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J. Paleont., 81(3), 2007, pp. 483–489 Copyright 䉷 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 ´ S SOLE´2 DAVID SHELLEY,3 AND JESU 1 Department of Geology, University of Dayton, Dayton, Ohio 45469-2364, ⬍[email protected]⬎, ⬍[email protected]⬎, 2Instituto de Geologıá, UNAM, Ciudad Universitaria, Me´xico, D. F. 04510, ⬍[email protected]⬎, ⬍[email protected]⬎, and 3Institute of Tectonic Studies, New Mexico State University, Las Cruces 88003, ⬍[email protected]⬎

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

P

from the Difunta Group in northeastern Mexico have yielded an important number of contributions on some of the groups that lived in shallow marine and paralic environments during Campanian through Eocene times. Several papers have been published on Campanian and Maastrichtian invertebrates, vertebrates, coprolites and plants (Bo¨se, 1913; Imlay, 1937; Murray and Wolleben, 1960; Wolleben, 1977; Vega and Perrilliat, 1989a, 1990; Vega and Feldmann, 1991; Hernańdez, 1992; Hernańdez and Kirkland, 1993; Rodrı´guez de la Rosa and Cevallos-Ferriz, 1994, 1995; 1998; Vega et al., 1994; Kirkland and Aguilloń-Martıńez, 1995; Vega et al., 1995; Rodrı´guez de la Rosa, 1996; Rodrı´guez de la Rosa et al., 1998). Lower Tertiary beds of the Difunta Group are also fossiliferous (Murray and 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 brachiopods have previously been reported from the Difunta Group. This paper documents two new species of brachiopods and the very peculiar paleoenvironmental setting in which they lived by the hundreds of thousands. ALEONTOLOGIC STUDIES

STRATIGRAPHY AND PALEOENVIRONMENT

The Difunta Group occurs in northeast Mexico (Fig. 1) and was subdivided between two sedimentary basins, known as Parras and La Popa (McBride et al., 1974). Most of the paleoenvironments for the eight Maastrichtian Formations of the Parras basin were interpreted as nearshore, deltaic plain, and fluviatile (McBride et al., 1975). The depositional history in the La Popa basin was different. Here, Maastrichtian and Paleocene units are predominantly marine, whereas Eocene Formations are mostly intertidal and fluviatile. Basin topography and deposition were influenced by local salt tectonics. Three diapirs and one weld have been recognized, with limestone lenses associated with the evolution of these structures (McBride et al., 1974; Laudon, 1984; 1996; Giles and Lawton, 1999; Lawton et al., 2002). The diapirs were active during the Late Cretaceous and Early Tertiary. Carbonate lenses formed on top of the topographic rise of the diapirs during episodes of low sediment input to the basin and are included in the Maastrichtian Lower Mudstone Member and the Paleocene Upper Mudstone Member, both from the Potrerillos Formation (Fig. 2). The specimens reported here were collected from lens number 6 of the El Papalote Diapir, located in the Canõn de Potrerillos

(Fig. 3). The locality has been registered in the Museo de Paleontologıá, Instituto de Geologıá, National Autonomous University of Mexico (UNAM), as locality IGM 3303. Lens number 1 is included in the Lower Mudstone Member. It includes red algae and Maastrichtian foraminifera (Hunnicutt, 1998). An unconformity between the Cretaceous and Tertiary has been documented (Vega et al., 1989; Giles et al., 1999; Lawton et al., 2002). A conglomerate discordantly overlies the uppermost Maastrichtian Delgado Sandstone Member, on top of the Middle Siltstone Member. This conglomerate includes reworked ammonoids and Paleocene nautiloids (Lawton et al., 2002). Previous paleontological reports from black shales of the Upper Mudstone Member of the Potrerillos Formation include the nautiloid Cimomia haltomi (Aldrich, 1931), an index fossil for the Paleocene in the Parras and La Popa basins. In both basins, this nautiloid and other invertebrates are found in concretions. The gastropod Elimia cf. E. trigemmata (Conrad, 1860) and the bivalve Venericardia (Baluchicardia) francescae (Gardner and Bowles, 1939) were also found in this same member (Vega and Perrilliat, 1995). 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 diapir (Lawton et al., 2002). The brachiopods are found in lens 6, included in the Upper Mudstone Member. The lens lies in direct contact with the gypsum diapir at its base, with a mean thickness of approximately 10 m. Echanove (1967) reported on benthic foraminifera, suggesting an Eocene age for this lens. Brachiopods are distributed uniformly along the lens. In thin section, benthic foraminifera, corals, small gastropods, and bryozoans were also observed. The limestone is very pure, and may represent calcium carbonate precipitation in a shadow effect area of the diapir. The precise stratigraphic age for lens 6 is still unknown. We made 87Sr/86Sr measurements of the well preserved calcitic shells of several specimens of the more abundant P. neoleonensis n. sp., as there is no discrete distribution of both species throughout the lens. The measurement of this ratio can be compared directly to the seawater 87Sr/86Sr curve for an age derivation. The table of Howarth and McArthur (1997) was used. The sample was extracted from the brachiopods by handpicking it with a pin. It was then pulverized with an agate mortar, acid-digested, catonic resin separated, and measured for Sr isotopic composition at the Isotopic Geochemestry Laboratory (LUGIS) of the National Autonomous University of Mexico. The 87Sr/86Sr measurement was done with a Finnigan 252 in static mode, giving a value of

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

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

0.707776 ⫹/⫺ 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 Ma) coincides well with the suggested age derived from previous biostratigraphic studies (Vega et al., 1989; Vega and Perrilliat, 1995). TECHNIQUES

Transverse serial sections have been taken to investigate the internal structures of Probolarina neoleonensis new species and P. papalotensis new species. A full description of the technique and equipment used in the preparation of serial sections was given by Sandy (1989), where additional references can be found. Serial sections are drawn with the dorsal valve oriented uppermost. SYSTEMATIC PALEONTOLOGY

Phylum BRACHIOPODA Dume´ril, 1806 Subphylum RHYNCHONELLIFORMEA Williams, Carlson, Brunton, Holmer, and Popov, 1996 Class RHYNCHONELLATA Williams, Carlson, Brunton, Holmer and Popov, 1996 Order RHYNCHONELLIDA Kuhn, 1949 Superfamily PUGNACOIDEA Rzhonsnitskaia, 1956 Family BASILIOLIDAE Cooper, 1959 Subfamily BASILIOLINAE Cooper, 1959 Genus PROBOLARINA Cooper, 1959 Type species.⎯Rhynchonella holmesii Dall, 1903, p. 1536, pl. 58, figs. 10, 12.

FIGURE 2—Composite stratigraphic section from La Popa basin, including position of carbonate lentils adjacent to El Papalote and Gordo diapirs. No vertical scale.

Discussion.⎯Tertiary rhynchonellid brachiopods are rarely found in North America and the genus Probolarina is represented by a small number of specimens. The genus was first described by Cooper in 1959, with the redescription of species P. salpinx and P. holmesii. Three additional species, P. brevirostris, P. holmesii santeenis, and P. transversa were described by Cooper in 1988. All of these species date from the Eocene and were located in the Castle Hayne and Santee Formations of North and South Carolina (Cooper, 1959, 1988). Harper (1993) described a single, poorly preserved individual from the Eocene in the Swanswick Formation of Jamaica and tentatively assigned it to the genus Probolarina. Wiedman et. al. (1988) collected a specimen from the Eocene in the La Mesta Formation from Seymour Island, Antarctica which bore resemblance to Probolarina but could not verify this assignment due to the the poor preservation of the specimens. The only other possible Paleocene occurrence of Probolarina 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 specimens of Probolarina located in the eastern coast of the Carolinas, P. chathamensis was found in much greater abundance.

KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO

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

PROBOLARINA

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 Geologıá, 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 NEOLEONENSIS

→ 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.

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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.

KLOSTERMAN ET AL.—NEW PALEOCENE BRACHIOPODS FROM MEXICO

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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 correspondingly 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 ⫻2.

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FIGURE 8—Transverse serial sections through a specimen of Probolarina papalotensis n. sp. Features observed include: pedicle collar (0.4 mm); development of dental plates (1.0 mm); beginning of socket ridge, narrow umbonal chambers are present (1.8 mm); teeth and sockets are fully visible, hinge plates are flat, nearly horizontal, dental plates are 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 are 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 ventral valve umbo. Cumulative distance from initialsection given in mm. Paratype IGM 7989. Collected in lens 6, Paleocene El Papalote diapir, La Popa basin, Me´xico. 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 species, with a thickness to length ratio of 0.72. In contrast, P. papalotensis has a thickness to length ratio of 0.61, and P. chathemensis has a ratio of approximately 0.66 (Lee, 1980). The internal features appear to be very similar to other described species of Probolarina. No pedicle collar was observed but the beak of the sectioned specimen (Fig. 6) appears to have been damaged. Several members of the subfamily Pamirorhynchiinae, including Orbirhynchia, Parthirhynchia, and Riorhynchia, are similar to P. papalotensis in terms of their more fully costate shells. Yet, the genera in this subfamily do not have the prominent, elongated beak associated with Probolarina. Parthirhynchia has an elongated-oval shape, atypical to the subfamily Pamirorhynchiinae (and Probolarina). Its subfamily assignment is due to its similarity to Orbirhynchia, but with a longer smooth stage. Orbirhynchia generally has an orbicular shell shape and falciform crura. In addition, the elements of cardinalia in Riorhynchia are all massive and thickened and the crura are hamiform. PROBOLARINA PAPALOTENSIS new species Figures 4, 7, 8 Diagnosis.⎯Triangular with low, broad costae beginning midvalve; 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 or two-thirds from posterior; dorsibiconvex profile; beak long, nearly straight to slightly incurved; small, oval foramen, submesothyrid, auriculate margins, visible conjunct deltidial plates; apical angle acute; lateral commissure straight, anterior commissure uniplicate; broad, low costae beginning at midlength; dorsal valve strongly convex, fold extending to anterior from midlength; ventral valve in lateral profile evenly and slightly convex, 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-third of length of valve; ventral valve interior with pedicle collar, strong, vertical dental plates separated from side wall by narrow umbonal chambers, small teeth.

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 Geologıá, 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 species identified by Cooper but appears within the size range of P. chathamensis. However, P. papalotensis has more costae and P. chathamensis has receding rather than vertical dental plates (Lee, 1980). ACKNOWLEDGMENTS

We are grateful to Drs. E. F. Owen and M. Mancen˜ido for reviews that improved the paper. 87Sr/86Sr isotopic determinations were made at the Laboratorio Universitario de Geoquimica Isotopica (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 images. MRS acknowledges the donors of The American Chemical Society Petroleum Research Fund for support. REFERENCES

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