Lower Mississippian (Osagean) brachiopods from the Santiago Formation, Oaxaca, Mexico: stratigraphic...

Lower Mississippian (Osagean) brachiopods from the Santiago Formation,

Oaxaca, Mexico: stratigraphic and tectonic implications

D. Navarro-Santillana,*, F. Sour-Tovara, E. Centeno-Garcıab

aMuseo de Paleontologıa de la Facultad de Ciencias, UNAM, Ciudad Universitaria, Mexico City, CP 04510, MexicobInstituto de Geologıa, UNAM, Ciudad Universitaria, Mexico DF, CP 04510, Mexico

Received 31 October 2001; accepted 31 January 2002

Abstract

Four species of spiriferid brachiopods from the Santiago Formation of Oaxaca are described. The species described belong to the orders

Athyridida (Actinoconchus lamellosus ), Spiriferida (Torynifer pseudolineatus ), and Spiriferinida (cf. Syringothyris sp. and Punctospirifer

gigas new species).

The association between A. lamellosus and T. pseudolineatus indicates an Early Mississippian (Osagean) age for the Santiago Formation.

The Carboniferous succession in the area was originally divided into the Santiago Formation (formed by lower and upper members) and the

Ixtaltepec Formation. The studied fauna belongs to the lower member of the Santiago Formation. Because Pennsylvanian brachiopods were

found in strata of the upper member, it is now considered to have more affinity with the Ixtaltepec Formation. Therefore, we propose a change

in the stratigraphy of the area to include the upper member of the Santiago Formation in the Ixtaltepec Formation. This change in the

stratigraphy is supported because a major low-angle normal fault places the former upper member of the Santiago Formation on the lower

member and indicates a tectonic contact between both members of the original Santiago Formation.

Brachiopods from the Santiago Formation are similar to those in the Midcontinent province located in the eastern and central regions of the

United States. The affinity of these Mississippian faunas has important tectonic implications, because Ordovician and Silurian faunas of

Oaxaquia have strong biogeographic affinities with Gondwanan–European provinces. Therefore, there may have been a geographic

connection between southeastern Mexico and North America during Lower Carboniferous times. This could change previous interpretations,

which suggest that the collision of Oaxaquia with North America occurred during Permian time. q 2002 Elsevier Science Ltd. All rights

reserved.

Keywords: Lower Mississippian; Osagean; Brachiopods; Oaxaco; Mexico; Santiago Formation

1. Introduction

The most complete Paleozoic section of south central

Mexico crops out north of Nochixtlan, Oaxaca (Fig. 1),

where it rests unconformably on the Grenvillean-age

Oaxaca complex (Robison and Pantoja-Alor, 1968). This

complex, together with the Huiznopala and Novillo

complexes, has been interpreted as a microcontinent called

Oaxaquia. This cratonic block seems to be allochthonous

with respect to the North America craton throughout its

Precambrian–Early Paleozoic evolution (Stewart et al.,

1993, 1999; Ortega-Gutierrez et al., 1995; Boucot et al.,

1997). The Paleozoic succession of Oaxaca that rests on

southern Oaxaquia is composed of basal Tinu Formation of

Tremadocian age (Robison and Pantoja-Alor, 1968; Orte-

ga-Gutierrez et al., 1995). This is overlain by the Santiago

Formation of Early Mississippian age, followed by the

Ixtaltepec Formation, which contains abundant Lower-

Middle Pennsylvanian invertebrates (Pantoja-Alor, 1970;

Sour-Tovar and Quiroz-Barroso, 1991; Quiroz-Barroso and

Perrilliat, 1997, 1998). The sequence continues into red

beds of the Yododene Formation of unknown age and is

capped by Cretaceous limestone.

The Santiago Formation was formally divided into two

members by Pantoja-Alor (1970), who considered it to be

Early Mississippian due to the presence of the brachiopods

Kitakamithyris and Rotaia. In recent paleontologic studies,

ichnofossils of the genus Scalarituba (Sour-Tovar and

Quiroz-Barroso, 1990, 1991) and the pelecypod ?Streblo-

chondria of Mississippian age have been recognized

(Quiroz-Barroso, 1995). The fossil assemblages of the

0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 89 5 -9 81 1 (0 2) 00 0 47 -0

Journal of South American Earth Sciences 15 (2002) 327–336

www.elsevier.com/locate/jsames

* Corresponding author.

E-mail address: [email protected] (D. Navarro-

Santillan).

http://www.elsevier.com/locate/jsames

Santiago Formation, their degree of preservation, and the

sedimentological features of the rocks indicate reef and

perireef communities (paleoenviroments) (Quiroz-Barroso,

1995) similar to those studied in other Carboniferous

outcrops of the world, such as the ‘Brachiopod Calcarenite

Community’ in England (McKerrow, 1978). Brachiopods

are the most abundant and diverse group among the Upper

Paleozoic fauna from the Oaxaca area. They are associated

with tabulate corals, sponges, bryozoans, pterioid bivalves,

gastropods, and crinoids. The purpose of this study is to

describe the spiriferid and athyridid brachiopods from the

Santiago Formation and discuss their stratigraphic and

paleobiogeographic significance. New paleontological find-

ings that lead us to redefine the stratigraphy of this unit are

also discussed.

2. Location

The Santiago Formation crops out in several canyons and

washes located between 178290 –178340N lat. and 978050 –

978080W long. (Figs. 1 and 2), all of them in the Nochixtlan

municipality, northwest of Oaxaca City. These sites can be

reached by Federal Highway 190. The type section is

Fig. 1. (a) Geographic distribution of Oaxaquia and its position with respect the North American craton and the Ouachita belt. (b) Location of Santiago

Ixtaltepec and distribution of terranes in southern Mexico (modified from Campa-Uranga and Coney, 1983; Ortega-Gutierrez et al., 1995).

Fig. 2. Geologic map of Paleozoic rocks of the Santiago Ixtaltepec region (modified from Pantoja-Alor, 1970; Centeno-Garcıa and Keppie, 1999) and location

of the fossiliferous localities.

D. Navarro-Santillan et al. / Journal of South American Earth Sciences 15 (2002) 327–336328

located in the Las Pulgas wash bed, 600 m north of the town

of Santiago.

3. Stratigraphy

Paleozoic sedimentary rocks from the Santa Maria Tinu

and Santiago Ixtaltepec localities were described by

Pantoja-Alor (1970), who stated that the Lower Paleozoic

is represented by the Tinu Formation of Cambrian–

Ordovician age (Figs. 2 and 3). This formation discordantly

overlies metamorphic rocks of the Precambrian Oaxaca

complex (Fig. 2) and is made up of interbedded limestone

and black shale containing abundant trilobites of Tremado-

cian age (Robison and Pantoja-Alor, 1968). Upper Paleo-

zoic rocks were grouped in three formations by

Pantoja-Alor (1970): the Santiago Formation of Early

Mississippian age, the Ixtaltepec Formation of Middle

Pennsylvanian age, and the Yododene Formation of Late

Pennsylvanian or Permian age (Fig. 3).

The Santiago Formation (Figs. 2 and 3) originally was

described as a series of marine rocks composed of

limestone, sandstone, siltstone, and shale that overlie, in

angular inconformity, clastic rocks of the Tinu Formation.

However, bedding-parallel shearing indicates that

this contact is a normal low-angle detachment fault

(Centeno-Garcıa and Keppie, 1999). Pantoja-Alor (1970)

suggested that the Santiago Formation transitionally

changes into overlying clastics of the Ixtaltepec Formation.

On the basis of lithologic differences, the Santiago

Formation was divided into two easily distinguishable

members (Pantoja-Alor, 1970): a lower calcareous member,

with interbedded thin strata of shale and marl in its upper

part, and an upper lutitic member, with interbedded strata of

siltstone, sandstone, and limestone.

As a part of this work, we measured the type section of

the Santiago Formation in the Las Pulgas canyon, 500 m

north of Santiago Ixtaltepec, and we reexamined the

stratigraphy and fossil content of this unit. We obtained a

thickness of 157 m instead of the 192 m previously

proposed (Pantoja-Alor, 1970). The lower 67 m belong to

the lower calcareous member, described by Pantoja-Alor

(1970). No Mississippian fossils were found in the

remaining 90 m of the upper lutitic member of the Santiago

Formation, but some specimens of the brachiopods

Rugosochonetes granulifer, Composita ovata, and Neospir-

ifer sp. were, which indicates a Pennsylvanian age for this

unit.

On the basis of the fossil content, we propose to reduce

the thickness of the Santiago Formation (Fig. 3) and extend

the lower limit of the Ixtaltepec Formation to include the

upper member of the Santiago Formation. We also suggest

Fig. 3. (a) Stratigraphic column proposed by Pantoja-Alor (1970) and (b) stratigraphic column proposed herein (showing changes made to the stratigraphy of

the Santiago and Ixtaltepec formations).

D. Navarro-Santillan et al. / Journal of South American Earth Sciences 15 (2002) 327–336 329

avoiding the use of the member divisions of the Santiago

Formation. The revised Santiago Formation, as defined

herein, is characterized in its lower part by limestone

containing tabulate coral banks associated with brachiopods

and demosponges belonging to the order Lithistida. These

communities are similar to those found in several ‘Osagean’

outcrops in France (D. Vachard, pers. comm.). The reef

banks are interbedded with calcareous sandstone nearly

12 m thick that contains the same fossil association. The

upper part of the unit is approximately 55 m thick and

contains most of the brachiopods described herein.

The contact between the Santiago and Ixtaltepec

Formations, as defined here, is a shear zone that strongly

deforms the shale and calcareous sandstone of the upper part

of the Santiago Formation and tectonically places the

limestone or siltstone of the Ixtaltepec Formation over the

sheared Santiago Formation. This slide is similar to that

observed at the Tinu–Santiago contact. Both structures

represent top-to-the-north sliding that places younger units

over older as a result of extensional tectonics (Centeno-

Garcıa and Keppie, 1999). The Ixtaltepec Formation

consists of interbedded shale and sandstone with some

limestone beds and contains abundant Lower-Middle

Pennsylvanian invertebrates, which indicate a shallow

marine environment (Sour-Tovar and Quiroz-Barroso,

1991; Quiroz-Barroso and Perrilliat, 1997, 1998).

4. Age and paleobiogeographic remarks

Brachiopods described herein were mainly collected

from the middle and upper parts of the lower member of the

Santiago Formation. Of the four species described here,

Actinoconchus lamellosus and Torynifer pseudolineatus

suggest an Osagean (Early Mississippian) age. These rocks

can be correlated with the outcrops of the Vicente Guerrero

Formation near Ciudad Victoria, Tamaulipas (Fig. 1)

(Carrillo-Bravo, 1959; Stewart et al., 1999) and with the

lower part of the Patlanoaya Formation, southeast of Izucar

de Matamoros, Puebla (Villasenor-Martınez et al., 1987).

Several other outcrops in North America yield similar

species, including the Escabrosa limestone of Bisbee

Quadrangle in Arizona (Girty, 1904), the Keokuk limestone

of the Mississippi River Valley in Illinois (Weller, 1914),

the Boone limestone of St. Joe, Arkansas (Girty, 1915), and

the Cuyahoga (Root et al., 1961) and Logan (Hyde, 1953;

Root et al., 1961) Formations in Ohio, as well as the

Chappel Formation in Texas (Plummer, 1950). The

association of A. lamellosus and T. pseudolineatus has

been found in all these units, except for the Patlanoaya, and

all have been dated as Osagean. The localities from the

United States are part of the Midcontinent province, except

for the Escabrosa limestone, which belongs to the

Appalachian province.

The finding of the A. lamellosus–T. pseudolineatus

association changes the paleogeographic and tectonic

interpretations of the geographic configuration of Mexico

during the Upper Paleozoic, as well as the possible origin of

its tectonostratigraphic terranes. During Early Mississippian

time, the lithospheric pieces where the Paleozoic outcrops

of Oaxaca are located had a latitudinal position very similar

to that of the epicontinental sea represented by the

Midcontinent province. Pennsylvanian fossils studied from

Oaxaca are also similar to those from the east central

regions of the United States (Sour-Tovar, 1994; Quiroz-

Barroso, 1995; Quiroz-Barroso and Perilliat, 1997, 1998).

Thus, we can propose that, during the Carboniferous, the

Midcontinent province may have extended to the south,

including the localities of Oaxaquia. Because marine

communities in Oaxaca have a faunistic association very

similar to those of the North American Midcontinent,

Oaxaquia is considered allochthonous with respect to North

America, at least until the Silurian (Stewart et al., 1993).

Some authors have proposed that its position in Permian

times was still allochthonous with respect to North America

because of the overlap of South America with southern

Mexico and Central America when Pangea is reconstructed

(e.g. Bullard et al., 1965). The similarity of Early

Mississippian faunas in Oaxaquia to those of south central

North America suggests a close approach of the Oaxaquia

block to Laurentia by this time. Reconstruction of Mexico

along the western margin of Pangea, as proposed by Pindell

(1985), places a collisional belt along Oaxaquia during the

Late Paleozoic. However, our data suggest that the

Oaxaquia block was either accreted before the Early

Mississippian or added to North America by other

mechanisms (transform faults?). Moreover, because the

faunal province of Oaxaca and the Midcontinent is different

from that of the Ouachitas, our data suggest that the

Oaxaquia block may not have collided with the Ouachita

belt.

5. Systematic paleontology

Fossil material studied here is deposited in the Museo de

Paleontologıa, Facultad de Ciencias, Universidad Nacional

Autonoma de Mexico, as part of the collection FCMP/E1.

5.1. A. lamellosus (Leveille, 1835)

Phylum Brachiopoda (Dumeril, 1806)

Class Articulata (Huxley, 1869)

Order Athyridida Boucot (Johnson and Staton, 1964)

Suborder Athyrididina Boucot (Johnson and Staton, 1964)

Superfamily Athyridoidea (Davidson, 1881)

Family Athyrididae (Davidson, 1881)

Subfamily Athyridinae (Davidson, 1881)

Genus Actinoconchus (M’Coy, 1844)

Type species Spirifera planosulcata (Phillips, 1836)

Actinoconshus Lamellosus (Leveille, 1835) Figs. 4.1–4.5.


5.1.1. Synonymy

Spirifer lamellosus (Leveille, 1835, p. 39, Figs. 21–23).

Athyris lamellosus (Leveille) (Davidson, 1857, pp.

79–80, plate 16, Fig. 1 and plate 17, Fig. 6).

Athyris lamellosa (Leveille) (Davidson, 1859, plate 16,

Fig. 1a,b).

Athyris (Actinoconchus ) cf. lamellosa (Leveille) (Sibly,

1906, p. 374, plate XXXII, Fig. 1a,b).

Actinoconchus cf. lamellosa (Leveille) (Minato, 1951,

pp. 380–381, plate 1, Fig. 6a,b).

Actinoconchus lamellosa (Leveille) (Minato, 1952, p.

173, plate 11, Fig. 6).

A. lamellosus (Leveille) (Brunton, 1980, pp. 225–226,

Figs. 16 and 17).

5.1.2. Description

Shell is up to 41.1 mm wide and 28.5 mm long;

transversely subelliptical in outline, moderately and sub-

equally biconvex, short hinge line; open delthyrium;

rounded auricles; and uniplicate anterior commisure.

Pedicle valve is slightly flattened along midline in the

posterior half of the shell. Flattening changes gradually to a

shallow, poorly defined mesial sulcus in anterior part;

anterior margin in this part of the shell sometimes folds over

opposite valve to produce a mesial linguiform extension of

moderate size. Posterior tip of the apex is small, curved, and

in close contact with the umbo of the brachial valve. Hinge

teeth are internally prominent, curved at the top, and

supported by very short dental plates; between plates, there

is a deep, transversely striated pedicle cavity.

Brachial valve is slightly flattened in the anterior part

along mesial line; this flattening gradually transforms the

anterior region into a shallow mesial fold that is very

prominent near the anterior margin.

Surfaces of both valves are marked by concentric,

parallel, lamelliform extensions, often mostly destroyed,

leaving only their bases. Internally, dental sockets are wide

and deep, and crura are long and convergent with primary

lamellae curving abruptly at their origin.

5.1.3. Discussion

A. lamellosus is easily recognized by its size and shell

form; the length of the hinge line is shorter than the widest

part of shell. It has rounded cardinal extremities and is

mainly characterized by strong and concentric subparallel

lamellar extensions separated by a distance of 3–5 mm.

The studied material included sixteen specimens: five

juvenile forms in different developmental stages, five

internal molds of pedicle valves (three fragmented and

two nearly complete), one internal mold of a brachial valve,

one external impression of a pedicle valve, and four external

impressions of brachial valves.

The specimen numbers are as follows: FCMP/E1-767,

E1783, E1-821a–b, E1-1462, E1-1882, E1-1883a–b, E1-

1884, E1-894, E1-1895, E1-1896, E1-1897, E1-1898, E1-

1899, E1-1900, E1-1901 and E1-1938.

5.2. T. pseudolineatus (Hall, 1858)

Order Spiriferida (Waagen, 1883)

Suborder Delthyridina (Ivanova, 1972)

Superfamily Reticularioidea (Waagen, 1883)

Family Elythidae (Frederiks, 1924)

Subfamily Toryniferinae (Carter, 1994)

Genus Torynifer (Hall and Clarke, 1894)

Type species Spirifer pseudolineatus (Hall, 1858) from the

Lower Mississippian of the United States.

Torynifer Pseudolineatus (Hall, 1858). Fig. 4.6–4.9.

5.2.1. Synonymy

Spirifer pseudolineatus (Hall, 1858, p. 645, plate 20, Fig.

4).

Spirifera pseudolineata (Hall, 1883, plate 36, Figs.

28–30).

Reticularia pseudolineatus (Hall) (Beede, 1906, p.

1317–1318, plate 21, Fig. 5 and plate 20, Fig. 6a).

Reticularia pseudolineata (Hall) (Grabau and Shimer,

1909, p. 339, Fig. 433).

Torynifer pseudolineata (Hall) (Cooper, 1944, p. 327,

plate 126, Figs. 4–8, 16, and 17).

T. pseudolineatus (Hall) (Ivanova, 1959, p. 54, text Fig.

6Z).

5.2.2. Description

The shell is large, with both valves strongly convex,

particularly in posterior region; transversely subelliptical in

outline; lateral margins symmetrically rounded, as are

cardinal extremities. Length of hinge line is about two-

thirds of the shell width.

Pedicle valve surface curves abruptly from umbonal

region to cardinal margin. Mesial sulcus is ill-defined,

shallow at its origination in umbonal region, and of

moderate width and obsolete at peak. Umbo is very small,

sharp, curved, and projecting beyond the brachial valve;

lateral margins are not well defined; surface is vertically

striated; cardinal area is very small and arched; delthyrium

is triangular and open; hinge teeth are supported by a pair of

strong dental plates that extend about one-fourth of valve

length, diverging at an angle of 308–408; between the dental

lamellae, a long medial septum extends half, or slightly

more, of the valve length, and reaches two or three times the

length of the dental plates; interior covered by fine radial

lines, which number 2–5 per mm.

Brachial valve is less convex than pedicle valve; surface

is abruptly curved from umbonal region and smooth in

anterior and lateral parts, where the shell is compressed

through cardinal extremities; mesial fold usually impercep-

tible or obsolete, except near anterior margin, where it is


low and poorly defined; umbo short and curved; umbonal

region projects beyond cardinal margin; cardinal area is

narrow and located almost in the plane of valve; the median

septum, more or less solid, extends about one-third of the

valve length; a pair of crural plates are united with medium

septum; muscle impressions poorly defined; entire internal

surface is covered by fine radial lines, similar to those in

pedicle valve.

External surfaces of both valves marked by concentric

lamellae with regularly disposed spines; successive rows

show an imbricated arrangement; spinule bases continue to

next concentric bands, as a slightly raised crest, which

makes the shell surface look as if it has both concentric and

radial markings, radial marks being more slender than

concentric ones; radial lines in the interior of valves are

often clearly visible in partially exfoliated specimens.

5.2.3. Discussion

The great size of T. pseudolineatus specimens from

Santiago Ixtaltepec and the presence of conspicuous internal

Fig. 4. Series of photographs of the brachiopods of Midcontinent affinity. (1–5) A. lamellosus (Leveille): 1. Juvenile stage, external impression FCMP/E1-767,

X2; 2a. brachial valve, internal mold, FCMP/E1-821a, X1; 2b. external impression, FCMP/E1-821b, X1; 3. brachial valve, external impression, FCMP/E1-

1882, X1; 4. pedicle valve, internal mold, FCMP/E1-1883b ( £ 1); and 5. juvenile stage, external impression, FCMP/E1-1884, X1. (6–9) T. pseudolineatus

(Hall): 6a. ventral view; 6b. lateral view, FCMP/E1-750, X1; 7. pedicle valve, external impression, FCMP/E1-1886, X1; 8. brachial valve, internal mold,

FCMP/E1-873, X1; and 9. pedicle valve, internal mold, FCMP/E1-1885, X1. (10–15) cf. Syringothyris sp.: 10a. pedicle valve, internal mold, oblique view;

10b. cardinal area, FCMP/E1-505, X1; 11a. pedicle valve, X1; 11b. detail, X2.9, FCMP/E1-289; and 12. brachial valve, internal mold, FCMP/E1-1887, X1.


radial lines distinguish them clearly from T. setigera, T.

salemensis (Weller, 1914), T. montanus (Shaw, 1962), T.

eufastigium (Carter, 1987), and T. spinosus (Stainbrook,

1947), all Mississippian species described from North

America. Moreover, in T. setigera and T. eufastigium, the

mesial sinus and fold are deeper, and apparently, T.

salemensis has no internal radial ribs. There are no other

comparable species of Torynifer.

The studied material included 27 specimens: two whole

specimens with articulated valves; two external impressions

and three internal molds of pedicle valves; eight internal

molds of brachial valves; 11 fragments of internal molds of

pedicle valves; and one fragment of an external impression.


E1-282, E1-731, E1-743, E1-750, E1-769, E1-780, E1-794,

E1-796, E1-838, E1-841, E1-849, E1-864, E1-868, E1-873,

E1-1885, E1-1886, E1-1902–1910, and E1-1911.

5.3. Cf. Syringothyris sp

Order Spiriferinida (Ivanova, 1972)

Suborder Spiriferinidina (Ivanova, 1972)

Superfamily Syringothyridoidea (Frederiks, 1926)

Family Syringothyrididae (Frederiks, 1926)

Subfamily Syringothyridinae (Frederiks, 1926)

Genus cf. Syringothyris (Winchell, 1863)

Type species Syringothyris typa (Winchell, 1863) from the

Lower Mississippian of Iowa, USA. cf. Syringothyris sp.

Figs. 4.10–4.12 and 5.1–5.3.

5.3.1. Description

Shell is large, biconvex, transverse, and impunctuate

with sharp cardinal extremities; widest part of the shell at

the cardinal line reaches 80 mm, whereas the length reaches

36.5 mm; median sulcus and fold are smooth and show only

microornamentation similar to the rest of the shell, which is

made up of pustules in textile-like pattern. Lateral slopes

have up to 17 costae that are regularly intercepted by

tenuous, concentric growth lamellae.

Umbonal apex of pedicle valve is slightly curved;

shallow medial sulcus originates near the umbo and

gradually widens to the anterior margin. Pedicle interarea

very wide, apsacline, and divided by triangular delthyrium,

with a height up to twice the width of base, where an

imperfect delthyrial plate can be observed. The delthyrium

sides are prolonged by dental plates that diverge toward the

posterior part of valve, dividing it into a central rostral zone

with two lateral cavities.

Internally, the brachial valve has a median septum that

extends to half the valve length.

5.3.2. Discussion

Syringothyris has been characterized by four principal

traits (Sass, 1960): (1) lack of ornamentation in the sinus

and fold, (2) microornamentation composed of textile-like

pustules, (3) a great development of interarea in the pedicle

valve with a mesial triangular delthyrium, and (4) the

presence of syrinx. Of these structures, the only one that

could not be observed was the syrinx, and its absence, if

clearly demonstrated, would mean that the material must be

assigned to the genus Pseudosyrinx. However, we assigned

our specimens to Syringothyris on the basis of comparisons

with Mississippian brachiopods collected in Tamaulipas, in

which a syrinx has been clearly observed. These specimens

from Tamaulipas show morphological patterns very similar

to the fossils from Oaxaca, and their association with other

brachiopod species is also very similar. The absence of an

observed syrinx may be due to poor preservation; the

internal molds lack subdeltidial plates, where the syrinx

develops.

Specimens from Oaxaca are clearly distinguishable from

other species by the general form of the shell, the number of

costae on each lateral slope, and their large size. The

Fig. 5. Other brachiopods identified at the Santiago Formation. (1–3) cf. Syringothyris sp.: 1. pedicle valve, internal mold, FCMP/E1-749, X1; 2a. brachial

valve, external view, FCMP/E1-1888, X1; 2b. pedicle valve, external view, FCMP/E1-1889, X1; and 3. pedicle valve, latex mold, FCMP/E1-1890, X1. (4–7)

P. gigas n. sp.: 4. posterior view, internal mold, FCMP/E1-1891, X1; 5. pedicle valve, external impression, FCMP/E1-1892, X1; 6. pedicle valve, internal

mold, FCMP/E1-1893, X1; and 7. pedicle valve, external mold, FCMP/E1-774, X2.


exception is S. transversa (Minato, 1951), from which they

differ only in the width–length ratio of the shell, which is

smaller in S. transversa. These comparisons suggest that the

material of cf. Syringothyris sp. may represent a new

species. However, because of the generic imprecision, it

was decided not to name it (Figs. 4 and 5).

The studied material included pedicle valves (seven

external molds, nine internal molds, and one external

impression) and brachial valves (two external molds and

one external impression). All specimens show different

development stages, and most are incomplete.


E1-505, E1-746, E1-749, E1-786, E1-788, E1-798, E1-800,

E1-803, E1-804, E1-808, E1-844, E1-857, E1-865, E1-881,

E1-1881, E1-1887–1890, E1-1912–1922, and E1-1923.

5.4. Punctospirifer gigas

Superfamily Pennospiriferinoidea (Dagys, 1972)

Family Punctospiriferidae (Waterhouse, 1975)

Genus Punctospirifer (North, 1920)

Type species Punctospirifer scabricosta (North, 1920) from

the Lower Carboniferous (Visean) of England.

Punctospirifer Gigas (Sour-Tovar and Navarro-Santillan

new species.)

Fig. 5.4–5.7.

5.4.1. Diagnosis

Shell is large for this genus, with dimensions up to

40.4 mm wide and 19.4 mm long for brachial valve and

37.3 mm wide and 18.9 mm long in pedicle valve; five ribs

on each side of sinus and fold, an average of six lamellar

growth lines for each five mm; shells lack plications in sinus

and a furrow on fold.

5.4.2. Description

Spiriferoid shell, clearly biconvex, is small in size but

large for genus; semicircular in outline; nearly twice as wide

as long; maximum width at hinge line; cardinal extremities

slightly rounded and mucronate auricles. Commissure

sulcate; sulcus of pedicle valve and fold of brachial valve

are well defined, moderately developed, and rounded;

lateral slopes of both valves with up to five costae and

five furrows on each side of fold and sulcus. There are up to

six radial growth lines for each 5 mm, perpendicular to the

costation, and crossing the fold and sulcus. This gives the

shell an imbricated appearance; microornamentation also

includes spines and granulations; shell punctate.

Pedicle valve is convex; narrow and well-defined

interarea; apex small and slightly curved; medial sulcus,

which originates at the apex, is shallow, rounded, narrow in

posterior region, and gradually widens to anterior region.

Internally, a pair of slightly divergent dental plates; middle

septum well developed and thick at base.

Brachial valve slightly less convex than pedicle valve;

small apex at umbo; mesial fold originates at umbo, striated,

rounded, and does not surpass the rest of shell; middle

septum extends half the valve length.

At the hinge line, there is a series of teeth forming the

hinge; three for each mm.

5.4.3. Discussion

P. gigas was compared with P. scabricosta (North, 1920;

Campbell, 1959), P. subtextus (Girty, 1915), P. kentuck-

yensis (Dunbar and Condra, 1932; Elias, 1957), P.

transversus (Elias, 1957; Lane, 1962; Easton, 1962;

Sour-Tovar, 1994), P. sulcifer, P. globosa (Sanders,

1958), P. amblys (Cvancara, 1958), P. solidirostris (Weller,

1914; Carter, 1972, 1987), P. campestris (Lane, 1962), P.

monroensis (Carter, 1990), P. (Reticulariina ) spinosa

(Easton, 1962), P. depressus (Root et al., 1961), P. cf.

similis (Hyde, 1953), and P. acutus (Carter, 1968) and

differs from all of them mainly by its larger size, its five

costae on each lateral slope (though some specimens of P.

globosa, P. depressus, and P. (Reticulariina ) spinosa have

five plications laterally), and the smaller number of growth

lamellae per millimeter. It is also distinguished from P.

globosa, P. sulcifer, and P. solidirostris, which have a small

plication in the sulcus and a small furrow in the fold.

The studied material included nine external impressions

and seven internal molds of pedicle valves, six external

impressions and three internal molds of brachial valves, and

one specimen that shows the posterior part of an internal

mold of one specimen.

5.4.4. Etymology

Gigas is a Latin word of Greek origin that refers to the

large size of individuals in this taxonomic unit.

The syntypes are as follows: FCMP/ E1-774, E1-1891,

E1-1892a, and E1-1893

The specimen numbers are as follows: FCMP/E1-72, E1-

301, E1-748, E1-784, E1-797, E1-861, E1-862, E1-885, E1-

897, E1-1892b, E1-1924–1936, and E1-1937.

Acknowledgments

The authors thank to H. Hernandez-Campos for photo-

graphic work and L. Chavez-Garcıa for his technical

assistance. They also express their appreciation to J.T.

Dutro Jr., S.A. Quiroz-Barroso, R.B. Blodgett, and F.J.

Vega-Vera for their critical reviews of the manuscript.

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