA 2 to inner BA 3 position. We will discuss in detail the environmental evidence for each locality.The fossils from both Vietnamese localities are almost in situ. This is chiefly based on the following evidence. 1) Preservation of fossils is excellent although the m atrix is relatively coarse; even micro-ornamentation is sometimes well preserved in this coarse matrix; brachiopod shells are always complete, with no broken valves or signs of abra­sion, indicating no strong w ater currents; 2) Relatively equal percentages of pedicle and bra­chial valves within most species (with little dispa­rity) among the brachiopods. At the locality near My Due the Retziella weberi collection contains 688 pedicle valves and 663 brachial valves; a t the Kien An area locality the ‘Howellella’ cf. lynxoides collec­tion contains 21 pedicle valves and 20 brachial valve, but things are different for this species in the My Due area where there are 451 pedicle valves and 234 brachial valves of this species; 3) The fos­sil associations we found in these two localities appear to be relatively in situ, without any eviden­ce of mixing of components of one community with those from another; 4) There is a complete size spectrum, large to small, for the relatively abun­dant taxa are all preserved in the place where they inhabited with an overall range in shell width from 2-4 mm to more than 20 mm for the species (Ret­ziella weberi, Retziella alaica, Nikiforovaena viet- namensis nov. sp., and ‘Howellella’ cf. lynxoides). The same thing characterizes the other fossils, including the bivalves and gastropods. Based on the statem ents above, it seems tha t there is no evi­dence for significant net directional transportation in these collections. Of course, this does not mean tha t there were no water currents present, because most of the shells are disarticulated.The fossils from the Kien An area, indicate a shal­lower environment than those from the My Due area. One of the evidences for demonstrating this conclusion, in particular, is tha t the bivalves from the My Due area (2.56 %) are much less abundant than those from the Kien An area (57.3 %), indica­

310i

3277821

145 5 1

182

3459

321

ting a shallower water environment for the latter. A higher proportion of the gastropods (9.4 %) in the Kien An area than those in the My Due area (2.27 %) again supports this conclusion. A lower diversi­ty (5 different higher taxa of fossils, and only 4 spe­cies of brachiopods) and high dominance of bivalves and Retziella weberi, which make up 90 % of the association in the Kien An area again suggests tha t the locality represents a nearshore, very shallow water environment (most likely BA 2).The My Due area locality, on the other hand, repre­sents a relatively deeper, normal marine environ­ment than th a t in the Kien An area, but still is considered a shallow water situation. This is m ain­ly based on the higher diversity of the entire faunal association (10 higher taxa), higher brachiopod diversity at the generic level, far fewer bivalves and gastropods, much more abundant brachiopod indi­viduals, and presence of some normal marine, shal­low water benthic invertebrates [such as rugose corals, bryozoans (?), tabulates, ostracodes and others which are lacking in the Kien An area] in the My Due area than in the Kien An area. Therefore, an assignment to inner BA 3 for the col­lection from the My Due area is suggested. This is consistent with all the records eisewhere in Asia and Australia for the Retziella Fauna which has been ascertained to have no deep water representa­tion in BA 4-5 (Rong et al. 1995). The new Vietna­mese data further support the conclusion th a t the Retziella Fauna is typicaliy a nearshore, shallow water benthic fauna, mainly BA 2 to shallower BA 3 position, a good environmental indicator for investigation of brachiopod communities in the Sino-Australian Province during the Late Silurian.

The overall abundance of bivalves a t Kien An, as stated above, suggests a very nearshore environ­ment, permissively in agreement with what the ecologically better known brachiopods suggest. The taxonomic composition of the bivalve assemblage is similar to th a t a t My Due, with the relative abun­dances also being not too different from each other (Goniophora and Schizodus are the abundant taxa, with Actinoptena absent a t My Due).The very rare, disarticulated vertebrate plates (Thanh et al. 1997) found in the My Due area pro­bably were transported into the Retziella weberi Community there, although whether from a marine or nonmarine fauna elsewhere is unknown.The My Due area fauna is assigned to the Retziella weberi Community, but the Kien An area fauna belongs to a bivalve-Retziella weberi unit, which might represent a mixture between a somewhat deeper water, i?etete//a-dominated fauna and a shallower water, nearer shore, bivalve-dominated fauna; in the absence of more samples from other localities we leave the Kien An area fauna unna­med.

BIOGEOGRAPHIC RELATIONSHIPS

The Retziella Fauna occurs in the North China Plate (Central Jilin and southern Inner Mongolia,

with a possible occurrence in North Korea), South China Plate (eastern Yunnan, eastern Guangxi, northern Vietnam, western Sichuan, and western Qinling), Tarim Plate (including southern Tien- shan, Central Asia), Indochina Plate and A ustra­lian Plate in the Late Silurian. In addition, there are some possible or doubtful occurrences of the genus Retziella in Central Pamir, Eastern Afgha­nistan, Eastern Iran, and the northwest corner of South Island, New Zealand (Rong et al. 1994). All of the definite areas are characterized by the pre­sence of the shallow water, normal marine, Retziel­la Fauna, indicating a possibly close biogeographic relationship among them in the Late Silurian, and a Sino-Australian Province has been proposed for the above regions th a t include the fauna (Rong et al. 1995). Both the Retziella Fauna and the Tu- vaella Fauna have been recognized as two major biogeographic units within the Uralian-Cordilleran Region (Boucot 1975; Rong et al. 1995).As one result of this study it has become apparent tha t those howellellids with one or more plications in the sulcus and corresponding grooves on the fold, a group of species th a t we suspect will end up in the genus Nikiforovaena (see taxonomic discussion of this genus), provides one more piece of evidence strengthening the concept of the Sino-Australian Late Silurian Province by providing a better understanding of the distribution and morphology of that genus.1) Relation to Central Asia. It is remarkable that Central Asia and Indochina possess the same spe­cies group of Retziella with the most abundant, com­mon species, R. weberi. The more interesting situa­tion is that R. weberi is associated with R. alaica N ikiforova [= Retzia (Retziella) weberi var. alaica N ikiforova , 1937] in both Central Asia (Nikiforova 1937) and Vietnam (this paper). Moreover, both regions have similar constituents not only at the generic, but also at the specific levels for the bra­chiopod fauna in which Retziella occurs.According to Nikiforova (1937, p. 7), Retziella webe­ri is one of the most im portant species of the Isfara fauna (Pridolian) of Central Asia in which is Eoreticularia tsehernyschewi N alivkin (probably Spirinella), Nikiforovaena ferganensis N ikiforova , and Lissatrypa camelina B u c h (should be Atry- poidea). As mentioned above R weberi is associated with R. alaica in Central Asia where diversity of the fauna is changeable: only a single species (R. weberi) (sec Nikiforova 1937, pi. 12, fig. 8) at one extremity, or at the other extremity more than 6 taxa as exemplified by an association at outcrop number 1552 collected by Weber in 1910, including R. weberi N ikiforova , Eoreticularia tschernyschewi mattschensis N ik ifo r o v a (probably Spirinella ), Eospirifer cf. togatus (B arrande), Atrypoidea came­lina (B u c h ), and Atrypa aspera aff. squamosa (So- w erby) (probably Gotatrypa) in 1552 m, and Schizo- phoria (Eoschizophoria) ferganensis N ikiforova in 1552i, and Nikiforovaena ferganensis (N ikiforova) in 1552 (with no ‘i’ or ‘m’ marked after ‘1552’) (Nikiforova 1937).The majority of these genera in the fauna in Central Asia are known from the My Due area on

322

the Indochina Plate, and some are known in the Kien An area of the South China Plate. Both Cen­tral Asia and Vietnam have R. weberi and R. alai- ca; ‘Howellella’ cf. lynxoides (N ikiforova); Nikiforo- vaena vietnamensis nov. sp. from the Kien An area and the My Due area are also very similar to H. lynxoides and N. ferganensis respectively from Central Asia. It should be kept in mind tha t the lat­ter four species are most abundant in the collec­tions from the Kien An area and the My Due area. Moreover, Tadschikia, a distinctive genus of rhyn- chonelloid in Central Asia, has also been recorded in Vietnam (Duong 1980, p. 68, pi. 34, figs 8a-d). Although some genera (such as Eospirifer, Eoschizophoria, Atrypa) occurring in Central Asia have not been found in Vietnam, and some [such as Morinorhynchus, Retieulatrypa and an undetermi­ned taxon (gen. and sp. indet. in this paper)] pre­sent in Vietnam have not been discovered in Cen­tra l Asia, this probably reflects different lithofacies or collection bias. It seems to the authors th a t the similarities between these two regions are more important than the differences.2) Relation to South China. There are two types of Retziella in eastern Yunnan, South China, one with a single rib, and another w ith two or more ribs in the pedicle sulcus, namely R. uniplicata (G rabau) and R. minor (H ayasaka) [= R. plicata (M ansuy) and many other synonyms] respectively. Remarkably, they are similar to R. weberi and R. alaica respec­tively from Central Asia and Vietnam, but they are different evolutionary stocks within the genus. This led us to conclude th a t the Retziella fauna descri­bed in this paper from Vietnam is closer to tha t of Central Asia than to South China, although, R. weberi and R. uniplicata are very similar to each other if they are not conspecific, even though diffe­rent tectonic plates are involved while keeping in mind tha t the Tarim Plate may represent a former northwestern portion of w hat is now the South China Plate with the Indochina Plate close to both. This conclusion is further supported by the follo­wing evidence. There are 5 genera (Eoschizophoria, Aesopomum, Atrypa, Protathyris, and Striispirifer) in South China th a t have not been found in Viet­nam, and Retieulatrypa and an undetermined taxon (gen. and sp. indet. in this paper) from Viet­nam th a t have not been discovered from South China. Additionally, abundance of some common genera is different in the two regions: Atrypoidea and Spirinella are common in South China but very rare in Vietnam. Moreover, specific consti­tuents of the fauna in Vietnam are evidently diffe­rent from those of South China. All of this suggests that the discrimination between them may have been related to slightly different environments, chiefly different substrates on which they lived. We are aware tha t both regions have many genera in common (Morinorhynchus, Atrypoidea, Retziella, ‘Howellella’, Nikiforovaena and Spirinella), indica­ting th a t they belonged to the same biogeographic province in the Late Silurian, even to the same sub­province (Rong et al. 1995), in spite of occurring in different lithofacies.Most recently, Rong et al. (1995) recorded an occur­rence of a very low diversity, high dominance Ret­

ziella Fauna with a single species of Retziella from Late Silurian rocks a t Chengxi, eastern Guangxi, geographically closer to the Kien An area of Vietnam, but considered to be within the Yunkai block (or terrane) rather than in the South China Plate, far from the eastern Yunnan region in the Late Silurian.It is notable th a t the bivalve species from both Kien An and My Due have much in common (4 out of 6 and 3 out of 5 respectively) with those from the Qujing area of eastern Yunnan, which further rein­forces the evidence provided by the brachiopods. The other species are either new, not known from elsewhere, or not identifiable specifically.3) Relation to eastern Australia. Since Molongia elegans capricornae M cK ellar from Queensland, New South Wales, Victoria and the Australian Capital Territory has been moved to Retziella (Rong et al. 1994), the occurrence of the Retziella Fauna in eastern Australia has been confirmed. It should be pointed out th a t R. capricornae (M cK ellar) possessing a single median rib in the sulcus is very similar to R. weberi or R. uniplicata if they are not conspecific. Besides, the Australian and Vietnamese faunas have some common genera including Retziella, Morinorhynchus, Atrypoidea, Atrypa, ‘Howellella’, Nikiforovaena, and Spirinella. All these facts suggest a faunal relationship bet­ween Vietnam and eastern Australia during the Late Silurian. However, the relation of Vietnam to eastern Australia was significantly less than that to Central Asia and South China. This is because many genera present in eastern Australia have not been discovered in Vietnam, including Aeginia, Pholidostrophia, Mitchellella, Protochonetes, Ali- conchidium, Notoconchidium, Molongia, and Strii­spirifer. The overall abundance of Notoconchidium in the Tasman region and its absence in Asia emphasizes the difference between these two geo­graphic entities. The occurrence of some endemic forms led Rong et al. (1995, p. 48) to establish a new subprovince w ithin the Sino-A ustralian Province for eastern Australia. A separate biogeo­graphic unit for the Australian Retziella faunas may eventually be needed.Iu summary, we position the Indochina Plate, including the My Due area, between the Tarim (including Central Asia) and South China Plates, and closer to the Tarim Plate than to South China particularly in terms of the same species group of Retziella in both the Tarim and Indochina Plates.Genera* Central S. Inner Eastern Kien An My Due Eastern

Asia Mongolia Yunnan Vietnam Vietnam Australii

1. Eoschizophoria X X X2. ‘Leptostrophia' X3 Aesopomum ? X4. Morinorhynchus X X5. Gypidula

(Gashaomiaoia) X6. Stegerhynchus X7. Linguopugnoides X8. Tadschikia X X X9. Atrypoidea X X X X X10. Retieulatrypa X11. Atrypa X X X12. Retziella XXX XXX XXX XXX XXX XXX13. Molongia X

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14. Protathyris X15. Eospirifer X 9 X16. Striispirifer X17. Howellella X X X X X X18. Nikiforovaena X X X X X19. Spirinella X X X X X

*The data came from Nikiforova (1937) for C entral Asia; Rong et al. (1985) for southern Inner Mongolia; Rong & Yang (1980) and Fang et al. (1985) for eastern Yunnan; th is paper for V ietnam; and McKellar (1969), S trusz e t al. (1984) for A ustralia. Some data are from Rong e t al. (1995). O ther gene­ra, such as Aegiria, Pholidostrophia, Mitchellella, Protocho- netes, Aliconchidium, and Notoconchidium, which only occur in A ustralia in the Late Silurian, are not included here. XXX means the most common species in the collection.

1) Morinorhynchus sp.; 2) Atrypoidea sp.; 3) Reti- culatrypa sp.; 4) Retziella weberi N ikiforova , 1937; 5) Retziella alaica N ikiforov a , 1937; 6) ‘Howellella’ cf. lynxoides (N ikiforov a , 1937); 7) Nikiforovaena vietnamensis B oucot & R o n g nov. sp.; 8) Spirinella ? sp.; and 9) Gen. and sp. indet.

BRACHIOPODA

Order STROPHOMENIDA Superfamily ORTHOTETOIDEA Waagen, 1884

Family CHILIDIOPSIDAE Boucot, 1959 Genus M orinorhynchus H a v licek , 1965

REGIONAL RELATIONS

The regional paleogeographic and lithofacies rela­tions of the Retziella Fauna are complex. The South China Plate includes northern Vietnam, and proba­bly the Kien An locality. A central, eastern Chinese locality yielding a very low diversity Retziella assemblage in eastern Guangxi (Chengxi) is consi­dered to be within the Yunkai block (or terrane), ratier than in the South China Plate in the Early Paleozoic. It m ight have been related to the Indochina Plate, but this remains to be decided. In southern Guangxi, and the Qujing area of eastern Yunnan, one finds Late Silurian or Early Devonian beds resting with angular unconformity on the Cambrian or Ordovician. The Retziella Fauna occurning in eastern Guangxi and in eastern Yunnan is interpreted to be present in ‘bays’ exten­ding inland into an overall nonmarine region. The Early Devonian faunas of this region are present in the Old Red Sandstone facies, locally interpreted as being nonmarine, and overlain by marine beds canying the Emsian, Early Devonian Rostrospirifer tonkinensis Fauna. In none of these areas is grapto- litic earlier Silurian recognized either above or below the angular unconformity. In the Hai Phong region, which includes the Kien An area, on the contraly, there is good evidence in the islands pre­sent to the northeast for graptolitic pre-Retziella Fauna, with nonmarine Devonian also occurring in this region. To the south, in the My Due area, cen­tral Vietnam region on the Indochina Plate, one also finds graptolitic, older Silurian overlain by the shal­lower water Retziella Fauna, overlain in turn by Old Red Sandstone facies beds tha t have flot yet yielded dated fossils. The overall faunal similarities between the Kien An area Retziella Fauna and that to the south in the My Due area suggests tha t they were not far apart geographically during the later Ludlow-Pridoli. A unifying theme here is tha t latest Silurian Pridoli, Downtonian) to Early Devonian Old Red Sandstone facies is present almost everyw­here.

SYSTEMATIC PALEONTOLOGY

R epository - The specimens figured and described here are deposited a t the Hanoi Geological Mu­seum, Collection Number: BT 178. 9 brachiopod species are described in this paper.

Morinorhynchus sp.Fig. 2.1-2, 15

M aterial - Only 3 specimens consisting of 2 pedicle valves w ith in ternal and external molds and 1 brachial valve w ith both molds.

Description - Shell small, 2.5-6.4 mm in length and 3.6-9.7 mm in width, transversely subrectan- gular in outline, gently biconvex in profile; shells not always symmetrical. Pedicle valve interarea high, catacline or slightly hypercline; brachial valve interarea anacline, much lower than pedicle valve. Ornament of fine costellae, increasing main­ly by insertion, costellae sharp, finer than inter­spaces; a few concentric growth lamellae developed near the anterior margin.Pedicle valve interior with short and very thin den­tal plates, widely divergent.Brachial valve interior with short socket ridges, widely divergent a t about 120 degrees; they connect with cardinal process which is bilobed, separated and extending evidently posteriorly above the hin- geline; no muscle scars seen.M easurem ents - Dimensions of the figured specimens of Morinorhynchus sp. (in mm).

№ of specimen

1. Internal pedicle mold2. Internal pedicle mold3. In ternal brachial mold

Length Width Height of interarea

2.63.96.4

3.65.69.7

1.5??

Comparison- The assignment of this undetermi­ned species to Morinorhynchus is considered to be better than to assign it to Aesopomum since it pos­sesses dental plates, although they are thin and short, and its cardinalia are much more like those of Morinorhynchus. This species is also characteri­zed by rather small individual size, whereas other species attributed to the genus are larger than the valves described here.

Order ATRYPIDA Superfamily LISSATRYPOIDEA Twenhofel, 1914

Family LISSATRYPIDAE Twenhofel, 1914 Genus A trypo idea M itc h ell & D u n , 1920

Atrypoidea sp.Fig. 2.9-14, 18-19

M aterial - 16 in ternal pedicle valve molds and 13 in ternal brachial valve molds.

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325

D escription - Shell small, width and length mea­sured less than 10 mm (see below), elongately oval in outline, gently dorsi-biconvex in profile; pedicle valve shows a very shallow median depression near the anterior margin of the shells; shell surface smooth. No dental plates in the pedicle valve. Bra­chial valve interior with separate hinge plates and widely divergent socket ridges. Muscle scars in both valves not observed.M e a su re m e n ts - Dimensions of the figured specimens of Atrypoidea sp. in mm.

№ of specimen Length Width

1. Internal pedicle mold 6.1 5.92. Internal pedicle mold ? 7.03. Internal pedicle mold 7.2 7.94. Internal brachial mold 3.9 4.25. Internal brachial mold 6.1 6.66. In ternal brachial mold 9.3 9.9

Rem arks - This taxon is assigned to Atrypoidea based on its smooth surface, its cardinalia, and the absence of dental plates. There are many species which have been attributed to Atrypoidea up to date. It is difficult to compare our Vietnamese spe­cimens with those of other species since 1) there are probably some synonyms within the genus and even different species which possess similar shell shape at the early stages; 2) there are no large indi­viduals of Atrypoidea sp. in the collection we stu ­died herein for comparison.

Superfamily ATRYPOIDEA Waagen, 1881 Family ATRYPIDAE Waagen, 1881

Genus R eticu la tryp a S avage, 1970

Reticulatrypa sp.Fig. 2.3,4

M a te ria l - Only one pedicle valve w ith both external and in ternal molds.

D escription - Shell small, 4.8 mm long and 5.4 mm wide, subcircular, pedicle valve gently convex with a median crest like a low fold; ornament of fine costeliae with about 7 regularly spaced concen­tric lamellae.Pedicle valve interior with thin, short, divergent (about 80 degrees) dental plates, and muscle scars discernible.R e m a rk s - Many features, such as outline, convexity and ornam entation of the shells, of th is undeterm inable species are consistent w ith those proposed by Savage (1970) for the

genus Reticulatrypa. We do not give a species name for the m aterial in th is paper because of the presence of only a single pedicle valve.

Order ATHYRIDIDA Boucot, Johnson & Staton, 1964 Superfamily RETZIELLOIDEA Modzalevskaya, 1996

R em arks - The retziellid group was assigned to Athyrisinacea in the 1965 Treatise on Invertebrate Paleontology Brachiopoda Volume which was followed by many paleontologists. Recently, Rong et al. (1994) thought th a t it is essentially different from the athyrisinids which are similar to athyridids and they attributed it to the Meristelloidea Waagen, 1883. Most recently, Alvarez, Rong and Boucot (1998) have further pointed out that the ret- ziellids are peculiar both in their external and internal struc­tures and agree with Modzalevskaya (1996) in promoting this group to superfamily rank. There is only one family, Retziellidae, involved in this superfamily. It includes five genera: Gissarina M enakova & N ikiforova , 1986; Metathyrisina R ong & Yang , 1981; Molongia M itchell , 1921; Qinlingia R ong , Z hang & C h e n , 1987; Retziella N ikiforova, 1937.The biogeographic significance of th is peculiar group has been discussed in Rong et al. (1995) and its fu rther implications are evaluated above.

Family RETZIELLIDAE Rhzonsnitskaya, 1974 (nom. transl. Rong, Strusz, Boucot, Fu, Modzalevskaya & Su,

1994 ex Retziellinae Rzhonsnitskaya, 1974, p. 54)

(= M etathyrisinidae W a n g , R ong & Y ang , in Rong & Yang, 1981, p. 245)

Genus R etz ie lla N ikiforov a , 1937T ype S pec ies - Retziella weberi N ik ifo r o v a , 1937, p. 57, pi.12, figs 8, pi. 14, figs 1-4; from U pper Isfara Horizon (Pridolian), Isfara River, northern slope of Alay M ountains, southwest Khirghizistan, C entral Asia. The holotype speci­men of th is species has been re-illustrated by Rong et al. (1994).

Em ended D iagnosis - Retziellidae with variably developed ventral sulcus and dorsal fold, both usually plicate (ribbed), with 1 to 3 (a few even 4 or 5) plicae in the sulcus and 3 to 7 on each flank, with thin, short dental plates and very narrow lateral cavities in the pedicle valve, and with a well-deve­loped apical septalium supported by a short, thin median septum in the brachial valve.R em arks - Having studied the internal structures, Rong et al. (1994) concluded that Protathyrisina Z h u , 1974, from South China, Gannania Fu, 1982 and Stegospira Fu, 1982 from Western Qinling should be regarded as junior subjective syno­nyms of Retziella. Some species formerly assigned to Athyrisina and Molongia should also be attributed to Retziella. All species of the genus Retziella have been listed in Rong et al. (1994). Retziella can be distinguished from Molongia in having a rib­bed pedicle sulcus and well developed septalium supported by

F ig u r e 2 - 1-2,15. Morinorhynchus sp. 1, ventral external impression; 2, ventral internal impression; 15, dorsal internal impression; all specimens x 5. 3, 4. Reticulatrypa sp., ventral external and internal impressions, x 4. 5. Spirinella ? sp., ventral internal impression, x 3. 6-8, gen. et sp. indet. 6, ventral internal impression. 7-8, ventral external and internal impressions, x 3. 9-14, 18-19. Atrypoidea sp., 9-11,19, internal dorsal impressions, x 3. 12-14 (11, 13, same specimen), 18,19, internal, external, ventral impressions, x 3. 16-17, 20­35. Retziella weberi N ik ifo ro v a , 16-17, two dorsal internal impressions, x 2. 20, 23, 27-28, steinkern, anterior, lateral, ventral, dorsal views, x 3. 21, 29-31, steinkern, anterior, dorsal, lateral, ventral views, x 3. 22, ventral internal impression, x 2. 24, dorsal internal impression, x 2. 25, dorsal external impression, x 2. 26, ventral internal impression, x 3. 32-34, three dorsal internal impressions, x 3. 35, dorsal external impression showing coarse, concentric growth lamellae, x 5 .1-2,15. Morinorhynchus sp. 1, moule externe ventral; 2, moule interne ventral; 15, moule interne dorsale; tous x 5. 3, 4, Reticulatrypa sp. moules externe et interne ventraux x 4. 5, Spirinella ? sp. moule interne ventral. 6-8, gen et sp. indet. 6, moule interne ventral; 7, 8, moules externe et interne ventraux. 9-14, 18-19. Atrypoidea sp. 9-11, 19, moules internes dorsaux x 3. 12-14 (11, 13, même spécimen); 18, 19, moules interne et externe ventraux x 3. 16-17, 20-35. Retziella weberi N ik ifo r o va . 16, 17, deux moules internes dorsaux x 2 . 20, 23, 27-28, moule interne en vues antérieure, latérale, ventra­le et dorsale x 3; 21, 29-31, moule interne en vues antérieure, dorsale, latérale et ventrale x 3; 22, moule interne ventral x 2; 24, moule interne dorsal X 2; 25, moule externe dorsal x 2; 26, moule interne ventral x 3; 32-34, trois moules internes dorsaux x 3; 35, moule exter­ne dorsal montrant les fortes lamelles de croissance concentriques x 5.

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a m edian septum , whereas Molongia has a smooth pedicle su l­cus, and no septalium in the brachial valve.

A ge and D istr ib u tion - Late Wenlock to Pridoli; K irghi­zistan and Tadzhikistan, C entral Asia, N orthern Tarim, South C hina, N orth China, N orth Korea (?), no rthern Vietnam, C entral V ietnam and E astern A ustralia.

Retziella weberi N ikiforova , 1937 Figs 2.16-17, 20-35; 3-18-20, 24-25

Retziella weberi N ik ifo r o v a , 1937, p. 57, pi. 12, fig. 8, pi. 14, figs 1-4; Nikiforova 1949, p. 19, pi. 7, fig. 4; Rzhonsnitskaya 1974, p. 61, pi. 11, fig. 8; Duong 1980, p. 68, pi. 34, figs 4-7; Rong et al. 1994, p. 566, pi. 2, figs 9, 12-19, 21-27; pi. 4, fig. 12; Rong et al. 1995, p. 41, Fig. 1, L,P,Q,V.

M aterials - All specimens of this species are preserved in a coarse sandy m atrix as external and in ternal molds. 1777 pedicle in ternal molds and 1391 brachial in ternal molds from the My Due area and 51 pedicle and 86 brachial molds from the Kien An area. Most of them are disarticulated, bu t 35 conjoined valves were recovered. The in ternal structures of both pedicle and brachial valves are well preserved w ith the exception of spiralia. We only obtained a single specimen w ith spiralia directed laterally, indicating its assignm ent to the rib­bed, im punctate athyridids.

D escription - Shell medium to large in average size (the minimum width of the shell only attains 3 mm, and the maximum more than 15 mm); chan­geable in outline, usually round to roundly penta­gonal, wider than long, and vice versa as well; gent­ly subequally ventri-biconvex in lateral profile; ventral beak prominent and relatively high, with a mesothyrid foramen and conjoined small deltidial plates underneath the foramen; shallow ventral sulcus and low brachial fold present, the sulcus expanded widely anteriorly. Ornament of promi­nent, usually rounded plications well preserved, interspaces between two plications wider than the latter; usually 4 plications, sometimes 3 and 5 on the flank area, only a single plica present in the sulcus and 2 on the fold; all plications s ta rt from the beak of the shells; widely located, strongly concentric lamellae well developed on the whole shell surface, in particular on the anterior half of the shells, one specimen (lenght 11.5 mm, width 12.5 mm) with 16 concentric lamellae, the lamellae crowded near the anterior marginal area.

Pedicle valve interior with thin, short, widely diver­gent dental plates, attaining 1/7-1/10 of shell leng­th; lateral cavities very narrow and small; muscle scars usually discernible, possibly limited to the pedicle chamber area.

Brachial valve interior with thinner outer hinge plates supported dorsally by a pair of plates conver­gent onto a thin median septum to form an apical septalium which is very small and shallow (see dis­cussion below); the median septum short, exten­ding anteriorly, gradually thinning, and attaining about 1/5-1/4 the length of the shells; spiralia orien­ted laterally.M easurem ents - Dimensions of the figured specimens of Retziella weberi N ik if o r o v a , 1937 (in mm). Abbreviations are length (L), w idth (W), thickness (T), num ber of la teral plica­tions (LP) and num ber of plications in the sulcus (SP) or fold (FP).

№.of specimens L W T LP SP FP

1. Internal brachial mold 3.1 3.2 3 22. Conjoined valves 7.4 7.8 2.9 4 1 23. Internal brachial mold 9.5 10.7 4 24. Internal brachial mold 9.8 14.2 3-4 25. Conjoined valves 10.0 11.7 7.0 4-5 1 26. Internal brachial mold 10.8 14.0 3 27. Internal pedicle mold 12.9 13.1 4 18. Internal pedicle mold 12.2 12.3 3-4 19. Internal pedicle mold 12.6 12.0 5 110. In ternal brachial mold 16.1 13.2 4-5 2

Comparison - Two species of the genus Retziella, R. weberi Nikiforova, 1937 from the Pridolian rocks of Central Asia and R. uniplicata (Grabau, 1931) from late Ludlow to early Pridoli strata of south­west China, are very sim ilar to each other. Externally they have the same shape and size range, plications well developed on flanks, and only 1 plication in the pedicle sulcus and 2 on the bra­chial fold, and the latter two variably developed in both species. They have great variation in shell shape and size in each population. For example, in addition to the shape of the shells, numbers of pli­cations are also variable in R. weberi: we have mea­sured both pedicle and brachial valves of 1) the holotype of the species (Nikiforova 1937), 2) two topotype specimens housed in the Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing (Rong et al. 1994), and 3) 13 specimens of this species from Central Asia housed in the U.S. National Museum, Washington, altogether 18 speci­mens with 36 valves: 19 valves with 4 plications (including the holotype and all other topotype speci­mens), 2 with 4-5 plications, 8 with 5 plications, 4 with 6 plications, and one each with 3, 3-4, and 5-6. But, we can see tha t the majority of the species pos­sess 4 plications on each flank. But, for R. uniplica­ta, we measured the neotype (NIGP 46666: length 9.0, width 9.1, and thickness 5.7 mm) and another

F ig u r e 3 - 1-3, 6-7,12-15, 21-23, 26-27. Retziella alaica N ik ifo ro v a , 1, dorsal internal impression, x 3. 2-3, two ventral internal impres­sions, x 3. 6-7, two ventral internal impressions, x 2. 12-13, two ventral internal impressions, x 2. 14, dorsal internal impression, x 2.15, ventral internal impression, x 2. 21, dorsal internal impression showing septalium, x 8. 22, dorsal internal impression, x 3. 23, ven­tral external impression showing fine filae, x 5 (on same specimen as bivalve holotype). 26-27, ventral external impressions showing fine flac, x 8. 18-20, 24-25. Retziella weberi N ik ifo ro v a , 18-20, 24, steinkern, dorsal, lateral, ventral, anterior of same specimen, x 3. 25, dorsal internal impression, x 2. 4-5, 8-11, 16-17. ‘Howellella’ cf. lynxoides (N ik ifo ro v a ) , 4, ventral internal impression, x 3. 5, rubber replica of ventral external impression, x 3. 8, 9, two dorsal internal impressions, x 3. 10, dorsal external impression, x 3. 11, ventral external impression, x 3. 16, 17, rubber replica of ventral external impression, x 3. 1-3 , 6-7, 1 2 -1 5 , 2 1 -2 3 , 2 6 -2 7 . Retziella alaica N ikifo ro va . 1, moule interne dorsal x 3; 2 -3 , deux moules internes ventraux x 3; 6-7, deux moules internes ventraux x 2; 12 -1 3 , deux moules internes ventraux x 2; 14 , moule interne dorsal x 2; 15 , moule interne ventral x 2; 2 1 , moule interne dorsal x 3; 2 3 , moule exter­ne ventral montrant les fines filae x 5 (sur le même échantillon que Vholotype); 2 6 -2 7 , moules externes ventraux montrant les filae x 8. 1 8 -2 0 , 2 4 -2 5 . Retziella weberi N ik ifo r o va . 1 8 -2 0 , 2 4 , moule interne, vues dorsale, latérale, ventrale et antérieure x 3; 2 5 , moule interne dorscil. 4-5 , 8 -1 1 , 1 6 -1 7 . ‘Howellella’ cf. lynxoides (N ikiforova). 4 , moule interne ventral x 3; 5, empreinte en latex d ’un moule externe ven­trale x 3; 8, 9 , deux moules internes dorsaux x 3; 10 , moule externe dorsal x 3; 11 , moule externe ventral x 3; 16 , 17, empreinte en latex d ’un moule externe ventral x 3.

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topotype (NIGP 46667: length 12.5, width 12.9, and thickness 8.0 mm) which have 6 and 5 in each of the valves. This feature can be used for distinguishing this species from R. uniplicata. Moreover, the inter­spaces between plications in R. weberi are usually wider than those in R. uniplicata.The Vietnamese specimens identified as R. weberi were originally described briefly and illustrated by Duong (1980) and are studied in detail in this paper. They are more like R. weberi of Central Asia than R. uniplicata from southwest China. We have measured lateral plications on the flanks of 10 figu­red specimens (two conjoined valves, 3 pedicle val­ves, and 5 brachial valves) with the following results: 3 and 3-4 plications on 2 valves, 4 plica­tions on 4 valves, 4-5 plications on 3 valves and 5 plications on one valve. The majority of the valves have 3-4 plications on each flank, a feature diffe­rent from R uniplicata. Moreover, the interspaces between two plications are wider in the Vietnamese specimens than in the Chinese specimens of R. uni­plicata.Retziella weberi N ikiforova is also similar to R. minor (H ayasaka, 1922) in many characters, inclu­ding general shell size, shape, profile, and plica­tions on the lateral areas. R. minor can be differen­tiated from R. weberi by having 2 or more (3-5) pli­cations in the pedicle sulcus, whereas Retziella weberi possesses only a single plica in the sulcus, and moreover R weberi has wider rib interspaces than R. minor.

Retziella alaica N ikiforov a , 1937 Fig. 3.1-3, 6-7, 12-15, 21-23, 26-27

Retzia (Retziella) weberi v a r . alaica N ik ifo r o v a , 1937, p . 58, p i. 14, figs 5-7.

M aterial - 175 pedicle in ternal valve molds and 145 brachial internal molds from the My Due area, and 10 pedicle and 11 brachial in ternal molds from the Kien An area.

D escription - Shell small to medium (the mini­mum width of the shell only attains 3 mm, and the maximum more than 16 mm); outline changeable, round to roundly pentagonal, wider than long or vice versa; usually gently subequally biconvex; ven­tral beak relatively high, pedicle sulcus shallow, narrow, or sometimes discernible during the whole ontogeny, and brachial fold low or sometimes indis­tinct. Ornament of prominent, usually low, rounded plications, interspaces between two plications pro- minantly wider than the latter; usually 14-16 pli­cations on the whole shell surface, 2 or more plica present in the sulcus and 3 or more on the fold; all plica originate a t the beak; numerous fine concen­tric fila well developed on the whole shell surface, a few strongly concentric growth lamellae only seen at the anterior margin.

Pedicle valve interior with a pair of thin, very short, widely divergent dental plates, lateral cavi­ties very narrow; muscle scars usually discernible, possibly limited to the pedicle chamber area.Brachial valve interior with outer hinge plates sup­ported dorsally by a pair of plates convergent onto a thin, short median septum to form an apical sep-

talium which is very small and shallow; the median septum attaining about 1/5 the shell length.M easurem ents - Dimensions of the figured specimens of Retziella alaica N ik if o r o v a , 1937 (in mm). Abbreviations are length (L), w idth (W), num ber of la teral plications (LP), and num ber of plications in the pedicle sulcus (SP) or brachial fold (FP).

№ of specimens L W LP SP FP

1. Internal pedicle mold 9.6 11.3 4-5 32. Internal brachial mold 10.2 10.8 4 (4) 53. Internal pedicie mold 10.4 10.5 5 34. Internal pedicle mold 15.6 13.8 4-5 35.Internal brachial mold 15.6 15.4 4-5 (3) 46. Internal pedicle mold 16.1 14.7 5 27. In ternal pedicle mold 16.4 14.5 5 2

Com parison - We promote Retzia (Retziella) webe­ri var. alaica N ikiforova , 1937, to specific level because alaica can be easily distinguished from R. weberi by the following characters: finer and more costae, much less developed (usually almost discer­nible) sulcus and fold, and more than 1 costa in the sulcus and more than 2 on the fold. Moreover, we examined the Vietnamese specimens of R. alaica studied in this paper which possess very fine, concentric fila on the whole shell surface, which are absent in R. weberi, and the concentric lamellae well developed in R. weberi are almost lacking or only located at the anterior marginal area of the shells in R. alaica.Retziella minor (Hayasaka) from the Miaokao Formation (late Ludlow-early Pridoli) of the Qujing area, southwest China, also has 2 plicae in the sul­cus, a character similar to R. alaica. They differ from each other in the following: R. minor pos­sesses smaller shell size on average, much more developed pedicle sulcus, and much less wide inter­spaces between two plicae than R. alaica. Regar­ding the synonymy question of species assigned to Retziella from South China there is a complicated story. What Grabau (1931) identified as Athyrisina plicata (Mansuy), Rong & Yang (1980) as Prota- thyrisinaplicata, and Rong et al. (1994) as Retziella plicata, can be differentiated from minor chiefly by having 3 plications in the sulcus. However, in Chinese specimens, those with 2 or 3 plications (or even more) in the sulcus occur in the same popula­tions, and may be better regarded as different phe­notypic forms of the same species. Based on the same reasoning, those with 4 plications in the sul­cus and identified as Retziella quadriplicata (Grabau) from the Miaokao Formation of the Qujing area (Zhu in Fang & Zhu 1974; Rong & Yang 1980) should also be assigned to Retziella minor (Hayasaka). It would be better to consider those forms with 2, 3, or 4 plications in the sulcus of various forms of the genus Retziella from South China as different phenotypes of the same species occurring in the same population. However, it should be pointed out that the holotype of Retzia plicata Mansuy, 1922, came from an unknown hori­zon at Lunan, Central Yunnan and is lost. Since its exact locality and horizon are unknown the topoty­pe specimens of this species cannot be restudied. It is also uncertain whether or not the internal struc­tures of Retzia plicata Mansuy are the same as those in Retziella minor. Meanwhile, the holotype

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specimen of Athyrisina quadriplicata G rabau , from the Middle Devonian Wangjia Beds of southern Gansu, North China (Grabau 1931), cannot be investigated as well because the holotypes have not been located. Therefore, it is also unknown whether or not A. quadriplicata G rabau and tha t identified as A. quadriplicata G rabau (in Rong et al. 1974) and Protathyrisina quadriplicata Z h u (in Fang & Zhu 1974) from the Miaokao Formation of the Qujing area are conspecific. Further investigation of these specimens is warranted.

Order SPIRIFERIDA Waagen, 1883 Superfamily DELTHYRIDOIDEA Phillips, 1841

Family DELTHYRIDIDAE Phillips, 1841 Genus H ow ellella KozLOW Sia, 1946

‘Howellella’ cf. lynxoides ( N ik if o r o v a , 1937)Fig. 3.4-5, 8-11, 16-17; 4-6, 18, 22-28

cf. Spirifer (Eospirifer) lynxoides N a livkin nomen nudum , in Nikiforova, 1937, p. 50, pi. 10, figs 7-1 1.

non Eospirifer lynxoides N a liv k in , Duong, 1980, p. 68, pi. 34, figs 3a-d.

M aterial - 21 pedicle and 20 brachial in ternal molds from the Kien An area, and 451 pedicle and 234 brachial valve molds from the My Due area. All preserved as in ternal and external molds.

D escription - Small to medium in size, transverse rhomboidal in outline, subequally biconvex in late­ral profile; wider than long, the maximum width at the hingeline, with acute extremities. Pedicle valve gently convex, interarea low, beak not higher, ove­rhanging the hingeline very much, umbo gently curved, not swollen; sulcus well developed, expan­ding anteriorly widely, occupying approximately 2/5 of the shell width. Brachial valve also gently convex, the most convex part of the shell near the mid line, median fold carinate, prominently above the flanks of the shells. Megascopic-ornament of about 3-5 (mostly 4 or 5), occasionally 6, simple, strong plications on the flanks, and a short, median rib in the pedicle sulcus with a corresponding median furrow on the brachial fold, in the smaller specimens the rib is absent, in a single specimen examined in the collection there are two weak, low plications present in the sulcus, in addition to the median one, there is another one lateral to the la t­ter, indicating an irregular phenomenon; micro­ornament is not usually well preserved in the col­lection, but a few specimens of this species clearly show concentric growth lamellae upon which are superimposed capillae tha t become fimbriate a t the anterior margin of the growth lamellae.Pedicle valve interior with short, thin, narrowly divergent dental plates, disposed ju st a t the outsi­de of the two strongest plications delimiting the sulcus, the la tte r extending anteriorly to expand widely and relatively deep; no muscle field seen.Brachial valve interior with a pair of widely diver­gent sockets, subparallel to the hingeline, crural plates almost lacking, striated cardinal process located at the end of small delthyrial platform; muscle fields discernible.

M easurem ents - Dimensions of the figured specimens of ‘Howellella’ cf. lynxoides (N ik i f o ro v a , 1937) (in mm). Abbre­viations are length (L), w idth (W), num ber of la teral plications (LP), and num ber of plications in the pedicle sulcus (SP) or brachial fold (FP).

№ of specimen L w LP SP FP

1. Internal pedicle mold 5.5 8.9 4 02. Internal pedicle mold 6.4 8.6 3 03. Internal pedicle mold 5.8 10.2 4 14. Internal pedicle mold 8.2 13.4 5 15. Internal pedicle mold 11.7 16.9 5 26. Internal pedicle mold <11.0 21.6(?) 6 17. Internal brachial mold 4 2 7.8 3 28. Internal brachial mold 6.2 12.0 4 29. Internal brachial mold 6.6 16.2 5 210. Internal brachial mold 7.1 13.8 4 211. In ternal brachial mold 7.4 13.4 4 2

R em arks - We do not consider th a t C arter et al.’s (1994) Suborder Delthyridina Ivanova, 1972, is needed, because the Delthyridoidea fit comfortably into the Suborder Spiriferidina in all regards. C arter et al.’s (1994) diagnoses of the two alle­ged suborders overlap in m any regards.

Comparison - There are more than 100, Silurian and Early Devonian species assigned to Howellella, but they almost all have a smooth sulcus without any sign of costae. Only very few species, very simi­lar in all other characters to Howellella, such as ‘Howellella’ tingi (G rabau , 1931) and ‘Howellella’ lynxoides (N ikiforova , 1937) have a median rib in the pedicle valve sulcus. Therefore, if further work is done, a new subgenus of Howellella or a new dis­tinctive genus based on the species ‘Howellella’ tingi (G ra ba u , 1931) or ‘Howellella’ lynxoides (N ikiforova , 1937) should be established if this new taxon and Nikiforovaena are not congeneric.‘Howellella’ cf. lynxoides is most like H. tingi (G rabau) from the Miaokao Formation, Qujing, eas­tern Yunnan, Southwest China in the presence of a median rib in the sulcus (Rong & Yang 1980) which is one of the main features of ‘H . lynxoides. It can be distinguished from H. tingi in having a rather more transversely extended shell shape, much more acute cardinal extremities, a wider ventral sulcus, more angular plications, and a more promi­nent median rib in the ventral sulcus than those in H. tingi.Externally, ‘Howellella’ cf. lynxoides is also similar to Spirifer (Eospirifer) balchaschensis N ikiforova (1937, p. 48, pi. 10, figs 20a, b, c, d) from the Late Silurian stra ta of the Pre-Balchasch region and southwestern Tien Shan, Central Asia. The latter has shell shape, outline, and size similar to ‘H . cf. lynxoides. But the latter species possesses more pli­cations on the flanks and usually one plication in the sulcus, whereas ‘Howellella’ balchaschensis (N ikiforova) has fewer (1-3) plications on the flanks and no median plica in the sulcus.It is clear tha t within the Sino-Australian Province there are a number of howellellid species with one or more plications in the sulcus, with corresponding grooves on the fold. In some cases, such as with ‘H .’ tingi G rabau (1926) (Rong & Yang 1978), there is a complete gradation within a single population from forms totally lacking a plication in the sulcus to ones with a short plication to still others with a long plication, i.e., this species is gradational in

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this regard between true Howellella and true Nikiforovaena.Again, the important point here is that H. tingi forms an evolutionary intermediate between the two genera. On the other hand we have species such as Nikiforovaena ferganensis, and our new species N. vietnamensis in which there is almost always a long plication in the sulcus. During ontogeny, of course, the small shells lack such a plication in the sulcus. Complicating this situation is the unfortu­nate fact tha t beginning with Nikiforova's (1937) original publication species with both eospiriferid and delthyrid type fine ornament have been first placed together into Eospirifer (N ikiforova , 1937) or later all placed into Nikiforovaena. It is now clear that two groups of species, belonging to two genera, themselves belonging to completely distinct families are involved. This situation requires careful popula­tion study of a number of topotype and type speci­mens, work tha t is beyond the scope of this paper. Finally, emphasizing the importance of this evolu- tionarily transitional situation is the fact that out­side of the Sino-Australian Province probably more than 100 species of Howellella have been described over the years, almost none of which have even a hint of a plication in the sulcus.

Genus N ikiforovaena B o u c o t, 1963

R em arks - The familial position of Nikiforovaena is uncertain because of the lack of well preserved specimens of the type spe­cies, N. ferganensis (N ikiforova, 1937), tha t show details of the micro-ornamentation. Boucot (1963) assigned Nikiforovaena to the eospiriferines when he erected it as a new genus following Nikiforova's earlier (1937) conclusion. This was accepted by P itrat in the 1965 Treatise volume oo Brachiopoda. However, C arter et al. (1994, p. 332) put Nikiforovaena into the Superfamily Spinelloidea Johnson 1970 [nom, transl. by Johnson & Hou, in C arter et al. 1994, p. 331) and further into Pinguispiriferinae Havlicek, 1971, saying th a t Nikiforovaena B oucot, 1963, with a previously unknown dorsal interior, was examined by preparing a dorsal internal mold of the type species; tha t revealed the presence of a ctenophoridium and short, diver­gent crural plates. However, it is now known tha t a striated car­dinal process (i.e. ctenophoridium) is a common structure within some Silurian and Early Devonian eospiriferines (such as Eospirifer, Striispirifer, Janius) and this structure cannot serve as an essential character for recognition of a higher taxonomic rank (such as a subfamily) (Ivanova 1971; Rong & Yang 1978). Further investigation of the micro-ornament in the type species of the genus Nikiforovaena needs to be done. 'This leaves us in the taxonomic quandary of having the type species of Nikiforovaena, N. ferganensis, w ith external gross morphology sim ilar to ‘Howellella’ tingi (G rabau) and our new species ‘H ’. cf. lynxoides, bu t fine ornam ent uncertain. If the fine ornam ent of N. ferganensis should upon further study

tu rn ont to be of delthyrid ra th e r th an eospiriferid type we would then reassign ‘i f . tingi and ‘i f . lynxoides to a new sub­genus of Howellella because the ir early ontogenetic stage is Howellella-like, i.e., completely lacking plications in the su l­cus, and their evolutionarily interm ediate position.

Nikiforovaena vietnamensis nov. sp. B oucot & R ong Fig. 4.1-5, 7-17, 19-21, 29

M a te ria l - All disarticulated specimens are molds w ith 441 pedicle and 335 brachial valves from the My Due area and 13 pedicle and 8 brachial valves from the Kien An area.

D iagnosis - Subcircular or round pentagonal shell outline, wider than long; shallow sulcus with a low, strong rib, or occasionally 3; coarse ornament of 4­6 simple, rounded plications on each flank, wider than interspaces. Dental plates short, thin, slightly divergent; crural plates very short, extending convergently to the valve floor.D escription - Medium in size, subcircular or round pentagonal in outline, subequally biconvex in lateral profile; wider than long, but with the occasional exception; the maximum width a t the hingeline, with generally rounded extremities. Pedicle valve gently convex, maximum convexity at the middle; beak prominent, slightly curved, evi­dently higher, overhanging the hingeline, interarea high, transversely triangular; umbo gently curved, not swollen; sulcus well defined but very shallow, anteriorly occupying 1/3 to 2/5 of the shell width. Brachial valve gently convex, the greatest convexi­ty near the mid line, median fold well defined, but low and not wide, slightly above the flanks of the shells. Coarse ornament of about 4-6, simple, rela­tively weak, rounded plications on the flanks; a low median rib starting from the umbo present in the pedicle sulcus w ith a corresponding shallow median furrow on the brachial fold, in the smallest specimen this rib is absent; occasionally, three weak and low plications present in the sulcus; micro-ornament of concentric growth lamellae and capillae.Pedicle valve interior with short, thin, slightly divergent or even subparallel dental plates, about one third the shell length; no muscle field seen.Brachial valve interior with very short crural plates extending convergently to the valve floor, striated cardinal process located at the end of small, convex part within the delthyrial platform; sockets widely divergent; muscle fields discernible.M e a s u re m e n ts - D im ensions of figured specimens of Nikiforovaena vietnamensis B o u c o t & R ong nov. sp. (in mm).

F ig u r e 4 - 1-5, 7-17, 19-21, 29. Nikiforovaena vietnamensis nov. sp. 1, ventral internal impression, x 3. 2-3, same individual, rubber replica of ventral external impression, ventral external impression, x 2. 4-5, two dorsal internal impressions, x 2. 7-9, three ventral internal impressions, x 2 .10 ,11 , two dorsal internal impressions, x 2. 12, rubber replica of ventral external impression, x 2.13, ventral internal impression, x 2.14-16, three dorsal internal impressions, x 2. 17,19-21, four ventral internal impressions, x 2, x 3, x 2, x 2. 29, ventral external impression slowing fine, concentric growth lines and spine fringe over the growth lines, x 5. 6, 18, 22-28. ‘Howellella’ cf. lynxoides (N ik ifo ro v a ) , 6, dorsal internal impression, x 3. 18, 22, two dorsal internal impressions showing cardinalia, x 8. 23, rubber replica of dorsal internal impression, x 2. 24-26, three dorsal internal impressions, x 3. 27, dorsal internal impression showing cardi­nalia, x 8. 28, ventral internal impression, x 3 .1-5, 7-27, 19-21, 29. Nikiforovaena vietnamensis nov. sp.. 1, moule interne ventral x 3; 2, 3, moule externe ventral et moulage en latex x 2; 4, 5, deux moules internes dorsaux x 2; 7, 9, trois moules internes ventraux x 2; 10, 11, deux moules internes dorsaux x 2; 12, latex d ’un moule externe ventral x 2; 13, moule interne ventral x 2; 14-16, trois moules internes dorsaux X 2; 17, 19-21, quatre moules internes ventraux x 2, x 3, X 2, x 2; 29, moule externe ventral montrant les fines lignes de croissance concentriques et la frange d ’épines. 6, 18, 22-28. ‘Howellella’ cf. lynxoides (N ikiforova). 6, moule interne dorsal x 3; 18, 22, deux moules internes dorsaux montrant les cardinalia x 8; 23, latex d ’un moule interne dorsal x 2; 24-26, trois moules internes dorsaux x 3; 27, moule interne dorsal montrant les cardinalia x 8; 28, moule interne ventral.

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Abbreviations are length (L), w idth (W), num ber of la teral pli­cations (LP), and num ber of plications in the pedicle sulcus (SP) or brachial fold (FP).

№ of specimen L W SW LP SP FP

1. Internal pedicle mold 5.8 8.2 2.9 3 02. Internal pedicle mold 10.3 11.8 4.1 6 13. Internal pedicle mold 11.2(?) 13.8 5.5 6 14. Internal pedicle mold 14.8 16.5 6.5 5-6 35. Internal pedicle mold 16.9 16.8 8.0 4 16 Internal pedicle mold 14.6 17.1 8.4 4 17. Internal brachial mold 3.2 4.3 1.4 2 18. Internal brachial mold 5.2 7.2 2.3 4 29. Internal brachial mold 9.2 12.3 5.1 4 210. Internal brachial mold 10.4 16.1 5.2 6 211. Internal brachial mold 13.6 18.4 8.0 5 212. In ternal brachial mold 14.3 21,0 8.1 6 2

Comparison - The Vietnamese specimens are relatively similar to those of Nikiforovaena ferga- nensis (N ikiforova) occurring in the Isfara Horizon (Pridolian), northern slope of the Alai Range, Central Asia based on the following three charac­ters: shell shape, relatively high, slightly curved pedicle interarea, and the presence of low, round plications with one in the sulcus (Nikiforova 1937, pi. 12, fig. 6a-c). Two or three plications can be pre­sent in the sulcus of N. ferganensis (N ikiforova), and there are also three plications in the sulcus in N. vietnamensis new species. But the Vietnamese specimens are different from Central Asian N. fer­ganensis mainly in having a more convex shell pro­file in particular in larger specimens, much stron­ger and wider median rib in the sulcus, and fewer plications on the flanks. The median rib in the sul­cus of N. ferganensis is sometimes hardly noti­ceable or faintly pronounced in the same popula­tion of this species (Nikiforova 1937, p. 48), but it is always very well developed in the Vietnamese spe­cimens.Nikiforovaena vietnamensis new species is also similar to N. borkoldoensis N ikiforova occurring with Retziella weberi N ikiforova in Central Asia (Nikiforova 1937, p. 54), but the latter possesses evidently larger shells attaining nearly 43.5 mm length, 39.0 mm width in the largest specimens (she measured 3 specimens of this species with the smallest one 26.7 mm long and 28.4 mm wide), on average, they are larger than Vietnamese speci­mens (the largest one is 14.3 mm long and 21 mm wide), much wider shell outline, and much more convex profile than those in N. borkoldoensis. Additionally, the micro-ornamentation of N. borkol­doensis is still unclear and Nikiforova (1937) stated th a t only some specimens preserve traces of concentric surface ornam entation for tha t species. Further investigation of th a t taxon is needed.Compare Spirifer (Delthyris ?) ferganensis N ik ifo ­rova, 1937, p. 137, pi. 12, fig. 4-7; Boucot 1963, p. 697, pi. 103, figs 1-6; P itra t 1965, p. H. 670, Fig. 545-2a-e with Eospirifer lynxoides N aliv kin , Duong 1980, p. 68, pi. 34, figs 3a-d, which are similar but for which there is inadequate material to be cer­tain.

Superfamily RETICULARIOIDEA Waagen, 1883 Family RETICULARIIDAE Waagen, 1883

Genus S p ir in e lla J o h n sto n , 1941

Spirinella ? sp.Fig. 2.5

M a te ria l - Only a single pedicle valve, external and in ternal molds.

D escription - Small shell, 5.5 mm long and 7.4 mm wide, semicircular in outline with a small, acute pedicle beak slightly extended posteriorly, and gently convex in profile; pedicle valve interarea low; sulcus narrow and shallow. Pedicle valve interior with short (only about 1.0 mm long) and divergent dental plates; no muscle scars seen.R e m a rk s - Since the m aterial of this species is so scarce we could not definitely identify it. There are no brachial valves in the collection and it is difficult to be sure w hether it is Spirinella or Eoreticularia. The former commonly occurs in the Retziella Fauna and we pu t a question m ark following our assignm ent of this taxon.

Order and Family UncertainGen. and sp. indet.

Fig. 2.6-8

M a te r ia l - Two pedicle valves include both external and in te r­nal molds.

D escription - Shell small to medium (one 4.1 mm long and 5.2 mm wide; another one 9.4 mm long and 12.4 mm wide), transversely extended, with rounded extremities, pedicle valve gently convex, interarea high, strongly apsacline, median sulcus shallow and wide, about 60 % of the maximum width of the shell. Ornament of several coarse, short plications on the flanks (2-3 and 3-4 in the smaller and larger specimens respectively), the central pair of plications are much stronger and longer thas the others, the others are short and not prominent in the posterior half part of the shell, 1 short, narrower plica in the sulcus of the larger spe­cimen and no plica in the sulcus of the smaller one.Pedicle valve interior with no dental plates and median septum; no muscle scars observed.R em arks - For the sake of completeness in describing the fauna, we include this taxon in our systematic paleontology. But we are unable to determine what family or even order this taxon belongs to since the specimens of this taxon are rare and their brachial interiors are unknown. It probably is a spiriferoid without dental plates and median septum in the pedicle valve, two features pre­sent in Proreticularia. But the latter genus has a smooth shell surface, whereas the specimens here have short plicae on the flanks.Further collecting is needed in order to look for more speci­mens, in particular brachial valves showing the cardinalia.

BIVALVES

Class BIVALVIA Linné, 1758 Subclass PTERIOMORPHIA Beurlen, 1944

Order PTERIOIDA Newell, 1965 Superfamily PTERIACEA Gray, 1847 Family PTERINEIDAE Miller, 1877

Genus P terinea G o l d fu ss , 1826T ype sp ec ie s - Pterinea laeuis G o l d fu ss , 1826, p . 133, p i. 119, fig. 1; by subsequent designation of Stoliczka, 1871.

Pterinea sp. aff. P. dianensis Guo, 1985 Fig. 5.4-6

333

Aff. Pterinea dianensis - Guo, 1985b, p. 132, pi. 3, fig. 10.

M aterial - Six incomplete in ternal molds and one external mold from two localities (My Due and Kien An) are on hand, including two left valves and four righ t valves.

D escription - Shell small, erect, suborbicular, a little longer than high; umbo subdued, subtermi­nal, not salient above hinge margin; anterior ear well-defined, semi-orbicular; posterior wing large, nearly flat; hinge margin nearly straight; anterior margin situated below anterior ear; ventral margin convex; surface covered by regularly arranged comarginal ridges.M easurem ents in mm

L La La/L H H/L

1. Right internal mold 11 2 0.18 >7 _2. Left internal mold 14+/- 3+/- 0.21+/- 13+/- 0.933. Right in ternal mold (unfigured) 14 3 0.21 12 0.86

Com parison - The present specimens are nearly identical to Pterinea dianensis Guo from the Upper Silurian of eastern Yunnan, China, except tha t the character of the posterior m argin is not clear in the Vietnamese specimens. In addition, the shell sculp­ture of P. dianensis is unknown, since the type, and only specimen is an internal mold. A more detailed comparison is therefore difficult to make.

Genus A ctin op teria H a l l , 1884Type sp ec ies - Avicula decussata H a l l , 1843, p . 203; by sub­sequent designation of Bassler 1915, p . 16.

Actinopteria mansuyi G ra b au , 1926 Fig. 5.7, 9

Pterinea lineata - Mansuy, 1912, p. 44, pi. 7, figs 2a-c (non Pterinea lineata G o l d fu ss , 1840).

Actinopteria m ansuyi - Grabau, 1926, p. 44, pi. 3, figs 3-5. Ptychopteria (Actinopteria) m ansuyi - Liu, 1976, p. 133, pi. 2,

fig. 10; Guo 1985a, p. 100, pi. 24, figs 7-9.

M aterial - There are 14 left valves from the Kien An locality, represented by in ternal and external molds. Most of them are incomplete or fragm entary, and more or less secondarily defor­med.

D escription - Shell small to medium in size, pro- socline, left valve strongly inflated; hirjge margin nearly straight, shorter than the length of shell; umbo salient above hinge margin, located near the anterior end; anterior ear small, subtrigonal, well- defined; posterior wing large, depressed; surface marked by reticulate sculpture (observed only in well-preserved external molds) made up of promi­nent radiating ribs and more obscure concentric ornament, radials somewhat irregularly spaced, from 5 to 7 ribs in 3 mm, near the ventral margin, with interspaces 2 to 3 times the width of the ribs, finer ribs sometimes intercalated; posterior wing with 7 or 8 ribs and a small number of intercalated finer ribs.M easurem ents in mm

L La La/L H H/L

1. Left composite mold 17 5 0.29 18 1.062. Left composite mold 21 7 0.33 20 0.953. Left internal mold 25 7 0.28 26 1.04

Subclass HETEROCONCHIA Hertwig, 1895 Order TRIGONIOIDA Dali, 1889

Superfamily TRIGONIACEA Lamarck, 1819 Family SCHIZODIDAE Newell & Boyd, 1975

Genus Schizodus d e V e rn e u i l & M u rch iso n , 1844Type sp ecies - Axinus obscurus J . S o w e rb y , 1821, p. 12, pi. 314; by subsequent designation of de Verneuil 1845, p. 308.

Schizodus kienanensis nov. sp. F an g Fig. 5.18, 20

M aterial - There are 50 specimens, 33 of which are left valves, represented by in ternal and external molds. All of them are from the Kien An area. The preservation usually is not very good and the hinge structure is poorly known. Only in a few specimens are faint traces of one cardinal tooth in each valve visible, bu t no further details are preserved.

D iagnosis - Subelliptical, with elongated postero- ventral extremity; umbonal ridge strong.

D escription - Shell medium sized, moderately in­flated, inequilateral, subelliptical, posteroventrally elongate; longer thas high with H/L ratio 0.7 or so; umbo located at about the anterior one-fourth of shell length, with prosogyrate beak; anterior m ar­gin well-rounded, continuing gradually into broad­ly rounded ventral margin; posterodorsal margin slightly convex, merging w ith ventral m argin through sharp curve; umbonal ridge well develo­ped, becoming rounded distally; surface nearly smooth.M easurem ents in mm

L La La/L H H/L

1. Left internal mold (holotype) 31 8 0.26 23 0.742. Left in ternal mold 30 7 0.23 22 0.733. Right in ternal mold 24 6 0.25 17 0.71

Comparison - The present specimens show the external morphological features ascribed to Schi­zodus by Newell and Boyd (1975, p. 95). Externally this species is much like Schizodus truemani J o h n s to n [1993, p. I l l , fig. 79, and the juveniles of S. oweni (Jo h n s to n , 1993, fig. 78D)] from the Lower Devonian of southeastern A ustralia, but the Australian specimens have an obliquely truncated posterior margin, forming an obtuse angle with the dorsal margin. An Upper Silurian species, Schizo­dus ? sp. from the Lower Ludlovian of Wales (Ne­well & Boyd 1975, p. 97, fig. 33) can be readily dis­tinguished by its subquadrate shape, absence of posteroventral elongation, obscure umbonal ridge and poorly defined corselet.

Schizodus ? myducensis nov. sp. F an g Fig. 5.14, 16

M aterial - All specimens of th is species are from the My Due area and preserved in a coarse sandy m atrix as external and internal molds. Three left valves and seven right valves on hand. Dentition and m uscular impressions are not preserved.

D iagnosis - Subcircular shell outline, umbonal ridge weak.

D escription - Shell small to medium in size, slightly inequilateral, subcircular; length slightly exceeding height by a ratio of about 0.9; umbo loca-

334

335

ted a t about the anterior one-third of shell length, with prosogyrate beak; all margins of shell well- rounded, umbonal ridge weak; surface nearly smoo­th.M easurem ents in mm

L La La/L H H/L

1. Left internal mold (holotype) 16 6 0.38 15 0.942. Right in ternal mold 19 6.5 0.34 18 0.953. Right in ternal mold 14 5 0.36 12 0.86

Com parison - The present specimens are ten tati­vely placed in the genus Schizodus because they look like shortened Schizodus kienanensis nov. sp. F ang without posteroventral elongation.

Subclass ANOMALODESMATA Dali, 1889 Order PHOLADOMYOIDA Newell, 1965

Superfamily PHOLADOMYACEA King, 1844 [nom. transl. Newell, 1965]

Family SANGUINOLITIDAE Miller, 1877 Subfamily SANGUINOLITINAE Miller, 1877 [nom.

transl. Morris et al. 1991]

Genus S angu inolites Mc’Coy, 1844T ype sp ec ies - Sanguinolites discors M c ’C oy , 1844, p . 49, p i. 8, fig. 4; by subsequent designation of Stolizcka 1871, p . XIX = Sanguinolaria ? angustata P h i l l i p s , 1836, p . 208, p i. 5, fig. 2). Hind (1900, p . 367) and Morris et al. (1991) concluded th a t S. discors 1844 is a junior subjective synonym of S. angustatus ( P h i l l ip s , 1836).

Sanguinolites sp.Fig. 5.12

M aterial - Only a single left composite mold from Kien An is available, nearly complete. No in ternal shell characters are preserved.

D escription - Shell medium in size, elongate ellip­tical; longer than high with H/L ratio of 0.55; umbo low, lying a t less than one-third of shell length from the anterior end; dorsal m argin nearly straight, ventral margin broadly convex; anterior end roun­ded, posterior end obliquely truncated; umbonal ridge well defined, post-umbonal area marked by one radial ridge, antero-umbonal sulcus faint; comarginal ornament fine, varying in strength, no granulation of shell surface observed. •

M easurem ents in mm - Length (L), 33; anterior length (La), 9; La/L, 0.27; height (H), 18; H/L, 0.55.

D iscussion - The author does not know any spe­cies comparable with the present specimen in the described Silurian and Devonian infaunal burro­wing bivalves. It has external shell characters simi­lar to those of Sanguinolites and may represent a new species.

Family GRAMMYSIIDAE Miller, 1877

Genus Sphenotus H a l l , 1885T ype sp ecies - Sanguinolites arcaeformis H a l l & W h i t f i e ld , 1869 (s e e H a l l 1885, p i. 65, f ig s 7-11), b y s u b s e q u e n t d e s ig n a ­t io n o f M il le r 1889, p . 513.

Sphenotus antecedens G rabau, 1926 Fig. 5.17

Sphenotus antecedens G r a b a u , 1926, p. 52, pi. 3, figs 15a, b. Goniophora m aritim a Guo, 1985a, p. 104, pi. 24, fig. 10. Goniophora maritim a socialis Guo, 1985a, p. 104, pi. 24, fig.

11.

M aterial - Two lefl valves are available from the My Due and Kien An areas respectively, represented by in ternal molds. Nothing is known of the in ternal structures.

D escription - Shell small, inequilateral, elongate subrectangular, length somewhat less than twice height; umbo salient above hinge margin, located at the anterior one-fourth of shell length; anterior margin rounded, posterior margin slightly convex or somewhat truncated; ventral margin broadly arcuate, subparallel to dorsal margin; umbonal ridge angular dorsally, becoming rounded and dying out ventrally; antero-umbonal sulcus weak and only variably present.M easurem ents in mm - Length (L), 17; anterior length (La), 4; La/L, 0.24; height (H), 9; H/L, 0.53.

D iscussion - Guo (1985a) proposed a new species and a new subspecies for two specimens which agree closely with Grabau's species. The author cannot find any im portant differences between them. Guo (1985a, b) often proposed new species based on individual specimens and left no room for intraspecific variation.

F igure 5 - 1-3, 13, Goniophora dianensis Guo, all x 2. 1, internal mold of left valve, Kien An. 2, internal mold of left valve, My Due; 3,13, internal mold of right valve, side view (3), dorsal view (13), My Due. 4-6. Pterinea sp. aff. P. dianensis Guo. 4, internal mold of right valve, x 5, My Due; 5, internal mold of left valve, x 3, My Due; 6, internal mold of left valve, x 3, Kien-An. 7-9. Actinopteria mansuyi G ra b a u , all x 3. 7, composite mold of left valve, K en An; 8, composite mold of left valve, Kien An; 9, internal mold of left valve, Kien- An. 10-11. Modiomorpha paracrypta nov. sp. F a n g , all x 2. 10, internal mold of right valve, paratype, My Due; 11, internal mold of left valve, holotype, My Due. 12. Sanguinolites sp., composite mold of left valve, x 2, Kien An. 14-16. Schizodus ? myducensis nov. sp. F a n g , all x 3. 14, internal mold of left valve, holotype, My Due; 15, internal mold of right valve, paratype, My Due; 16, internal mold of right valve, paratype, My Due. 17, Sphenodus antecedens G ra b a u , internal mold of left valve, x 3, My Due. 18-20. Schizodus kienanensis nov. sp. F a n g , all x 2. 18, internal mold of right valve, paratype, Kien An; 19, internal mold of left valve, paratype, Kien An; 20, internal mold of left valve, holotype, Kien An. 1-3,13, Goniophora dianensis Guo, tous x 2 .1, moule interne d ’une valve gauche, Kien An; 2, moule interne d ’une valve gauche, My Due; 3, 13, moule interne d ’une valve droite, vues latérale (3) et dorsale (13), My Duc. 4-6. Pterinea sp. aff. P. dianensis Guo. 4, moule interne d ’une valve droite x 5, My Duc; 5, moule interne d ’une valve gauche x 3, My Duc; 6, moule inter­ne d’une valve gauche x 3, Kien An. 7-9, Actinopteria mansuyi G r ab au , tous x 3; 7, moule composite d ’une valve gauche, Kien An; 8, moule composite d ’une valve gauche, Kien An; 9, moule interne d ’une valve gauche, Kien An. 10-11, Modiomorpha paracrypta nov. sp. F an g , tous x 2; 10, moule interne d ’une valve droite, paratype, My Duc; 11, moule interne d ’une valve gauche, holotype, My Duc. 12, Sanguinolites sp., moule composite d ’une valve gauche x 2, Kien An. 14-16, Schizodus ? myducensis nov. sp. F a n g , tous x 3; 14, moule interne d ’une valve gauche, holotype, My Duc; 15, moule interne d ’une valve droite, paratype, My Duc; 16, moule interne d ’une valve droi­te, paratype, My Duc. 17, Sphenodus antecedens G r abau , moule interne d ’une valve gauche x 3, My Duc. 18-20, Schizodus kienanensis nov. sp. F an g , tous x 2; 18, moule intenre d ’une valve droite, paratype, Kien An; 19, moule interne d ’une valve gauche, paratype, Kien An; 20, moule intenre d ’une valve gauche, holotype, Kien An.

336

F igure 6 - 1 . Diagram showing the mea­surements of Pterinea. Abbreviations are length (L), length of anterior (La), height (H), ratio of anterior length (La) to length (L) (La/L), ratio of height (H) to length (L) (H7L). 2. Diagram showing the measure­ments of Modiomorpha. 1. Diagramme présentant les mesures de Pterinea. Abréviations: longueur (L), longueur anté­rieure (La), hauteur (H),; rapport entre longueur antérieure et longueur (LrIL) et rapport entre hauteur et longueur (HIL). 2. Diagramme présentant les mesures de Modiomorpha.

hUH

H

1— La—|

H

Superfamily MODIOMORPHINEA Miller, 1877 emend. Fang & Morris, 1997 [norm, transl. Newell,

1965]Family MODIOMORPHIDAE Miller, 1877

(= Permophoridae van de P o e l, 1959, pro Pleurophoridae D a l l , 1895)

Subfamily MODIOMORPHINAE Miller, 1877 (nom, transl. Fang & Morris, 1997)

(= Permophorinae v an d e P o e l , 1959, nom. transl. Chavan, 1969)

Genus M odiom orpha H a l l & W h itf ie ld , 1869Type sp ec ies - Pterinea concentrica C o n r a d , 1838, p. 116; by subsequent designation of Hall, 1885, p. XXIV.

Modiomorpha paracrypta nov. sp . F an g Fig. 5 .10,11

M aterial - This species is a t p resent represented by only two right valves and one left valve. All of them are from the My Due area and preserved as external and in ternal molds. In ternal shell characters are not visible.

D iagnosis - Obliquely elliptical, with nasute ante­rior end, posteriorly expanded; antero-umbonal sul­cus prominent.D escription - Shell medium in size, very inequila­teral, obliquely elliptical, posteriorly expanded; lon­ger than high with H/L ratio of about 0.7; umbo salient above hinge margin, located at about te anterior one third of shell length; hinge margin straight; anterior end nasute, regularly rounded into ventral m argin which shows a prominent concavity near the center of the length; posterior margin broadly arcuate,' rounding more abruptly into ventral margin; umbonal ridge scarcely defi­ned; antero-umbonal sulcus very prominent, cau­sing the median concavity of ventral margin; surfa­ce nearly smooth.M easurem ents o f mm

L La La/L H H/L

1. Left in ternal mold (holotype) '31 10 0.32 22 0.712. Right in ternal mold 30 8 0.27 20 0.67

Comparison - Externally this species is very simi-lar to Modiomorpia crypta (G rab a u , 1926, p. 47, pi. 3, figs 7-11) from the Upper Silurian of eastern Yunnan, China, but in the latter, the antero-umbo­nal sulcus is very faint and the anterior end is more rounded. Modiomorpia paradoxa (Zhang) Pojeta et al. 1986, p. 72, pi. 13, figs 1-7; pi. 14, figs 7,10) from the Lower Devonian of Guangxi, China resembles

the new species in posteriorly expanded and a pro­minent antero-umbonal sulcus, but it has stronger comarginal ornament and a more rounded anterior end.

Genus Goniophora P h ill ip s , 1848Type sp ec ies - Cypricardia cymbaeformis S o w e rb y , 1839 (in Murchison 1839, p. 602, pi. 3, fig. 10).

Goniophora dianensis Guo, 1985 Fig. 5.1, 3, 13)

Goniophora dianensis Guo, 1985b, p. 168, pi. 3, fig. 16.

M aterial - There are 39 specimens represented by in ternal and external molds, 19 left valves and 8 righ t valves from the My Due area and 6 specimen of each valve from the Kien An area. All of them are disarticulated and have been more or less crushed secondarily. In ternal shell features are not preserved.

D escription - Shell medium in size, inequilateral, subtrapezoidal, posteriorly elongated; longer than high nearly two times; umbo salient above hinge margin, lying at about one-fourth shell length from anterior end; anterior end rounded, posterior m ar­gin obliquely truncated with an obtuse posterodor- sal angle; umbonal ridge strong, angular, exten­ding from umbo to posteroventral extremity; some­times post-umbonal area provided with a radial ridge near the dorsal margin (Fig. 5.3, 13); ventral margin slightly concave caused by the antero- umbonal sulcus.M easurem ents in mm

L La La/L H H/L 1

1. Left in ternal mold 34 8 0.26 17 0.50 102. Left in ternal mold 35 8 0.23 17 0.49 83. Right in ternal mold 30 6 0.20 17 0.57 6

Com parison - The Vietnamese specimens are almost identical in external features with the type specimen of this species, except for the radial ridge in the post-umbonal area which only occurs in some specimens and is not a stable character.

Acknowledgem ents and Responsibility - Tong-Dzuy Thanh acknowledges the support of the Vietnamese Program on Fundamental Research in the Field of Natural Science and Boucot the National Geographic Society's Research Fund and the National Science Foundation. In 1994 Boucot made the My Due and Kien An Retziella fauna collections under Tong-Dzuy Thanh’s guidance, followed by preparation, sorting and preliminary iden­tification in Corvallis. Rong Jia-yu then visited Corvallis in 1997 to check the identifications and make revisions based on his knowledge of the similar Qujing, South China Retziella fauna. Boucot then spent time in Nanjing in 1997 working with Rong Jia-yu on the brachiopod descriptions, at whidh time Fang Zong-

337

jie prepared the bivalve descriptions based on the materials pre­viously prepared and sorted by Boucot. Tong-Dzuy Thanh is res­ponsible for the account of the local geology and stratigraphy. We all collaborated on the overall paleogeographic and lithofacies relations, as well as on varied aspects of the environments invol­ved. Boucot and Rong Jia-yu are responsible for the synecological and biogeographic treatment. Boucot is responsible for the gas­tropod determinations. Goujet and Janvier (Tong-Dzuy Thanh et al. 1997) are responsible for the vertebrate information and deter­minations. This work was also supported by the Major Basic Research Project of MTS, China (G2000077708) to Rong Jia-yu and Fang Zong-jie. -

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T. TONG-DZUYLaboratory of Geology, V ietnam N ational University

334 Nguyen Trai Road Thanh Xuan, Ha Noi, Vietnam

A.J. BOUCOTD epartm ent of Zoology

Oregon S tate University Corvallis, Oregon 97331 USA

J.Y. RONG & Z.J. FANGN anjing Institu te of Geology and Palaeontology

Academia Sinica 39 Beijing Road

N anjing 210008, China