Revista Mexicana de Ciencias Geológicas,Decapod v. crustaceans 26, núm. 3, from2009, the p. 745-763Paleocene of Central Texas, USA 745 Decapod crustaceans from the Paleocene of Central Texas, USA Adam Armstrong1, Torrey Nyborg2, Gale A. Bishop3, Àlex Ossó-Morales4, and Francisco J. Vega5,* 1 306 Hilltop Road, Keene, TX 76059, U.S.A. 2 Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, California 92350, U.S.A. 3 Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460, U.S.A. 4 Josep Vicenç Foix, 12-H, 1er-1ª 43007 Tarragona, Catalonia, Spain. 5 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, México DF 04510, Mexico. * [email protected] ABSTRACT Fourteen species of decapods crustaceans are described from a single locality near Mexia, Texas, where Paleocene sediments of the Mexia Clay Member of the Wills Point Formation crop out. The species are represented by Hoploparia sp., Linuparus wilcoxensis Rathbun, 1935, an unnamed paguroid, Kierionopsis nodosa Davidson, 1966, Pithonoton cardwelli new species, Caloxanthus sp., Macroacaena johnsoni (Rathbun, 1935), new combination, Raninoides bournei (Rathbun, 1928), R. treldenaesensis (Collins and Jakobsen, 2003), Prehepatus sp., Tehuacana americana (Rathbun, 1935), new combination, Costacopluma texana new species, Paraverrucoides alabamensis (Rathbun, 1935) and Viapinnixa mexiaensis new species. Morphological details unknown for previously described species are included. New systematic placements are offered based on recent research. Intraspecifi c morphological variation is documented for the goneplacoid crab Tehuacana americana. Costacopluma texana is the second Paleogene species of that genus in southeastern USA, and because of its abundance represents the most completely described species for the genus. This fauna represents one of the most diverse assemblages of Paleocene crustacean decapods. Key words: Paleocene, Crustacea, Mexia, Texas. RESUMEN Se describen catorce especies de crustáceos decápodos colectados en una sola localidad cercana a Mexia, Texas, en donde afl oran sedimentos del Miembro Arcilloso Mexia de la Formación Wills Point. Las especies están representadas por Hoploparia sp., Linuparus wilcoxensis Rathbun, 1935, un paguroideo indeterminado, Kierionopsis nodosa Davidson, 1966, Pithonoton cardwelli especie nueva, Caloxanthus sp., Macroacaena johnsoni (Rathbun, 1935), combinación nueva, Raninoides bournei (Rathbun, 1928), R. treldenaesensis (Collins y Jakobsen, 2003), Prehepatus sp., Tehuacana americana (Rathbun, 1935) combinación nueva, Costacopluma texana especie nueva, Paraverrucoides alabamensis (Rathbun, 1935) y Viapinnixa mexiaensis especie nueva. Se incluyen detalles morfológicos, no descritos para especies previamente reportadas. Se ofrece una sistemática actualizada, basada en investigaciones recientes. Armstrong, A., Nyborg, T., Bishop, G.A., Ossó-Morales À., Vega, F.J., 2009, Decapod crustaceans from the Paleocene of Central Texas, USA: Revista Mexicana de Ciencias Geológicas, v. 26, núm. 3, p. 745-763. 746 Armstrong et al. Se documenta la variación morfológica intraespecífi ca para el cangrejo goneplacoideo Tehuacana americana. Costacopluma texana es la segunda especie del género en el Paleógeno del sureste de EUA, y debido a la abundancia de ejemplares, es la especie descrita con mayor detalle del género Costacopluma. Esta fauna representa una de las asociaciones más diversas de crustáceos decápodos del Paleoceno. Palabras clave: Paleoceno, Crustacea, Mexia, Texas. INTRODUCTION decapods herein reported. Biostratigraphy is primarily based on foraminifera with good index foraminifera indicative of Decapod crustaceans were collected from a quarry site a Paleocene age (Kellough, 1959; 1965). about two miles west of Mexia, Texas from the Paleocene Specimens, referred to in this study, are deposited Mexia Clay Member of the Wills Point Formation (Figure at the Texas Natural Science Center, Austin (previously 1). The Wills Point Formation is part of the Paleocene Paleontological Collection of the Bureau of Economic Midway Group of the East Texas Basin (McGowen et al., Geology, University of Texas), under acronym NPL, and 1972; Oldani, 1988; Barnes et al., 1992). Deposition of Museu de Geologia del Seminari de Barcelona, under acro- the Midway Group occurred during the Early Paleocene nym MGSB. Classifi cation follows that of Ng et al. (2008) (Danian) associated with a major transgression following and Števčić (2005). the Cretaceous-Tertiary event. The maximum advance of this transgression is represented by the outer neritic deposits of the Mexia Clay Member (Kellough, 1965; Oldani, 1988). SYSTEMATIC PALEONTOLOGY The Midway Group is the lowermost stratigraphic Tertiary unit within the East Texas Basin, separated from the underly- Order Decapoda Latreille, 1802 ing Gulfi an Cretaceous Navarro Group by a biostratigraphic Infraorder Astacidea Latreille, 1802 and depositional hiatus. The Mexia Clay Member is con- Superfamily Nephropoidea Dana, 1852 formably overlain by the Paleocene-Eocene Wilcox Group Family Nephropidae Dana, 1852 (Oldani, 1988). In east Texas, the Midway Group consists Genus Hoploparia McCoy, 1849 of two formations (Kincaid and Wills Point formations), each with three members (Oldani, 1988) (Figure 2). The Type species. Astacus longimanus Sowerby, 1826, by sub- thickness of the Midway Group, within the East Texas Basin, sequent designation of Rathbun, 1926. varies between 120 and 550 m; it is more than 300 m at the border of Limestone and Navarro counties, near the town of Hoploparia sp. Tehuacana (Figure 1), and thins to about 120 m at the study Figures 3.1, 3.2 quarry (Oldani, 1988). The Wills Point Formation consists of the basal clays of the Mexia Clay Member, overlain by Description of study material. Cephalothorax small, the silty shales of the Kerens Member, which grade into the elongated, surface covered by small scale-like tubercles, interbedded sands and shales of the Solomon Creek Member much fi ner toward ventral and posterior margins; posterior (Oldani, 1988). Like the Midway Group, the thickness of margin curved, rimmed; postcervical groove deep, slightly the Wills Point Formation varies between 60 and 300 m sinuous; cervical groove one-third the length of postcervi- (Oldani, 1988). In Navarro County it averages 150 to 200 cal groove. m (Oldani, 1988). The Mexia Clay Member consists of dark Material examined. One incomplete cephalothorax and gray, thinly laminated clay shale, bearing pyrite nodules, one fragment of a cheliped, hypotypes NPL31148 and limonite, siderite and calcareous concretions (Kellough, NPL31149. 1965; Oldani, 1988). The Mexia Clay Member averages Measurements (in mm). Hypotype NPL31148 cephalotho- 50 meters making up the bottom quarter of the Wills Point rax length = about 26.0, height = 15.9; hypotype NPL31149 Formation (Kellough, 1965). partial palm length = >21.6, height = >22.0. Abundant decapods, horn corals, scaphopods, pelecy- Discussion. The carapace fragment differs from Hoploparia pods, gastropods, nautiloids, foraminifera, fi sh teeth, and johnsoni Rathbun, 1935 from the middle Eocene of Alabama, some shark teeth occur within the lower part of the Mexia in bearing bigger tubercles directed forward. H. buntingi Clay Member (Kellough, 1965), 6-10 meters above the (Feldmann and Holland, 1971) from the Paleocene of North Tehuacana Limestone. At the study quarry, the basal Mexia Dakota is larger and has a straight postcervical groove. Clay Member is represented by a series of siltstone/claystone H. klebsi Noetling, 1885 from the Paleogene of northern lenses with abundant yellow, brown, and black calcareous Germany is larger and shows coarser tubercles (see also nodules that preserve abundant fossils, including the fossil Förster and Mundlos, 1982). The specimens are too incom- Decapod crustaceans from the Paleocene of Central Texas, USA 747 Discussion. Carapace fragment most resembles the cara- pace features as seen in Linuparus wilcoxensis from the Lower Eocene of Wilcox, Alabama. Compared with the type specimen figured by Rathbun (1935, fig. 16.13), the specimen from Mexia is smaller and ridges of the gastric area define a lanceolate shape, instead of being subparallel. A more complete specimen reported by Vega et al. (2007, fig. 4.9) from the Paleocene of Coahuila, northeastern Mexico, show a lanceolate shape for the ridges of the gastric area. More complete specimens are necessary to define a specific affinity. Infraorder Anomura MacLeay, 1838 Superfamily Paguroidea Latreille, 1802 Paguroidea, gen. and sp. indet. Figures 3.4, 3.5 Figure 1. Location map of fossil locality east of Mexia, Limestone County, Texas. Description of study material. Right palm small, rectan- gular, one-third longer than high, narrow at junction with plete to confirm species affinity. carpus; outer surface convex, covered by coarse punctae; Although most species of Hoploparia are docu- inner surface flat, covered with aerolations that become mented from Cretaceous deposit, it survived in Paleogene scarce toward lower margin; fixed finger incomplete, tropical and subtropical areas. According to Schweitzer and slightly curved inwards, outer surface covered by strong Feldmann (2005), the nearly cosmopolitan distribution, tubercles, inner surface with punctae. Occlusal surface particularly in higher latitudes during Cretaceous times, covered by punctae. was an important fact to buffer the effects of the K/P event Material examined.