J. Paleont.. 76(3), 2002, pp. 486^94 Copyright © 2002, The Paleontological Society 0022-3360/02/0076-486$03.00 CRUSTACEAN-BEARING CONTINENTAL DEPOSITS IN THE PETROLIA FORMATION (LEONARDIAN SERIES, LOWER PERMIAN) OF NORTH-CENTRAL TEXAS NICHOLAS HOTTON, III,'* RODNEY M. FELDMANN,^ ROBERT W. HOOK,^ AND WILLIAM A. DIMICHELEi 'Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, ^Department of Geology, Kent State University, Kent, OH 44242, {[email protected]), and 'Texas Memorial Museum, Vertebrate Paleontology Lab, University of Texas at Austin, Austin 78758-4445, *Deceased, Rodney M. Feldmann is the corresponding author ABSTRACT—Numerous pygocephalomorph crustaceans occur with conchostracans, plants, fishes, amphibians, and amniotes in the Petrolia Formation (Leonardian Series, Lower Permian) of Baylor and Archer counties, Texas. Two pygocephalomorph species are represented; Mamayocaris serendipitous, new species, by hundreds of specimens that appear to be molted exoskeletons, and Paulocaris schrami, new species, by only a few specimens. Mamayocaris has been reported previously from the Lower Permian of Texas and South Dakota and the Upper Carboniferous of Illinois; Paulocaris was previously known only from South America. Associated plant assemblages are dominated by conifers accompanied by other Early Permian and some Late Carboniferous elements. Accompanying vertebrate remains include aquatic to fully terrestrial forms with close taxonomic ties to genera or families recorded in Upper Carbon- iferous deposits. The fossils are preserved in local deposits of thin (<2 cm), lenticular to nodular beds of limestone and thin (<15 cm) intervals of dark-gray claystone. These deposits accumulated in abandoned, standing-water segments of suspended-load fluvial channels. The archaic nature of these plant and animal assemblages supports previous interpretations that the Permian Petrolia Formation contains paleoenvironmentally isolated biotic elements characteristic of the Carboniferous and underscores prior depictions of the assemblages as relictual. INTRODUCTION The sediments of the Petrolia Formation were deposited in a THE TERRESTRIAL Lower Permian rocks of North-Central Texas broad coastal plain along the landward margin of the Eastern are a well-known source of vertebrate remains, but few non- Shelf of the Midland Basin. The approximately 130 m thick, mud- marine invertebrates have been reported from this region. How- stone-dominated Petrolia Formation is bounded by thin (<1 m) ever pygocephalomorph crustaceans, which are otherwise rare in marine carbonates and consists mainly of paleosol sequences that the Permian (Brooks, 1962), have been found in abundance at include pedogenic carbonates. Sandstones are quartzose and re- several localities in the Petrolia Formation (Wichita Group, Leon- stricted to fine to very-fine grained, high-sinuosity, suspended- ardian Series) of Baylor and Archer counties (Fig. 1). These in- load channel deposits and to minor tabular deposits of limited vertebrates are accompanied by plants and aquatic to terrestrial lateral extent (Hentz, 1988). vertebrates, all preserved in a distinctive, recurring depositional Seven pygocephalomorph-bearing localities were sampled in setting. The purpose of this work is to describe two new pygo- the Petrolia Formation of the Archer-Baylor county-line area (Fig. cephalomorph crustacean species from the Petrolia Formation, re- 1). These occur in local mudstone-dominated, channel-form de- cord the associated biota, describe the geologic context of these posits that are underlain by thin (<12 cm) channel-lag conglom- deposits, and consider the paleoenvironmental and evolutionary erates. Localities 4, 5, and possibly 6, may represent coeval de- significance of the biota. posits, but their lateral continuity cannot be demonstrated. At all A single pygocephalomorph, Mamayocaris jepseni Brooks, localities other than locality 3, numerous pygocephalomorphs are 1962, was described previously from the Lower Permian of Texas preserved in thin (<2 cm), resistant, lenticular to nodular beds of (Brooks, 1962). That specimen was from the middle Clear Fork nonbedded, light- to dark-gray limestone that weathers to white, Group of Taylor County, approximately 180 km south-southwest buff, or fight brown. The upper surface of each bed is marked of our field area. Olson and Mead (1982) described the geology locally by small-scale cracks that may reflect synaeresis, and the and diverse vertebrate assemblage of this deposit, and Mamay limestone breaks with a conchoidal fracture. In thin section, the (1976) summarized associated plant remains. limestone is a pelmicrite that consists mainly of irregularly round- ed grains that probably represent invertebrate fecal pellets. At GEOLOGY localities 2 and 3, a few pygocephalomorphs were collected from The Petrolia Formation of the Wichita Group was established finely laminated, gray to dark gray claystone units approximately by Hentz and Brown (1987) for continental rocks that are ap- 15 cm thick. Similar claystones found in association with the fos- proximate equivalents of the Elm Creek Formation and undivided siliferous limestones at the other five localities did not yield py- Jagger Bend and Valera formations of the marine Albany Group gocephalomorphs. (Hentz, 1988). Within our field area, the Petrolia Formation is At locality 2, the most complete exposure (Fig. 2), the lime- identical to the more widely known but now obsolete Belle Plains stone extends laterally for approximately 30 m and is distinctively Formation as mapped in detail by Romer (1974). The Petrolia U-shaped in cross section. The fossiliferous limestone occurs Formation is regarded as early Leonardian in age mainly on the within the lowermost storey of a repetitive, fluvial channel se- basis of ammonoids that occur in limestones at the base of the quence. This storey begins with a basal conglomerate that scoured formation (Bose, 1917; Kemp, 1962). In a study of Late Paleozoic into red mudstones containing paleosol features. The conglom- floral zones. Read and Mamay (1964) placed the Leonard-Wolf- erate is overlain by a claystone-dominated fossiliferous interval, camp boundary in North-Central Texas at the base of the Belle which contains spirorbid worm tubes, abundant conchostracans, Plains Formation on the basis of the first occurrence of gigantop- plant debris, fusain, and a pygocephalomorph-bearing limestone. terid remains. A second fossiliferous claystone interval coarsens upward to non- 486 HOTTON ET AL—PERMIAN CRUSTACEA FROM TEXAS 487 'III Pbb )^ 7- cc e^ cc o 4-6 < DC HI O o_l CL LL 3 DC ,^. ^'-^9^^ o 5 J ^^ DC ov °3 O A^^ LL ?^ CL 33°40'- ^ X o LL § 2 3- 2- LU Q. |S 1- O DC DQ O 10m I i I i FIGURE 1—Lithostratigraphy and location of Lower Permian pygocepiialomorpli deposits in Nortii-Central Texas. 1, Generalized stratigraphic column; Wichita Group formations abbreviated as NF, Nocona Formation, PF, Petrolia Formation, and WRF, Waggoner Ranch Formation; 2, generalized Petrolia Formation stratigraphic column with pygocephalomorph deposits (1-7) indicated; Pbb = Beaverburk Limestone; 3, map of study area; inset shows location within Texas. Geology adapted from Romer (1974) and Hentz and Brown (1987). fossiliferous, gray siltstone. The overlying second storey fines up- SYSTEMATIC PALEONTOLOGY ward from very fine grained sandstone to siltstone; it contains Class MALACOSTRACA Latreille, 1802 lateral-accretion bedding, and is nonfossiliferous. The capping Order PYGOCEPHALOMORPHA Beurlen, 1930 third storey is a fine-grained, medium-scale trough-crossbedded, Family PYGOCEPHALIDAE Brooks, 1962 rippled sandstone that contains pecopterid impressions and small tetrapod trackways. Discussion.—Transverse grooves characterize most of the py- Many or all of the sedimentologic characteristics of the lower gocephalomorphs. Within this order, the Tealliocaridae typically storey at locality 2 are found at the other pygocephalomorph de- have longitudinal carinae in the branchial region and tend to lack posits. Such organic-rich, claystone-dominated channel fills are anterolateral spines, and the Notocaridae are crab-like in outline interpreted as floodplain lakes that originated as cutoffs in high- with a nearly equidimensional carapace and an abdomen that is sinuosity channel belts. Although deposits such as these comprise carried beneath the carapace. The Pygocephalidae, to which the a minor fraction of the continental Lower Permian of North-Cen- Petrolia Formation specimens are referred, possess a broadened tral Texas, they are the primary source of plant, invertebrate, and carapace with anterolateral spines and lacking longitudinal cari- vertebrate remains in the region (Hook et al., 1989; DiMichele nae. and Hook, 1992; Chaney et al., 1993, 1997; DiMichele et al., Recently, Taylor et al. (1998) described some new pygoce- 1997). phalomorphs from China and performed a cladistic analysis of 488 JOURNAL OF PALEONTOLOGY, V. 76, NO. 3, 2002 of Pygocephalus given by Brooks (1962) and Schram (1974) pre- clude placement in that taxon. Pygocephalus possesses a cervical groove that crosses the carapace at a high angle and a telson with y STOREY I small caudal furcae situated in the distalmost of two abrupt mar- PECOPTERID IMPRESSIONS, TETRAPOD TRACKWAY ginal steps. By contrast, the cervical groove of Mamayocaris crosses the midline at a low angle, and the telson bears a single pair of large furcae articulated at the position of the more