New Paleocene Rhynchonellide Brachiopods from the Potrerillos Formation, Northeast Mexico

J. Paleont., 81(3), 2007, pp. 483–489 Copyright ᭧ 2007, The Paleontological Society 0022-3360/07/0081-483$03.00

NEW PALEOCENE RHYNCHONELLIDE BRACHIOPODS FROM THE POTRERILLOS 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 JESU´ S SOLE´ 2 1Department of Geology, University of Dayton, Dayton, Ohio 45469-2364, Ͻ[email protected]Ͼ, Ͻ[email protected]Ͼ, 2Instituto de Geologı´a, UNAM, Ciudad Universitaria, Me´xico, D. F. 04510, Ͻ[email protected]Ͼ, Ͻ[email protected]Ͼ, and 3Institute of Tectonic Studies, New Mexico State University, Las Cruces 88003, Ͻ[email protected]Ͼ

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

INTRODUCTION (Fig. 3). The locality has been registered in the Museo de Pa- ALEONTOLOGIC STUDIES from the Difunta Group in northeast- leontologıá, Instituto de Geologıá, National Autonomous Univer- P ern Mexico have yielded an important number of contribu- sity of Mexico (UNAM), as locality IGM 3303. Lens number 1 tions on some of the groups that lived in shallow marine and is included in the Lower Mudstone Member. It includes red algae paralic environments during Campanian through Eocene times. and Maastrichtian foraminifera (Hunnicutt, 1998). An unconfor- Several papers have been published on Campanian and Maastrich- mity between the Cretaceous and Tertiary has been documented tian invertebrates, vertebrates, coprolites and plants (Bo¨se, 1913; (Vega et al., 1989; Giles et al., 1999; Lawton et al., 2002). A Imlay, 1937; Murray and Wolleben, 1960; Wolleben, 1977; Vega conglomerate discordantly overlies the uppermost Maastrichtian and Perrilliat, 1989a, 1990; Vega and Feldmann, 1991; Hernań- Delgado Sandstone Member, on top of the Middle Siltstone Mem- dez, 1992; Hernańdez and Kirkland, 1993; Rodrı´guez de la Rosa ber. This conglomerate includes reworked ammonoids and Paleo- and Cevallos-Ferriz, 1994, 1995; 1998; Vega et al., 1994; Kirk- cene nautiloids (Lawton et al., 2002). land and Aguilloń-Martıńez, 1995; Vega et al., 1995; Rodrı´guez Previous paleontological reports from black shales of the Upper de la Rosa, 1996; Rodrı´guez de la Rosa et al., 1998). Lower Mudstone Member of the Potrerillos Formation include the nau- Tertiary beds of the Difunta Group are also fossiliferous (Murray tiloid Cimomia haltomi (Aldrich, 1931), an index fossil for the and Wolleben, 1959; Echanove, 1967; Vega and Perrilliat, 1989b, Paleocene in the Parras and La Popa basins. In both basins, this 1989c, 1992, 1995; Perrilliat and Vega, 1993; Vega et al., 1999). nautiloid and other invertebrates are found in concretions. The Despite this abundance and diversity of fossil groups, no bra- gastropod Elimia cf. E. trigemmata (Conrad, 1860) and the bi- chiopods have previously been reported from the Difunta Group. valve Venericardia (Baluchicardia) francescae (Gardner and This paper documents two new species of brachiopods and the Bowles, 1939) were also found in this same member (Vega and very peculiar paleoenvironmental setting in which they lived by Perrilliat, 1995). the hundreds of thousands. Lenses 2–6 are included in the Upper Mudstone Member (Fig. 2). They include abundant mollusk and echinoid fragments, and represent shallow-marine, subtidal environments adjacent to the STRATIGRAPHY AND PALEOENVIRONMENT diapir (Lawton et al., 2002). The brachiopods are found in lens The Difunta Group occurs in northeast Mexico (Fig. 1) and 6, included in the Upper Mudstone Member. The lens lies in direct was subdivided between two sedimentary basins, known as Parras contact with the gypsum diapir at its base, with a mean thickness and La Popa (McBride et al., 1974). Most of the paleoenviron- of approximately 10 m. Echanove (1967) reported on benthic fo- ments for the eight Maastrichtian Formations of the Parras basin raminifera, suggesting an Eocene age for this lens. Brachiopods were interpreted as nearshore, deltaic plain, and fluviatile (Mc- are distributed uniformly along the lens. In thin section, benthic Bride et al., 1975). The depositional history in the La Popa basin foraminifera, corals, small gastropods, and bryozoans were also was different. Here, Maastrichtian and Paleocene units are pre- observed. The limestone is very pure, and may represent calcium dominantly marine, whereas Eocene Formations are mostly inter- carbonate precipitation in a shadow effect area of the diapir. tidal and fluviatile. Basin topography and deposition were influ- The precise stratigraphic age for lens 6 is still unknown. We enced by local salt tectonics. Three diapirs and one weld have made 87Sr/86Sr measurements of the well preserved calcitic shells been recognized, with limestone lenses associated with the evo- of several specimens of the more abundant P. neoleonensis n. sp., lution of these structures (McBride et al., 1974; Laudon, 1984; as there is no discrete distribution of both species throughout the 1996; Giles and Lawton, 1999; Lawton et al., 2002). The diapirs lens. The measurement of this ratio can be compared directly to were active during the Late Cretaceous and Early Tertiary. Car- the seawater 87Sr/86Sr curve for an age derivation. The table of bonate lenses formed on top of the topographic rise of the diapirs Howarth and McArthur (1997) was used. The sample was ex- during episodes of low sediment input to the basin and are in- tracted from the brachiopods by handpicking it with a pin. It was cluded in the Maastrichtian Lower Mudstone Member and the then pulverized with an agate mortar, acid-digested, catonic resin Paleocene Upper Mudstone Member, both from the Potrerillos separated, and measured for Sr isotopic composition at the Iso- Formation (Fig. 2). topic Geochemestry Laboratory (LUGIS) of the National Auton- The specimens reported here were collected from lens number omous University of Mexico. The 87Sr/86Sr measurement was 6 of the El Papalote Diapir, located in the Canõn de Potrerillos done with a Finnigan 252 in static mode, giving a value of 483 484 JOURNAL OF PALEONTOLOGY, V. 81, NO. 3, 2007

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

0.707776 ϩ/Ϫ 0.000038. Using the above cited table, this value can correspond either to the Late Cretaceous, late Paleocene, or Middle Eocene. However, the first and last ages are not consistent with local stratigraphy, and therefore, the late Paleocene age (57.4 FIGURE 2—Composite stratigraphic section from La Popa basin, includ- Ma) coincides well with the suggested age derived from previous ing position of carbonate lentils adjacent to El Papalote and Gordo biostratigraphic studies (Vega et al., 1989; Vega and Perrilliat, diapirs. No vertical scale. 1995). TECHNIQUES Discussion.⎯Tertiary rhynchonellid brachiopods are rarely Transverse serial sections have been taken to investigate the found in North America and the genus Probolarina is represented internal structures of Probolarina neoleonensis new species and by a small number of specimens. The genus was first described P. papalotensis new species. A full description of the technique by Cooper in 1959, with the redescription of species P. salpinx and equipment used in the preparation of serial sections was given and P. holmesii. Three additional species, P. brevirostris, P. hol- by Sandy (1989), where additional references can be found. Serial mesii santeenis, and P. transversa were described by Cooper in sections are drawn with the dorsal valve oriented uppermost. 1988. All of these species date from the Eocene and were located in the Castle Hayne and Santee Formations of North and South SYSTEMATIC PALEONTOLOGY Carolina (Cooper, 1959, 1988). Harper (1993) described a single, Phylum BRACHIOPODA Dume´ril, 1806 poorly preserved individual from the Eocene in the Swanswick Subphylum RHYNCHONELLIFORMEA Williams, Carlson, Brunton, Formation of Jamaica and tentatively assigned it to the genus Holmer, and Popov, 1996 Probolarina. Wiedman et. al. (1988) collected a specimen from Class RHYNCHONELLATA Williams, Carlson, Brunton, Holmer the Eocene in the La Mesta Formation from Seymour Island, Ant- and Popov, 1996 arctica which bore resemblance to Probolarina but could not ver- Order RHYNCHONELLIDA Kuhn, 1949 ify this assignment due to the the poor preservation of the spec- Superfamily PUGNACOIDEA Rzhonsnitskaia, 1956 imens. The only other possible Paleocene occurrence of Family BASILIOLIDAE Cooper, 1959 Probolarina which has been described is P. chathamensis (Lee, Subfamily BASILIOLINAE Cooper, 1959 1980) from the Chatham Islands (Late Paleocene to Early Eocene) Genus PROBOLARINA Cooper, 1959 near New Zealand. In contrast to the sparse number of specimens Type species.⎯Rhynchonella holmesii Dall, 1903, p. 1536, pl. of Probolarina located in the eastern coast of the Carolinas, P. 58, figs. 10, 12. chathamensis was found in much greater abundance. KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO 485

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

PROBOLARINA NEOLEONENSIS new species Figures 4–6 Diagnosis.⎯Roundly triangular with narrow costae beginning near umbo; dorsal valve interior with narrow sockets and nearly erect socket ridges; ventral valve interior with strong, vertical dental plates; subfalciform crura. Description.⎯Medium size, subpentagonal to triangular outline, length slightly greater than width, both greater than thickness, maximum width at midvalve to two-thirds from posterior; dorsibiconvex profile; beak moderately long, nearly straight to slightly incurved; small, oval foramen, submesothyrid, auriculate margins, visible conjunct deltidial plates; apical angle varies from acute to obtuse; lateral commissure straight, anterior commissure uniplicate; 8–12 narrow costae beginning near umbo; dorsal valve strongly convex, low fold extending to anterior from midlength and merging into the lateral slopes without disrupting outline in frontal view; ventral valve in lateral profile evenly and slightly convex, shallow sulcus in anterior portion of valve. Dorsal valve interior with narrow sockets and nearly erect socket ridges, no median septum, crural bases attached to socket ridges by flat outer hinge plates, subfalciform crura long, crescentic, concave outward, reaching to one-quarter of length of valve; ventral valve interior with strong, subparallel, vertical dental plates separated from side wall by narrow umbonal chambers, small teeth. Etymology.⎯Nuevo Leoń, Mexico, where this fossil was collected. Types.⎯Holotype IGM 7979; paratypes IGM 7980 to IGM 7986, deposited in the Museo de Paleontologıá, Instituto de Geo- logıá, UNAM. Occurrence.⎯Locality IGM 3303. Limestone lens 6, Upper Mudstone Member, Potrerillos Formation, Paleocene, northeast edge of El Papalote Diapir, Nuevo Leoń, Me´xico. Discussion.⎯Probolarina neoleonensis differs from other described species of Probolarina due to the more prominent, narrow

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FIGURE 4—Plots of length versus thickness (1), length versus width (2), and width versus thickness (3) for specimens of Probolarina neoleonensis n. sp. (triangles) and Probolarina papalotensis n. sp. (circles). Trendline calculated using the ‘‘least squares’’ method. Dimensions in mm. 486 JOURNAL OF PALEONTOLOGY, V. 81, NO. 3, 2007

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 ϫ2. KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO 487

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FIGURE 6—Transverse serial sections through a specimen of Probolarina neoleonensis n. sp. Features observed include: development of dental plates (0.4 mm); beginning of socket ridge (1.0 mm); teeth and sockets begin to form (1.3 mm); teeth and sockets fully visible, hinge plates are nearly ﬂat and horizontal, dental plates are short in proportion to height of pedicle valve, socket ridges are narrow, teeth are correspond- ingly small (1.4 mm); teeth and dental plates are diminishing, crural bases 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 (2.1 mm); crura traced to 3.6 mm. Initial section at 0.0 mm is at tip of ventral valve umbo. Cumulative distance from initial section given in mm. Paratype IGM 7980. Collected in lentil 6, Paleocene El Papalote diapir, La Popa basin, Mexico. Dimensions of specimen: length 14.4 mm; 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 ϫ2. 488 JOURNAL OF PALEONTOLOGY, V. 81, NO. 3, 2007

Etymology.⎯El Papalote Diapir, locality where this fossil was collected. Types.⎯Holotype IGM 7987; Paratypes IGM 7988 to IGM 7992, deposited in the Museo de Paleontologıá, Instituto de Geo- logıá, UNAM. Occurrence.⎯Locality IGM 3303. Limestone lens 6, Upper Mudstone Member, Potrerillos Formation, Paleocene, northeast edge of El Papalote Diapir, Nuevo Leoń, Me´xico. Discussion.⎯Probolarina papalotensis has the broad, low costae which appear to be more similar to other described species of Probolarina. The overall size of P. papalotensis is larger than the FIGURE 8—Transverse serial sections through a specimen of Probolarina papalotensis n. sp. Features observed include: pedicle collar (0.4 mm); species identified by Cooper but appears within the size range of development of dental plates (1.0 mm); beginning of socket ridge, nar- P. chathamensis. However, P. papalotensis has more costae and row umbonal chambers are present (1.8 mm); teeth and sockets are P. chathamensis has receding rather than vertical dental plates fully visible, hinge plates are flat, nearly horizontal, dental plates are (Lee, 1980). short in comparison to height of pedicle valve, narrow socket ridges, small teeth (2.3 mm); teeth and sockets are diminishing (3.0 mm); crura ACKNOWLEDGMENTS are free and form a concave outline facing the dorsal valve (3.6 mm); We are grateful to Drs. E. F. Owen and M. Mancen˜ido for crura traced to 3.9 mm. Initial section at 0.0 mm is at tip of ventral 87 86 valve umbo. Cumulative distance from initialsection given in mm. reviews that improved the paper. Sr/ Sr isotopic determinations Paratype IGM 7989. Collected in lens 6, Paleocene El Papalote diapir, were made at the Laboratorio Universitario de Geoquimica Iso- La Popa basin, Me´xico. Dimensions of specimen: length 12.0 mm; topica (LUGIS) of UNAM. Special thanks to G. Solis and J. J. width 11.0 mm; thickness 8.3 mm. 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 Chemical costae which begin at the umbo. It is also thicker than other spe- Society Petroleum Research Fund for support. cies, with a thickness to length ratio of 0.72. In contrast, P. papalotensis has a thickness to length ratio of 0.61, and P. chathe- REFERENCES mensis has a ratio of approximately 0.66 (Lee, 1980). The internal ALDRICH, T. H. 1931. Description of a few Alabama Eocene species and features appear to be very similar to other described species of remarks on varieties. Alabama Geological Survey, Museum Paper, 12: Probolarina. No pedicle collar was observed but the beak of the 1–21. sectioned specimen (Fig. 6) appears to have been damaged. BO¨ SE, E. 1913. 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Philosophical Transactions of the Royal Society, London, B, 351:1171– Plant-bearing coprolites from the El Cerro del Pueblo Formation, 1193. southeastern Coahuila. American Journal of Botany (Supplement), 82: WOLLEBEN, J. A. 1977. Paleontology of the Difunta Group (Upper Cre- 260. taceous-Tertiary) in northern Mexico. Journal of Paleontology, 51:373– RODRIGUEZDELAROSA, R., AND S. R. S. CEVALLOS-FERRIZ. 1998. 398. Vertebrates of the El Pelillal locality (Campanian, Cerro del Pueblo ACCEPTED 25 JANUARY 2006