Senckenbergiana lethaea 88 (1) 11–21 4 text-figs, 1 pl. Frankfurt am Main, 30.06.2008

Theearliestostracods:thegeologicalevidence

With 4 Text-figures and 1 Plate

Mark Williams, David J. Siveter, María José Salas, Jean Vannier, Leonid E. Popov & Mansoureh Ghobadi Pour

Abstract

The oldest assumed ostracods appear in the fossil record from the Tremadocian Paltodus deltifer cono-dont Biozone. Although geographically widespread these early ostracods have no obvious Cambrian an-tecedents. Their first appearance at ca. 485 Ma contrasts with molecular evidence that suggests a much earlier (latest Proterozoic or Cambrian) origin for ostracods. Some Cambrian bivalved arthropods such as Altajanella and Vojbokalina, conventionally referred to the Bradoriida, have carapace morphologies that resemble Ordovician palaeocopid ostracods, though such a relationship is unproven without soft part anatomy. Evidence from preserved soft anatomy demonstrates that Bradoriida, such as Kunmingella, and Phosphatocopida, essentially the Cambrian ‘ostracod’ record of traditional usage, belong outside the Eucrustacea. Early Ordovician ostracods appeared first in shallow marine, oxygenated environments on shelf margins, in a similar setting to other elements of the ‘Paleozoic fauna’. Their biodiversity was low (3 named genera and ca. 12 species), though some taxa such as Nanopsis and Eopilla achieved widespread dispersal between major Ordovician palaeocontinents. As bradoriids were largely extinct by the Late Cam-brian, ostracods do not appear to have directly competed with them for shallow marine environments. The rapid colonisation of these settings by ostracods may have been facilitated by the available ecospace vacated by Bradoriida.

Key words : Ostracoda, Bradoriida, Phosphatocopida, Tremadocian, biogeography

Introduction

There are an estimated 65,000 living and fossil species of the crustacean Class Ostracoda, divided in some schemes into two major subclasses, the Podocopa and the Myodocopa (e.g. Horne et al. 2002). Despite the abundance of ostracods in the fossil record, their origin(s) remains enigmatic. Evidence from molecular studies suggest that ostracods lie at or near the base of the Pancrustacea (for definition see Mallatt & Giribet 2006; Regier et al. 2005), suggesting an origin perhaps during the latest Proterozoic (see Regier et al. 2005: 395, 400). The oldest undoubted ostracods – with their soft anatomy preserved – are from 425 million year old deposits

in the Silurian of England (Text-fig. 1; Siveter et al. 2003a, 2007). The record of putative fossilized ostracod carapaces (the term ‘carapace’ is used in the sense of Siveter et al. 2003) suggests an older history for the group extending back at least to the Early Ordovician (Tinn & Meidla 2004; Salas et al. 2007; see also Newman 2005). Two arthropod groups with bivalved carapaces that have traditionally been suggested as the forerunners of ostracods (e.g. Sylvester-Bradley 1961), namely the bradoriids and phosphatocopids, are abundant in Cambrian rocks worldwide. These two groups first appeared in the Early Cambrian (Text-fig. 1; Williams et al. 2007) and were largely extinct by the earliest Ordovician. Although pos-sessing a bivalved carapace with inner and outer lamellae, evi-

Dr Mark Williams (Corresponding author <mri@le.ac.uk>) & Prof. Dr David J Siveter, Department of Geology, University of Leicester, Leicester LE1 7RH, UK. – Dr María José Salas, Centro de Investigaciones Paleobiológicas CIPAL, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Avenida Vélez Sarsfield 299, 5000 Córdoba, Argentina. – Dr Jean Vannier, Université Lyon 1, UMR 5125 Paléoenvironnements & Paléobiosphère, Bâtiment GEODE - 2, rue Raphaël Dubois, 69622 Villeurbanne cedex, France. – Dr Leo-nid E. Popov, National Museum of Wales, Department of Geology, Cathays Park, Cardiff CF10 3NP, UK. – Dr Mansoureh Ghobadi Pour, Department of Geology, Faculty of Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan 49138-15739, Iran.

© E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), 2008, ISSN 0037-2110

12 The earliest ostracods: the geological evidence

dence from soft-part anatomy indicates that phosphatocopids are sister-group to the Eucrustacea and cannot be ancestors of ostracods (Text-fig. 2; Siveter et al. 2001, 2003a; Maas et al. 2003). The only known bradoriid species with detailed soft anatomy preserved, Kunmingella douvillei (Text-fig. 2; Hou et al. 1996, 2002; Shu et al. 1999), has a morphology which places it on the stem-line of the Crustacea. Appendages have

also been reported in a possible oepikalutid bradoriid, but have not been described in detail (Hinz-Schallreuter 1993). An-other group of putative ostracods that are well represented in the Early Palaeozoic, the ‘Order Leperditicopida’, have large bivalved carapaces typically 5-50 mm in length. However, as their soft anatomy is unknown, their biological affinities are uncertain (for a review of the group see Vannier et al. 2001).

The fossil record of small bivalved arthropods through the Cambrian-Ordovician transition poses important questions for the origins of the ostracods and even perhaps for other ar-thropod groups (see Newman 2005). Were some bradoriids ancestral to the Ostracoda? Where and when do the first podo-cope or myodocope ostracods appear? What was their early biogeographical distribution? What kinds of environments did they evolve in? And, did they supplant earlier bivalved arthro-pod faunas?

Recognising ostracods in the fossil record

The phylogenetic position and classification of the Ostracoda based on carapace and soft part morphology (e.g. Horne et al. 2002, 2005) and molecular evidence (e.g. Yamaguchi & Endo, 2003) is in a state of flux. Indeed, a monophyletic versus polyphyletic origin for the Ostracoda is debated (e.g. Regier et al. 2005; Newman 2005), and even the systematic relation-ships of such major taxa as the Palaeozoic Order Palaeocopida (placed by some within the ostracod Subclass Podocopa; e.g. Horne et al. 2002) are still to be resolved (Yamaguchi & Endo 2003). Three areas of investigation provide clues to the earliest occurrence of ostracods: evidence from molecular biol-ogy; recognition of fossilised soft anatomy; and recognition of fossilised hard tissues (the carapace).

Molecular evidence supports the Ostracoda and Branchiu-ra either as a paraphyletic clade at the base of the Pancrustacea, or separates the Podocopa at the base of a clade that includes cirrepedes and copepods, with the Myodocopa grouped with the Branchiura at the base of the Pancrustacea (Regier et al.

Text-fig. 2. Figures 1–3. Phosphatocopid (1, 2) and Bradoriid arthropods (3) preserved with soft anatomy. Museum abbreviations are: OUM, Oxford University Museum; RCCBYU, Re-search Centre for Chengjiang Biota, Kunming University, Yunnan, China. 1, 2, Klausmuelle-ria salopiensis Siveter, Waloszek & Williams, 2003, anterior aspect (stereo-pair) and oblique posterior views (RV to bottom) respectively, × 260-OUM A.2209. 3, Kunmingella douvillei (Mansuy, 1912), carapace in butterfly orienta-tion, LV to bottom, × 7.5-RCCBYU 10258.

Text-fig. 1. Main events in the appearance of ostracods in the fossil record. The dates of the events should be considered approximate. 1, Origin of the Pancrustacea (Regier et al. 2005); 2, oldest Bradoriida (Hou et al. 2002; Williams et al. 2007); 3, major extinction of the Bradoriida during the latest mid Cambrian (age calibrated from Shergold & Cooper 2004); 4, oldest ostracods (age calibrated from Cooper & Sadler 2004); 5, youngest bradoriids (see Ghobadi Pour et al. 2007, age calibrated with Cooper & Sadler 2004); 6, earliest myodocopes (Gabbott et al. 2003, age calibrated with Cooper & Sadler 2004); 7, oldest ostracods (myodocopes) with preserved soft anatomy (Siveter et al. 2003b, 2007).

13The earliest ostracods: the geological evidence

2005). Although molecular evidence suggests that Ostracoda may be present in the latest Proterozoic or Early Cambrian (cf. Regier et al. 2005), there is no fossil evidence to support this.

It is largely soft anatomy that defines the Ostracoda, and its subdivision into the Myodocopa and Podocopa (see Horne et al. 2002), but fossil ostracods with preserved soft anatomy are extremely rare (Smith 2000; Siveter et al. 2003b, 2007). Daunting for recognising the relationships of early fossil os-tracods is the realisation that carapace design can be very mis-leading: Siveter et al. (2007) have shown in Silurian mate-rial that a myodocope body can reside in a palaeocopid-like carapace. Indeed, modern myodocopes display a great variety of carapace design although their morphology remains consis-tently similar. The pre-Silurian fossil record of ostracods can-not currently resolve these debates as there are no Cambrian or Ordovician ostracods preserved with soft tissues. However, some rock successions, such as the Early Ordovician Merevale Shales of England, do yield ostracod-like bivalved arthropods with soft-tissue preservation (Plate 1; Siveter et al. 1995) and deserve further exploration.

Where fossil material lacks soft tissue preservation, struc-tures such as muscle scars, preserved on the inner surface of the valves are of considerable value and have been used exten-sively in taxonomy (e.g. Siveter & Vannier 1990). As there is no soft tissue or muscle scar preservation in Early Ordovician fossil material, recognition of the occurrences of ostracods is based on carapace morphology alone. Even then, the earliest Ordovician ostracods lack such distinctive carapace structures as domicilar or extradomicilar dimorphism, or evidence about hinge articulation, features that are significant for Palaeozoic ostracod taxonomy (e.g. Henningsmoen 1953; Jaanusson 1957).

The characters used herein for recognising the earliest (Tremadocian) ostracods invoke uniformitarian principles, that is, morphological similarity of the carapace to post-Tremado-cian ostracod faunas (Plate 1; see also Tinn & Meidla 2004). On this basis these early Ordovician supposed ostracods ap-pear to be assignable to the orders Palaeocopida and Binodi-copida (some authors would subsume the Binodicopida into the Palaeocopida, see ‘Superfamily Drepanellacea’ of Whatley et al. 1993). The criteria we use to recognise the earliest ostra-cods are: 1) a primary mineralised calcium carbonate carapace; 2) a bivalved carapace (typically around 0.5-1.5 mm in length, and much smaller than most Bradoriida); 3) lobation in which the lobes are oriented dorsal to ventral (like the palaeocopid Tetradella), or which involve inflation of the ventral part of the valve (characteristic for many Palaeocopida); and 4) devel-opment of a distinct adductorial sulcus and associated post- and pre-adductorial nodes. Some of these characters overlap with those of Bradoriida and Phosphatocopida: the latter have a number of carapace features that are not seen in ostracods, particularly the development of an interdorsum in Phosphato-copida, or relatively large size in Bradoriida (typically 0.25-1.7 cm long; e.g. see Siveter & Williams 1997 and Williams & Siveter 1998).

In the following discussion we focus on documenting the earliest binodicopids and palaeocopids. We have not included the Leperditicopida. Although conventionally treated as os-tracods (e.g. Whatley et al. 1993), their systematic affinities are unclear, compounded by the absence of post-Devonian

representatives of the group and of evidence from soft tissue preservation (Vannier et al. 2001). Leperditicopids may have appeared first in the Late Cambrian (Frederickson 1946), overlapping with Bradoriida. Confirmed representatives of the group appear in rocks of Floian age (Ordovician) in Spitsber-gen (Williams & Siveter 2008).

The oldest ostracods

Most major groups of the ‘Paleozoic fauna’ first appear in the Cambrian (Sepkoski, 1990). However, the record of ostracods from this period is contentious. The Cambrian to Early Or-dovician bradoriid and phosphatocopid bivalved arthropods were part of the Cambrian radiation (e.g. Hou et al. 2002; Williams et al. 2007). These two groups have traditionally been considered as the earliest representatives of the Ostracoda (e.g. Sylvester-Bradley 1961; Müller 1964, 1979; Jones & McKenzie 1980) and some authors maintain that opinion (Hinz 1993; Gozalo & Hinz-Schallreuter 2002). These ar-guments centre on similarities between the ostracod, bradoriid and phosphatocopid carapace (see Plate 1), all of which may be bivalved, lobate, and possess an inner lamella (e.g. Williams et al. 1994; Siveter & Williams 1997; Siveter et al. 2001). However, it is generally accepted that a bivalved carapace has been acquired convergently by a number of different malacos-tracan, branchiopod and maxillopod crustaceans.

Bradoriids first appear in Lower Cambrian rocks, slightly below the oldest trilobites (Williams et al. 2007). Like trilo-bites, Bradoriida emerge highly differentiated into a number of species and with a global distribution, suggesting a period of evolution for which there is no fossil record. Bradoriida have long been regarded as polyphyletic (Jones & McKenzie 1980), and encompass a wide range of carapace designs. Nevertheless, several species have been resolved that may be closely related (Williams et al. 2007). These have a bivalved carapace, with the dorsal portion of the two valves apparently connected by a thin band of cuticle (without an articulating hinge as in the palaeocopids or binodicopids), an amplete or postplete shape, a concentration of lobes anteriorly or anterodorsally (though other lobes can be developed more centrally or posteriorly), and a lateroadmarginal ridge (see Plate 1, figs 1, 6, 8, 11). The only known bradoriid with well preserved soft part anatomy is Kunmingella and its anatomy, particularly the undifferentiated 2nd to 4th cephalic appendages, precludes its placement within the Ostracoda or even the Eucrustacea (Hou et al. 1996, 2002; Shu et al. 1999). Appendages of a possible oepikalutid bra-doriid have also been figured but are not described in detail (Hinz-Schallreuter 1993). Soft part anatomy is unknown in the type bradoriid, Bradoria (see Plate 1, fig. 8).

A number of Cambrian bivalved arthropods that have been referred to the Bradoriida s.l. (for characterisation see Williams et al. 2007) have carapace designs that strongly re-semble post-Cambrian ostracods, particularly Vojbokalina and Altajanella (Williams et al. 2007). These bradoriids possess a straight dorsal outline, have multilobate valves, with the lobes arranged dorso-ventrally like the Ordovician tetradellid pal-aeocopids (see Plate 1, figs 10, 12; see also Henningsmoen 1953) and are small (millimetric) in scale. It is possible that some taxa referred to the Bradoriida s.l. might be ostracods, though without information from soft anatomy across a range

14 The earliest ostracods: the geological evidence

Plate 1. Early Palaeozoic bivalved arthropods. Bradoriida (1, 6, 8, 10–12), Ostracoda (2-5?, 7?, 9, 13–15) and Phosphatocopida (16). Museum abbreviations are: MCZ, Museum of Comparative Zoology, Harvard University, USA; BGS, British Geological Survey, Keyworth, Nottingham, UK; SM, Sedgwick Museum, Cambridge University, UK; PIN, Palaeontological Institute, Moscow, Russia; NHM, Natural History Museum, London, UK; RCCBYU, Research Centre for Chengjiang Biota, Kunming University, Yunnan, China; CORD-MP, Museo de Paleontología, Universidad Nacional de Córdoba, Argentina.

Fig. 1. Beyrichona triceps (Matthew, 1903). An open carapace, RV to bottom × 16. Tremadocian, Arenaceous Beds, Shineton Shale Formation, Shropshire, England. – BGS RX2277.

Fig. 2. Rivillina? sp. An open carapace, LV to bottom × 18.5. Tremadocian, Shineton Shale Formation, Shropshire, England. – SM A.29023.

15The earliest ostracods: the geological evidence

of different bradoriid taxa, the relationships between the earli-est Ostracoda and some ‘ostracod-like’ Bradoriida remain hy-pothetical.

Phosphatocopids also appear in the Early Cambrian (e.g. Siveter et al. 2001), and like bradoriids were global in their distribution. Details of their soft anatomy, known from sev-eral different taxa including falitids, hesslandonids and vestro-gothiids (Müller 1964, 1979, 1982), clearly precludes them from being ostracods (Siveter et al. 2001, 2003a; Maas et al. 2003).

The earliest occurrence of ostracods based on carapace alone is during the Early Ordovician, (herein; see also Tinn & Meidla, 2004; Salas et al. 2007), from rocks about 485 million years old. These ostracods are multi-lobate forms such as the palaeocopids Eopilla and Nanopsis, and the binodicopid Kimsella (see Salas et al. 2007). They have a straight dorsal outline and a small (millimetre-scale) carapace (see Plate 1, figs 9, 13). The geological record of myodocopes is somewhat younger. The oldest myodocopes with preserved soft anatomy are those of the Silurian Herefordshire Lagerstätte (ca. 425 Ma; Siveter et al. 2003b), though records of myodocopes based on carapace morphology extend back to the Late Ordovician (Gabbott et al. 2003).

Stratigraphical occurrence of the earliest ostracod faunas: the Tremadocian

The base of the Tremadocian Stage (and of the Ordovician System) is defined at the level of the first occurrence of the conodont Iapetognathus fluctivagus (Cooper & Sadler 2004). Radiometric dates for the Stage place its lower boundary at 488.3 ± 1.7 Ma, and its upper boundary at 478.6 ± 1.7 Ma

(Cooper & Sadler 2004). The Tremadocian is characterised by a global marine transgression, following a prolonged period of a lowstand during the Late Cambrian (Artyushkov et al. 2000). Well developed graptolite and conodont biozonations permit correlation of Tremadocian strata worldwide (Cooper & Sadler, 2004). Where we use local Tremadocian stratigra-phy we indicate how this correlates with the standard grapto-lite and conodont biozonations (Cooper & Sadler 2004).

Ostracods first appear at about the level of the P. deltifer conodont Biozone. This appearance coincides with fundamen-tal changes in marine biota following the demise of much of the ‘Cambrian fauna’ through the interval of the C. andreis to C. angulatus conodont biozones (Sturesson et al. 2005). Os-tracods of Tremadocian age are known from southern Britain, Scandinavia, Iran, Argentina, China and possibly Australia. Despite having extensive deposits of Tremadocian age, no ear-liest Ordovician, pre-Floian age ostracods have been reported from North America or Russia.

SouthernBritain

A poorly preserved ostracod species (Plate 1, fig. 2) charac-terises the upper part of the Shineton Shale Formation at the level of the ‘Mignetian Stage’ C. salopiensis trilobite Biozone in Shropshire (Williams & Siveter 1998; see Rushton et al. 1999, fig. 6.2 for stratigraphy). This species may be conspe-cific with Shropshire material recorded as ‘primitiae’ (Jones & Holl 1869) and Primitia spp. (Stubblefield & Bulman 1927). Much of the locally defined ‘Mignetian Stage’ strata are non-graptolitic, but in the British sequence the C. salopiensis Biozone immediately precedes an interval bearing the zonal graptolite A. murrayi (see Cooper & Sadler 2004). Indeter-

Figs 3–5, 7. Ostracod-like arthropod, Tremadocian, central England (see Siveter et al. 1995). 3. Anterior part of carapace with two valves slightly displaced (close-up of anterior of Fig. 5), × 20. Southam Borehole.-BGS

BDM4841 4. Presumed LV lateral view, × 15.5. Berry Fields Farm Borehole.-BGS B7195. 5. Carapace with two valves slightly displaced and anterior appendages, × 15. Southam Borehole.-BGS BDM4841 7. BGS BDM4842 (counterpart of BDM4841), × 15.5. Southam Borehole.Fig. 6. Septadella jackmanae Stubblefield, 1933. LV lateral view, × 27.5. Tremadocian, Breadstone Shales, Gloucestershire, England.-

BRSMG Cs 1813 Fig. 8. Bradoria scrutator Matthew, 1886. RV lateral view, × 7.7. Middle Cambrian, Dugald Brook Formation, Dugald Brook, Nova

Scotia, Canada. – USNM 483189.Fig. 9. Kimsella luciae Salas, Vannier & Williams, 2007. RV lateral view, × 46. Tremadocian, lower part of the Parcha Formation, Abra

de Sococha Section, Province of Salta, Argentina. – CORD-MP 11186.Fig. 10. Altajanella costulata Melnikova, 1992. RV lateral view (of silicon rubber mould), × 46. Upper Cambrian, Tandoshka Formation,

Gorny Altay. – PIN N4346/1.Fig. 11. Tsunyiella luna Zhang, 1974. Open carapace, LV to bottom, × 8.3. Lower Cambrian, Guizhou Province, China. – RCCBYU

00101. Fig. 12. Vojbokalina magnifica Melnikova, 1984. Carapace, left lateral view, × 36. Middle Cambrian, Leningrad Region, Russia – PIN

N4341/6.Fig. 13. Nanopsis coquena Salas, Vannier & Williams, 2007. LV lateral view, × 39. Tremadocian, Upper Member of the Coquena

Formation, Quebrada Chalala Section, Cordillera Oriental, Argentina. – CORD-MP 11179.Fig. 14. Tetradellina henningsmoeni Harris, 1957. Carapace, right lateral view, × 118. Upper Ordovician, Bromide Formation, Oklahoma,

USA. MCZ 4643.Fig. 15. ‘Bolbozoe’ sp. nov. A of Siveter, Vannier & Palmer, 1987. RV, lateral view, × 15. Upper Silurian, Long Mountain Silstone

Formation, Powys, Wales. – NHM OS13060.Fig. 16. Cyclotron sp. C of Williams & Siveter, 1998. LV lateral view, × 20. Upper Cambrian, Outwoods Shales Formation, Warwickshire,

England. – BGS BDA1261.

16 The earliest ostracods: the geological evidence

minate ‘ostracods’ spp. also occur in deposits of Tremadocian age in boreholes in central England (Williams & Siveter 1998), as does an ‘ostracod-like’ bivalved arthropod with ap-pendages preserved (Siveter et al. 1995). The latter is derived from strata in the ‘R. flabellimormis Biozone’, suggesting an early Tremadocian age. Although only the anntenules of this arthropod are preserved, its carapace morphology is suggestive of a myodocope (Plate 1, figs 3-5, 7).

Scandinavia

Ostracods occur in the Bjorkasholmen Formation (previously known as the Ceratopyge Limestone) of Norway and the Alum Shale of Sweden at the level of the P. deltifer conodont Biozone. The fauna includes one described palaeocopid, Nanopsis nanel-la (Henningsmoen 1954; Tinn & Meidla 2004).

Iran

In Iran, the oldest ostracods occur in the Lashkarak Formation of the Alborz region (Text-fig. 3) and are associated with the trilobite Psilocephalina lubrica, indicating a mid Tremadocian age, and likely below the level of the ‘Ceratopyge limestone’ of Sweden. Ostracods also occur at the level of the lowermost subzone of the P. proteus conodont Biozone (Drepanoistodus aff. amoenus subzone, approximately equivalent to the lower A. murrayi graptolite Biozone), indicating a late Tremadocian age (Ghobadi Pour 2006). These ostracods include possible

palaeocopids and podocopids but are too poorly preserved to permit a firm identification.

Argentina

Ostracods are recorded from the Salta and Jujuy provinces, Cordillera Oriental, northwestern Argentina (Salas et al. 2007). They are preserved in rocks from the southern exten-sion of the Central Andean Basin and include Nanopsis, Eo-pilla and Kimsella. The ostracods come from three different localities in the Lower Ordovician. In the Quebrada Chalala section (Purmamarca area) the ostracods are from the Coquena Formation (Benedetto & Carrasco 2002) and are associated with the trilobite Notopeltis orthometopa, and the brachiopods Tarfaya? brachymyaria, Lipanorthis andinus, and Astraborthis quebradensis. The N. orthometopa Biozone is now under revi-sion, but a late Tremadocian age for the ostracod-bearing levels is widely accepted. In the Abra Santa Laura (Sierra de Mojoto-ro) region the ostracods are from the Floresta Formation asso-ciated with bivalves and trilobites (Sánchez & Vaccari 2003). The presence of the trilobite Parabolinella sp. (Vaccari, pers. comm., 2005 in Salas et al. 2007), which occurs elsewhere in the Rupasca Formation within the upper part of the P. deltifer Biozone (P. deltifer pristinus Subzone; Zeballo et al. 2003; Ze-ballo & Tortello 2005), indicates an early late Tremadocian age. Ostracods also occur in the Abra de Sococha (Parcha-In-camayo area) within the Parcha Formation. The ostracod-rich levels belong to the upper part of the A. murrayi graptolite Biozone and the P. proteus and A. deltatus conodont biozones

Text-fig. 3. Ostracod valves of Tremadocian age dispersed over a bedding surface of the Lashkarak Formation in the Simeh-Kuh Section, 13 km north-west of Damghan, Simeh-Kuh, eastern Alborz Mountains, Iran (see Ghobadi Pour 2006). National Museum of Wales collection NMW2004.22G.207. Scale bar 1 mm.

17The earliest ostracods: the geological evidence

(Ortega & Albanesi 2003; Salas & Albanesi 2003) and can reasonably be attributed to the late Tremadocian (Salas et al. 2007).

Australia

The palaeocopid Eopilla ingelorae Schallreuter, 1993a, from the lower part of the Emanuel Formation of the Canning Ba-sin of Northwest Australia, is associated with the ostracods Eo-dominina and Conchoprimitia (see Schallreuter 1993b, and pers. comm., 2005 in Salas et al. 2007). Although these rocks were earlier assigned a late Tremadocian age (e.g. Schall-reuter 1993a), more recent stratigraphical evaluation of the Emanuel Formation suggests an early Floian age (late Lance-fieldian – early Bendigonian Australian regional stages; Brock & Holmer 2004). We include discussion of this material here, as it indicates the persistence of the late Tremadocian ostracod fauna into the Floian.

China

Apparently diverse assemblages of ostracods have been recov-ered from the Fenhsiang Formation (= Ichang Formation, Hou 1953a) of western Hubei, and the Yehli Formation (Hou 1953b) of Liaoning Province, NE China, both of possible late

Tremadocian age (see summary in Salas et al. 2007). The ex-act age of these ostracod-bearing horizons requires confirma-tion from new field studies, and the fauna itself needs revision. However, the illustrations of Hou (1953a) indicate possible palaeocopids (“Sinoprimitia” hupeiensis Hou, 1953a resembles Nanopsis), binodicopids (“Sinoprimitia” sinensis Hou, 1953a, “Primitia” tumiduformis Hou, 1953a), leiocopids (“Bythocy-pris” subcircularis Hou, 1953a), and eridostracans (“Primitia” ichangensis Hou, 1953a resembles Conchoprimitia). Schall-reuter (1993b) synonymized Ctenobolbina sinensis and C. taitzehoensis and tentatively assigned them to Eopilla.

Biogeography of the earliest ostracods

The wide geographical occurrence of ostracods and their al-most simultaneous appearance on several palaeocontinents suggests rapid dispersal, tolerance of a range of different cli-matic belts, or a much reduced latitudinal temperature gradi-ent from low to high southern latitude, that is consistent with interpretations of Tremadocian climate as being ‘warm’ (Coo-per & Sadler 2004, fig. 12.3). The earliest record of ostracods is in the mid Tremadocian of Baltica with the occurrence of the palaeocopid Nanopsis (Tinn & Meidla 2004), and contem-poraneously in Gondwana (Cordillera Oriental of Argentina) with Nanopsis, Eopilla and Kimsella (Salas et al. 2007). The

Text-fig. 4. Distribution of ostracods in the late Tremadocian (palaeogeography modified from Popov et al. [2003], relative position of Baltica and Gondwana after Cocks & Torsvik [2002]). The icon indicates the occurrence of ostracods but does not denote a particular group. Ostracods are widely distributed from their early appearance in the P. deltifer Biozone. Genera such as Nanopsis range between Baltica and Gondwana, whilst Eopilla appears in Gondwana and possibly North China. 1, Coquena, Floresta and Parcha formations, Argentina (Salas et al. 2007); 2, Bjorkasholmen Formation of Norway and Alum Shale of Sweden (Tinn & Meidla, 2004); 3, Shineton Shales Formation, England (Williams & Siveter 1998); 4, Lashkarak Formation, mid-late Tremadocian of Iran (this paper); 5, Fenhsiang Formation (= Ichang Formation) of western Hubei (Hou 1953a); 6, Emanuel Formation, northern Western Australia (Schallreuter 1993a, b) – this record is now considered earliest Arenigian; 7, Yehli Formation, Liaoning Province, NE China (Hou 1953b).

18 The earliest ostracods: the geological evidence

presence of indeterminate ostracods in Avalonia and Alborz at about the same time indicates the widespread distribution of the group (Text-fig. 4). If the faunas of China are included in this analysis, a latitudinal range from the tropical zone (North China palaeoplate) to the southern high latitude Avalonia mi-croplate is indicated (Text-fig. 4).

Individual genera are widespread in shallow marine litho-facies, with Nanopsis present in Baltica and Gondwana (Argen-tina; Text-fig. 4) during the P. deltifer Biozone, whilst Eopilla appears to have distributed rapidly through Gondwana occur-ring in the late Tremadocian of Argentina, and shortly after-wards in Australia. Eopilla may even have extended to North China (Text-fig. 4).

Ostracods of Tremadocian age are not recorded from the major palaeoplates of Laurentia or Siberia (see Text-fig. 4). In these areas ostracods become common in rocks of Dapingian age (= ‘Third Stage’ sensu Cooper & Sadler 2004) in North America (e.g. Harris 1957; Berdan 1988) or become abun-dant in strata of Darriwilian age in Siberia (see Abushik et al. 1990): there are records of ostracods from earlier horizons in Siberia, but these are post-Tremadocian (e.g. Kanygin 1971; see also Tinn & Meidla 2004). In other Ordovician palaeo-geographical terranes the earliest ostracods so far documented appear to be post-Tremadocian, for example in Kazakhstan (Melnikova 1986), Novaya Zemlya (Schallreuter et al. 2001), and Armorica (e.g. Vannier 1983). Notwithstanding the patchy record, the present known distribution of Tremado-cian ostracods hints at an earliest occurrence of the group cen-tred on Gondwana/Baltica (Text-fig. 4).

Biogeography of terminal bradoriids compared with the earliest ostracods

Bradoriida had global distribution during the early and middle Cambrian, but their biodiversity was curtailed by a major ex-tinction event at the end of the middle Cambrian (Williams et al. 2007). The distribution of remnant early Tremadocian bradoriids shows stratigraphical overlap with ostracods only in Avalonia (Williams & Siveter 1998). In Baltica the bradoriid ‘Eremos bryograptorum’ is reported from an early Tremadocian level in Scania (Hinz-Schallreuter 1993: 432), but the pres-ervation of the material is poor. Even in Avalonia, only one species of bradoriid, Beyrichona triceps, persisted into the late Tremadocian. Here bradoriids and ostracods may have co-ex-isted in the shelf marine environments of the Shineton Shales Formation. There are records of rare bradoriids post-dating the Tremadocian (e.g. Ghobadi Pour et al. 2007), but these are sporadic, might be reworked, and do not suggest the persis-tence of widespread Bradoriida in the Ordovician.

The earliest ostracods colonised the same kinds of benthic marine shelf environments as bradoriids (for a summary of the habitats of Bradoriida see Williams et al. 2007). However, there is no evidence of ostracods displacing bradoriids into deeper marine facies, an ecological shift that is possibly the case with other Ordovician groups (e.g. Barnes et al. 1994: 145). Indeed, the rapid, widespread global and latitudinal dis-tribution of ostracods at their first appearance may reflect an availability of ecospace following the demise of the bradoriid faunas. Wide distribution may also have been facilitated by

the ‘warm’ climate of the Tremadocian (Cooper & Sadler 2004).

Environmental distribution of the earliest ostracods

The ‘Paleozoic fauna’ appears to have originated in shelf ma-rine environments (Sheehan 2001) and ostracods followed this pattern. The earliest ostracods occupied well oxygenated marine shelf sequences, both in carbonate- and clastic-rich settings. Nanopsis occurs in marine carbonate-rich sequences of the Bjorkasholmen Formation which formed in a shallow epicontinental sea (Dronov & Holmer 1999). The strong association of certain palaeocopids with carbonate platforms is a pattern that would continue throughout the Ordovician (see Vannier et al. 1989). Nanopsis is also present in clastic-rich environments, in the Coquena Formation in Argentina (Benedetto & Carrasco 2002), where grey, bioturbated mudstones were deposited in an assumed inner-shelf setting (Salas et al. 2007). Given the wide latitudinal distribution and varied lithofacies, early ostracods like Nanopsis may have been environmental generalists. Ostracods like Kimsella and Eopilla also occur within shallow marine settings of the lower part of the Parcha Formation of Argentina, in calcarenitic lev-els (Astini 2002, 2003; Salas et al. 2007). A few early os-tracods may have colonised somewhat deeper shelf settings, as for example those in the Shineton Shales Formation (Wil-liams & Siveter 1998). Myodocope-like bivalved arthropods are also reported from the laterally equivalent Merevale Shales Formation of England (Siveter et al. 1995). In Iran, ostracods occur in mudstones at two stratigraphical levels in the Lash-karak Formation, both interpreted as being deposited in outer shelf settings, and associated with the trilobites Psilocephalina lubrica and Asaphellus, respectively in nilied and raphiophorid trilobite biofacies (Ghobadi Pour 2006).

Post-Tremadocian Ordovician ostracod diversity

Ostracod diversity remained low during the Tremadocian, with just three named genera (Kimsella, Eopilla and Nanopsis) and a handful of species globally (perhaps 12 depending on the affinities of British, Iranian and Chinese material). Ostracods underwent a major diversification after the Tremadocian. In the best studied faunas, those of Baltoscandia, some 33 spe-cies are reported from the post-Tremadocian Billingen through Kundan regional stages, including many geographically wide-spread Ordovician genera such as Laccochilina, Euprimites and Ogmoopsis (Tinn & Meidla 2004). This post-Tremadocian increase in ostracod biodiversity tracks that of other benthic marine organisms (Hammer 2003), though is uneven in its development. In southern Britain (palaeocontinental Ava-lonia) for example, ostracod biodiversity remained low until the Darriwilian (e.g. Botting 2002) and in North America (Laurentia) until a level in the Dapingian or early Darriwilian (‘Whiterockian’ ostracod faunas, e.g. Harris 1957; Berdan 1988). Many post-Tremadocian ostracod morphologies can be traced back to ancestors like Nanopsis (Tinn & Meidla 2004),

19The earliest ostracods: the geological evidence

but also form clear lineages that continue through the Ordovi-cian (Williams et al. 2003), resulting in an ostracod biodiver-sity with tens to hundreds of species in individual formations (e.g. Meidla 1996) by the Late Ordovician. Although there is local evidence for a link between ostracod diversification and regional volcanism (Botting 2002), the global nature of this biodiversification event and its implications for the develop-ment of early Palaeozoic benthic marine communities remains to be fully investigated.

AcknowledgmentsMark Williams thanks the University of Leicester for study leave. We are grateful to Ewa Olempska (Warsaw) and An-dreas Maas (Ulm) for their constructive reviews of this paper. Leonid Popov acknowledges support from the National Mu-seum of Wales. Mansoureh Ghobadi Pour was supported by the University of Agricultural Sciences and Natural Resources, Gorgan. We thank Ludmila Melnikova (Moscow), Hou Xianguang (Kunming) and Mark Florence (Smithsonian Institution, Washington DC) for access to specimens in their care. This is UMR 5125 contribution number UMR5125-08.011.

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Manuscript received: 05 September 2007Reviewed manuscript accepted: 21 February 2008