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J. Paleont., 81(3), 2007, pp. 483–489Copyright � 2007, The Paleontological Society0022-3360/07/0081-483$03.00

NEW PALEOCENE RHYNCHONELLIDE BRACHIOPODS FROM THEPOTRERILLOS FORMATION, NORTHEAST MEXICO

SUSAN L. KLOSTERMAN,1 MICHAEL R. SANDY,1 FRANCISCO J. VEGA,2 KATHERINE A. GILES,3 KYLE GRAF,3

DAVID SHELLEY,3 AND JESUS SOLE2

1Department of Geology, University of Dayton, Dayton, Ohio 45469-2364, �Sue.Klosterman@notes.udayton.edu�,�Michael.Sandy@notes.udayton.edu�, 2Instituto de Geologıa, UNAM, Ciudad Universitaria, Mexico, D. F. 04510, �vegver@servidor.unam.mx�,

�jsole@geologia.unam.mx�, and 3Institute of Tectonic Studies, New Mexico State University, Las Cruces 88003, �kgiles@nmsu.edu�

ABSTRACT—Two new species of the rhynchonellid brachiopod Probolarina are described, Probolarina neoleonensis new species andProbolarina papalotensis new species. They were collected from a Paleocene limestone lens associated with a diapir in the La Popabasin, northeastern Mexico. Thousands of these brachiopods occur in this lens and constitute the first report of brachiopods for theDifunta Group, from which a diverse paleobiota has been previously reported. This occurrence represents the oldest record for thegenus in the Western Hemisphere, as the only other Paleocene occurence of this genus was reported from New Zealand. Recent studiessuggest that the carbonate lentil from which the brachiopods were collected were deposited in the shadow-effect area adjacent to thediapir, which affected the sediment influx into the basin.

INTRODUCTION

PALEONTOLOGIC STUDIES from the Difunta Group in northeast-ern Mexico have yielded an important number of contribu-

tions on some of the groups that lived in shallow marine andparalic environments during Campanian through Eocene times.Several papers have been published on Campanian and Maastrich-tian invertebrates, vertebrates, coprolites and plants (Bose, 1913;Imlay, 1937; Murray and Wolleben, 1960; Wolleben, 1977; Vegaand Perrilliat, 1989a, 1990; Vega and Feldmann, 1991; Hernan-dez, 1992; Hernandez and Kirkland, 1993; Rodrıguez de la Rosaand Cevallos-Ferriz, 1994, 1995; 1998; Vega et al., 1994; Kirk-land and Aguillon-Martınez, 1995; Vega et al., 1995; Rodrıguezde la Rosa, 1996; Rodrıguez de la Rosa et al., 1998). LowerTertiary beds of the Difunta Group are also fossiliferous (Murrayand Wolleben, 1959; Echanove, 1967; Vega and Perrilliat, 1989b,1989c, 1992, 1995; Perrilliat and Vega, 1993; Vega et al., 1999).Despite this abundance and diversity of fossil groups, no bra-chiopods have previously been reported from the Difunta Group.

This paper documents two new species of brachiopods and thevery peculiar paleoenvironmental setting in which they lived bythe hundreds of thousands.

STRATIGRAPHY AND PALEOENVIRONMENT

The Difunta Group occurs in northeast Mexico (Fig. 1) andwas subdivided between two sedimentary basins, known as Parrasand La Popa (McBride et al., 1974). Most of the paleoenviron-ments for the eight Maastrichtian Formations of the Parras basinwere interpreted as nearshore, deltaic plain, and fluviatile (Mc-Bride et al., 1975). The depositional history in the La Popa basinwas different. Here, Maastrichtian and Paleocene units are pre-dominantly marine, whereas Eocene Formations are mostly inter-tidal and fluviatile. Basin topography and deposition were influ-enced by local salt tectonics. Three diapirs and one weld havebeen recognized, with limestone lenses associated with the evo-lution of these structures (McBride et al., 1974; Laudon, 1984;1996; Giles and Lawton, 1999; Lawton et al., 2002). The diapirswere active during the Late Cretaceous and Early Tertiary. Car-bonate lenses formed on top of the topographic rise of the diapirsduring episodes of low sediment input to the basin and are in-cluded in the Maastrichtian Lower Mudstone Member and thePaleocene Upper Mudstone Member, both from the PotrerillosFormation (Fig. 2).

The specimens reported here were collected from lens number6 of the El Papalote Diapir, located in the Canon de Potrerillos

(Fig. 3). The locality has been registered in the Museo de Pa-leontologıa, Instituto de Geologıa, National Autonomous Univer-sity of Mexico (UNAM), as locality IGM 3303. Lens number 1is included in the Lower Mudstone Member. It includes red algaeand Maastrichtian foraminifera (Hunnicutt, 1998). An unconfor-mity between the Cretaceous and Tertiary has been documented(Vega et al., 1989; Giles et al., 1999; Lawton et al., 2002). Aconglomerate discordantly overlies the uppermost MaastrichtianDelgado Sandstone Member, on top of the Middle Siltstone Mem-ber. This conglomerate includes reworked ammonoids and Paleo-cene nautiloids (Lawton et al., 2002).

Previous paleontological reports from black shales of the UpperMudstone Member of the Potrerillos Formation include the nau-tiloid Cimomia haltomi (Aldrich, 1931), an index fossil for thePaleocene in the Parras and La Popa basins. In both basins, thisnautiloid and other invertebrates are found in concretions. Thegastropod Elimia cf. E. trigemmata (Conrad, 1860) and the bi-valve Venericardia (Baluchicardia) francescae (Gardner andBowles, 1939) were also found in this same member (Vega andPerrilliat, 1995).

Lenses 2–6 are included in the Upper Mudstone Member (Fig.2). They include abundant mollusk and echinoid fragments, andrepresent shallow-marine, subtidal environments adjacent to thediapir (Lawton et al., 2002). The brachiopods are found in lens6, included in the Upper Mudstone Member. The lens lies in directcontact with the gypsum diapir at its base, with a mean thicknessof approximately 10 m. Echanove (1967) reported on benthic fo-raminifera, suggesting an Eocene age for this lens. Brachiopodsare distributed uniformly along the lens. In thin section, benthicforaminifera, corals, small gastropods, and bryozoans were alsoobserved. The limestone is very pure, and may represent calciumcarbonate precipitation in a shadow effect area of the diapir.

The precise stratigraphic age for lens 6 is still unknown. Wemade 87Sr/86Sr measurements of the well preserved calcitic shellsof several specimens of the more abundant P. neoleonensis n. sp.,as there is no discrete distribution of both species throughout thelens. The measurement of this ratio can be compared directly tothe seawater 87Sr/86Sr curve for an age derivation. The table ofHowarth and McArthur (1997) was used. The sample was ex-tracted from the brachiopods by handpicking it with a pin. It wasthen pulverized with an agate mortar, acid-digested, catonic resinseparated, and measured for Sr isotopic composition at the Iso-topic Geochemestry Laboratory (LUGIS) of the National Auton-omous University of Mexico. The 87Sr/86Sr measurement wasdone with a Finnigan 252 in static mode, giving a value of

484 JOURNAL OF PALEONTOLOGY, V. 81, NO. 3, 2007

FIGURE 1—Location map of the La Popa basin in northeastern Mexico.

FIGURE 2—Composite stratigraphic section from La Popa basin, includ-ing position of carbonate lentils adjacent to El Papalote and Gordodiapirs. No vertical scale.

0.707776 �/� 0.000038. Using the above cited table, this valuecan correspond either to the Late Cretaceous, late Paleocene, orMiddle Eocene. However, the first and last ages are not consistentwith local stratigraphy, and therefore, the late Paleocene age (57.4Ma) coincides well with the suggested age derived from previousbiostratigraphic studies (Vega et al., 1989; Vega and Perrilliat,1995).

TECHNIQUES

Transverse serial sections have been taken to investigate theinternal structures of Probolarina neoleonensis new species andP. papalotensis new species. A full description of the techniqueand equipment used in the preparation of serial sections was givenby Sandy (1989), where additional references can be found. Serialsections are drawn with the dorsal valve oriented uppermost.

SYSTEMATIC PALEONTOLOGY

Phylum BRACHIOPODA Dumeril, 1806Subphylum RHYNCHONELLIFORMEA Williams, Carlson, Brunton,

Holmer, and Popov, 1996Class RHYNCHONELLATA Williams, Carlson, Brunton, Holmer

and Popov, 1996Order RHYNCHONELLIDA Kuhn, 1949

Superfamily PUGNACOIDEA Rzhonsnitskaia, 1956Family BASILIOLIDAE Cooper, 1959

Subfamily BASILIOLINAE Cooper, 1959Genus PROBOLARINA Cooper, 1959

Type species.⎯Rhynchonella holmesii Dall, 1903, p. 1536, pl.58, figs. 10, 12.

Discussion.⎯Tertiary rhynchonellid brachiopods are rarelyfound in North America and the genus Probolarina is representedby a small number of specimens. The genus was first describedby Cooper in 1959, with the redescription of species P. salpinxand P. holmesii. Three additional species, P. brevirostris, P. hol-mesii santeenis, and P. transversa were described by Cooper in1988. All of these species date from the Eocene and were locatedin the Castle Hayne and Santee Formations of North and SouthCarolina (Cooper, 1959, 1988). Harper (1993) described a single,poorly preserved individual from the Eocene in the SwanswickFormation of Jamaica and tentatively assigned it to the genusProbolarina. Wiedman et. al. (1988) collected a specimen fromthe Eocene in the La Mesta Formation from Seymour Island, Ant-arctica which bore resemblance to Probolarina but could not ver-ify this assignment due to the the poor preservation of the spec-imens. The only other possible Paleocene occurrence ofProbolarina which has been described is P. chathamensis (Lee,1980) from the Chatham Islands (Late Paleocene to Early Eocene)near New Zealand. In contrast to the sparse number of specimensof Probolarina located in the eastern coast of the Carolinas, P.chathamensis was found in much greater abundance.

485KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO

FIGURE 3—Location map of fossil locality at lens 6, northeast of ElPapalote diapir.

→

FIGURE 4—Plots of length versus thickness (1), length versus width (2),and width versus thickness (3) for specimens of Probolarina neoleo-nensis n. sp. (triangles) and Probolarina papalotensis n. sp. (circles).Trendline calculated using the ‘‘least squares’’ method. Dimensions inmm.

PROBOLARINA NEOLEONENSIS new speciesFigures 4–6

Diagnosis.⎯Roundly triangular with narrow costae beginningnear umbo; dorsal valve interior with narrow sockets and nearlyerect socket ridges; ventral valve interior with strong, vertical den-tal plates; subfalciform crura.

Description.⎯Medium size, subpentagonal to triangular out-line, length slightly greater than width, both greater than thick-ness, maximum width at midvalve to two-thirds from posterior;dorsibiconvex profile; beak moderately long, nearly straight toslightly incurved; small, oval foramen, submesothyrid, auriculatemargins, visible conjunct deltidial plates; apical angle varies fromacute to obtuse; lateral commissure straight, anterior commissureuniplicate; 8–12 narrow costae beginning near umbo; dorsal valvestrongly convex, low fold extending to anterior from midlengthand merging into the lateral slopes without disrupting outline infrontal view; ventral valve in lateral profile evenly and slightlyconvex, shallow sulcus in anterior portion of valve.

Dorsal valve interior with narrow sockets and nearly erect sock-et ridges, no median septum, crural bases attached to socket ridgesby flat outer hinge plates, subfalciform crura long, crescentic, con-cave outward, reaching to one-quarter of length of valve; ventralvalve interior with strong, subparallel, vertical dental plates sep-arated from side wall by narrow umbonal chambers, small teeth.

Etymology.⎯Nuevo Leon, Mexico, where this fossil was col-lected.

Types.⎯Holotype IGM 7979; paratypes IGM 7980 to IGM7986, deposited in the Museo de Paleontologıa, Instituto de Geo-logıa, UNAM.

Occurrence.⎯Locality IGM 3303. Limestone lens 6, UpperMudstone Member, Potrerillos Formation, Paleocene, northeastedge of El Papalote Diapir, Nuevo Leon, Mexico.

Discussion.⎯Probolarina neoleonensis differs from other de-scribed species of Probolarina due to the more prominent, narrow

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FIGURE 5—Probolarina neoleonensis n. sp. Collected in lens 6, Paleocene El Papalote diapir, La Popa basin, Mexico. 1–4, Holotype IGM 7979,dorsal,ventral, lateral, and anterior views; 5–8, paratype IGM 7981, dorsal, ventral, lateral, and anterior views; 9–12, paratype IGM 7982, dorsal,ventral, lateral, and anterior views; 13–16, paratype IGM 7983, dorsal, ventral, lateral, and anterior views. 17–20, paratype IGM 7984, dorsal,ventral, lateral, and anterior views; 21–24, paratype IGM 7985, dorsal, ventral, lateral, and anterior views; 25–28, paratype IGM 7986, dorsal,ventral, lateral, and anterior views. All �2.

487KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO

←

FIGURE 6—Transverse serial sections through a specimen of Probolarinaneoleonensis n. sp. Features observed include: development of dentalplates (0.4 mm); beginning of socket ridge (1.0 mm); teeth and socketsbegin to form (1.3 mm); teeth and sockets fully visible, hinge platesare nearly flat and horizontal, dental plates are short in proportion toheight of pedicle valve, socket ridges are narrow, teeth are correspond-ingly small (1.4 mm); teeth and dental plates are diminishing, cruralbases are attached to socket ridges by flat outer hinge plate (2.0 mm);crura are free and form a concave outline facing the dorsal valve (2.1mm); crura traced to 3.6 mm. Initial section at 0.0 mm is at tip ofventral valve umbo. Cumulative distance from initial section given inmm. Paratype IGM 7980. Collected in lentil 6, Paleocene El Papalotediapir, La Popa basin, Mexico. Dimensions of specimen: length 14.4mm; width 15.7 mm; thickness 10.2 mm.

FIGURE 7—Probolarina papalotensis n. sp. Collected in lens 6, Paleocene El Papalote diapir, La Popa basin, Mexico. 1–4, Holotype IGM 7987,dorsal, ventral, lateral, and anterior views; 5–8, paratype IGM 7988, dorsal, ventral, lateral, and anterior views; 9–12, paratype IGM 7990, dorsal,ventral, lateral, and anterior views; 13–16, paratype IGM 7991, dorsal, ventral, lateral, and anterior views; 17–20, paratype IGM 7992, dorsal,ventral, lateral, and anterior views. All �2.

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FIGURE 8—Transverse serial sections through a specimen of Probolarinapapalotensis n. sp. Features observed include: pedicle collar (0.4 mm);development of dental plates (1.0 mm); beginning of socket ridge, nar-row umbonal chambers are present (1.8 mm); teeth and sockets arefully visible, hinge plates are flat, nearly horizontal, dental plates areshort in comparison to height of pedicle valve, narrow socket ridges,small teeth (2.3 mm); teeth and sockets are diminishing (3.0 mm); cruraare free and form a concave outline facing the dorsal valve (3.6 mm);crura traced to 3.9 mm. Initial section at 0.0 mm is at tip of ventralvalve umbo. Cumulative distance from initialsection given in mm.Paratype IGM 7989. Collected in lens 6, Paleocene El Papalote diapir,La Popa basin, Mexico. Dimensions of specimen: length 12.0 mm;width 11.0 mm; thickness 8.3 mm.

costae which begin at the umbo. It is also thicker than other spe-cies, with a thickness to length ratio of 0.72. In contrast, P. pa-palotensis has a thickness to length ratio of 0.61, and P. chathe-mensis has a ratio of approximately 0.66 (Lee, 1980). The internalfeatures appear to be very similar to other described species ofProbolarina. No pedicle collar was observed but the beak of thesectioned specimen (Fig. 6) appears to have been damaged.

Several members of the subfamily Pamirorhynchiinae, includ-ing Orbirhynchia, Parthirhynchia, and Riorhynchia, are similarto P. papalotensis in terms of their more fully costate shells. Yet,the genera in this subfamily do not have the prominent, elongatedbeak associated with Probolarina. Parthirhynchia has an elon-gated-oval shape, atypical to the subfamily Pamirorhynchiinae(and Probolarina). Its subfamily assignment is due to its similar-ity to Orbirhynchia, but with a longer smooth stage. Orbirhynchiagenerally has an orbicular shell shape and falciform crura. In ad-dition, the elements of cardinalia in Riorhynchia are all massiveand thickened and the crura are hamiform.

PROBOLARINA PAPALOTENSIS new speciesFigures 4, 7, 8

Diagnosis.⎯Triangular with low, broad costae beginning mid-valve; dorsal valve interior with narrow sockets and nearly erectsocket ridges; ventral valve interior with strong, vertical dentalplates; subfalciform crura.

Description.⎯Medium size, subpentagonal to triangular out-line, length slightly greater than width, both greater than thick-ness, maximum width at midvalve or two-thirds from posterior;dorsibiconvex profile; beak long, nearly straight to slightly in-curved; small, oval foramen, submesothyrid, auriculate margins,visible conjunct deltidial plates; apical angle acute; lateral com-missure straight, anterior commissure uniplicate; broad, low cos-tae beginning at midlength; dorsal valve strongly convex, foldextending to anterior from midlength; ventral valve in lateral pro-file evenly and slightly convex, sulcus in anterior portion of valve.

Dorsal valve interior with narrow sockets and nearly erect sock-et ridges, no median septum, crural bases attached to socket ridgesby flat outer hinge plates, subfalciform crura long, crescentic, con-cave outward, reaching to one-third of length of valve; ventralvalve interior with pedicle collar, strong, vertical dental platesseparated from side wall by narrow umbonal chambers, smallteeth.

Etymology.⎯El Papalote Diapir, locality where this fossil wascollected.

Types.⎯Holotype IGM 7987; Paratypes IGM 7988 to IGM7992, deposited in the Museo de Paleontologıa, Instituto de Geo-logıa, UNAM.

Occurrence.⎯Locality IGM 3303. Limestone lens 6, UpperMudstone Member, Potrerillos Formation, Paleocene, northeastedge of El Papalote Diapir, Nuevo Leon, Mexico.

Discussion.⎯Probolarina papalotensis has the broad, low cos-tae which appear to be more similar to other described species ofProbolarina. The overall size of P. papalotensis is larger than thespecies identified by Cooper but appears within the size range ofP. chathamensis. However, P. papalotensis has more costae andP. chathamensis has receding rather than vertical dental plates(Lee, 1980).

ACKNOWLEDGMENTS

We are grateful to Drs. E. F. Owen and M. Mancenido forreviews that improved the paper. 87Sr/86Sr isotopic determinationswere made at the Laboratorio Universitario de Geoquimica Iso-topica (LUGIS) of UNAM. Special thanks to G. Solis and J. J.Morales, whose help was valuable during the laboratory process.A. Altamira, Instituto de Geologia, UNAM, helped with the im-ages. MRS acknowledges the donors of The American ChemicalSociety Petroleum Research Fund for support.

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ACCEPTED 25 JANUARY 2006