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, {rfeldman@kent.edu), 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 TERRESTRIAL Lower Permian rocks of North-Central Texas are a well-known source of vertebrate remains, but few non- marine invertebrates have been reported from this region. How- ever pygocephalomorph crustaceans, which are otherwise rare in the Permian (Brooks, 1962), have been found in abundance at several localities in the Petrolia Formation (Wichita Group, Leon- ardian Series) of Baylor and Archer counties (Fig. 1). These in- vertebrates are accompanied by plants and aquatic to terrestrial vertebrates, all preserved in a distinctive, recurring depositional setting. The purpose of this work is to describe two new pygo- cephalomorph crustacean species from the Petrolia Formation, re- cord the associated biota, describe the geologic context of these deposits, and consider the paleoenvironmental and evolutionary significance of the biota. A single pygocephalomorph, Mamayocaris jepseni Brooks, 1962, was described previously from the Lower Permian of Texas (Brooks, 1962). That specimen was from the middle Clear Fork Group of Taylor County, approximately 180 km south-southwest of our field area. Olson and Mead (1982) described the geology and diverse vertebrate assemblage of this deposit, and Mamay (1976) summarized associated plant remains. GEOLOGY The Petrolia Formation of the Wichita Group was established by Hentz and Brown (1987) for continental rocks that are ap- proximate equivalents of the Elm Creek Formation and undivided Jagger Bend and Valera formations of the marine Albany Group (Hentz, 1988). Within our field area, the Petrolia Formation is identical to the more widely known but now obsolete Belle Plains Formation as mapped in detail by Romer (1974). The Petrolia Formation is regarded as early Leonardian in age mainly on the basis of ammonoids that occur in limestones at the base of the formation (Bose, 1917; Kemp, 1962). In a study of Late Paleozoic floral zones. Read and Mamay (1964) placed the Leonard-Wolf- camp boundary in North-Central Texas at the base of the Belle Plains Formation on the basis of the first occurrence of gigantop- terid remains. The sediments of the Petrolia Formation were deposited in a broad coastal plain along the landward margin of the Eastern Shelf of the Midland Basin. The approximately 130 m thick, mud- stone-dominated Petrolia Formation is bounded by thin (<1 m) marine carbonates and consists mainly of paleosol sequences that include pedogenic carbonates. Sandstones are quartzose and re- stricted to fine to very-fine grained, high-sinuosity, suspended- load channel deposits and to minor tabular deposits of limited lateral extent (Hentz, 1988). Seven pygocephalomorph-bearing localities were sampled in the Petrolia Formation of the Archer-Baylor county-line area (Fig. 1). These occur in local mudstone-dominated, channel-form de- posits that are underlain by thin (<12 cm) channel-lag conglom- erates. Localities 4, 5, and possibly 6, may represent coeval de- posits, but their lateral continuity cannot be demonstrated. At all localities other than locality 3, numerous pygocephalomorphs are preserved in thin (<2 cm), resistant, lenticular to nodular beds of nonbedded, light- to dark-gray limestone that weathers to white, buff, or fight brown. The upper surface of each bed is marked locally by small-scale cracks that may reflect synaeresis, and the limestone breaks with a conchoidal fracture. In thin section, the limestone is a pelmicrite that consists mainly of irregularly round- ed grains that probably represent invertebrate fecal pellets. At localities 2 and 3, a few pygocephalomorphs were collected from finely laminated, gray to dark gray claystone units approximately 15 cm thick. Similar claystones found in association with the fos- siliferous limestones at the other five localities did not yield py- gocephalomorphs. At locality 2, the most complete exposure (Fig. 2), the lime- stone extends laterally for approximately 30 m and is distinctively U-shaped in cross section. The fossiliferous limestone occurs within the lowermost storey of a repetitive, fluvial channel se- quence. This storey begins with a basal conglomerate that scoured into red mudstones containing paleosol features. The conglom- erate is overlain by a claystone-dominated fossiliferous interval, which contains spirorbid worm tubes, abundant conchostracans, plant debris, fusain, and a pygocephalomorph-bearing limestone. A second fossiliferous claystone interval coarsens upward to non- 486 HOTTON ET AL?PERMIAN CRUSTACEA FROM TEXAS 487 )^ cc e^ cc o < DC HI O _l o CL LL 3 DC o DC 5 O LL ^ CL X o LL ? 2 LU Q. |S O DC DQ O 'III 7- 4-6 3- 2- 1- I i I i Pbb J 10m ,^. ^'-^9^^ ?^ A^^ ov ^^ ?3 33?40'- 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- ward from very fine grained sandstone to siltstone; it contains lateral-accretion bedding, and is nonfossiliferous. The capping third storey is a fine-grained, medium-scale trough-crossbedded, rippled sandstone that contains pecopterid impressions and small tetrapod trackways. Many or all of the sedimentologic characteristics of the lower storey at locality 2 are found at the other pygocephalomorph de- posits. Such organic-rich, claystone-dominated channel fills are interpreted as floodplain lakes that originated as cutoffs in high- sinuosity channel belts. Although deposits such as these comprise a minor fraction of the continental Lower Permian of North-Cen- tral Texas, they are the primary source of plant, invertebrate, and vertebrate remains in the region (Hook et al., 1989; DiMichele and Hook, 1992; Chaney et al., 1993, 1997; DiMichele et al., 1997). SYSTEMATIC PALEONTOLOGY Class MALACOSTRACA Latreille, 1802 Order PYGOCEPHALOMORPHA Beurlen, 1930 Family PYGOCEPHALIDAE Brooks, 1962 Discussion.?Transverse grooves characterize most of the py- gocephalomorphs. Within this order, the Tealliocaridae typically have longitudinal carinae in the branchial region and tend to lack anterolateral spines, and the Notocaridae are crab-like in outline with a nearly equidimensional carapace and an abdomen that is carried beneath the carapace. The Pygocephalidae, to which the Petrolia Formation specimens are referred, possess a broadened carapace with anterolateral spines and lacking longitudinal cari- nae. Recently, Taylor et al. (1998) described some new pygoce- phalomorphs from China and performed a cladistic analysis of 488 JOURNAL OF PALEONTOLOGY, V. 76, NO. 3, 2002 y STOREY I PECOPTERID IMPRESSIONS, TETRAPOD TRACKWAY 2''? STOREY 1" STOREY PYCOCEPHALOMORPHS, CONCHOSTRACANS, PALAEONISCOIDS ?- PYCOCEPHALOMORPHS, CONCHOSTRACANS, CONIFERS CONCHOSTRACANS, PLANT DEBRIS SPIRORBIDS CONCHOSTRACANS, PLANT DEBRIS, FUSAIN CONCHOSTRACANS, SPIRORBIDS, PLANT DEBRIS A PALEOSOL TROUGH CROSSBEDS LATERAL ACCRETION RIPPLE LAMINATION qXJC>-^ FOSSILIFEROUS LIMESTONE S i \ PEDOCENIC CARBONATE PEBBLES PEDOCENIC SLICKENSIDES FIGURE 2?Measured section of locality 2. Left vertical scale is in meters. the pygocephalomorphs and lophogastrids. Their results generally supported the unity of the Pygocephalidae as defined by Brooks (1962). Genus MAMAYOCARIS Brooks, 1962 Type species.?Mamayocaris jepseni Brooks, 1962, p. 189, text pi. 5, pi. 44, figs. 1-3, pi. 45, fig. 5. Diagnosis.?Pygocephalid with carapace lacking hepatic spines, dorsal carinae indistinct; short anterolateral spines fol- lowed by variable number of branchiostegal serrations; cervical groove directed posteriorly; telson subtriangular; females with seminal receptacles (Brooks, 1969; Schram, 1974). Discussion.?The genus Mamayocaris Brooks, 1962, seems to be the most reasonable placement for this material. Mamayocaris is in many regards similar to Pygocephalus, but the descriptions 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 small caudal furcae situated in the distalmost of two abrupt mar- 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 prox- imal marginal step. As noted above, a single specimen referred to this genus was reported from the middle Clear Fork Group, Leonardian Series, Lower Permian, of Taylor County (Brooks, 1962). He also noted a collection of 168 paratypes from the roughly correlative Opeche Shale of Pennington County, South Dakota. The South Dakota specimens are preserved as compressed, articulated individuals and do not appear to be exuviae. Attempts to recollect this site have been unsuccessful. MAMAYOCARIS SERENDIPITOUS new species Figure 3 Diagnosis.?Generally large Mamayocaris, with convex-for- ward cervical groove; strongly developed axial convergence; uro- pod base with deeply concave distal margin, posterolateral mar- gins of telson with step-like taper; large furcae that extend beyond the tip of the telson spine, and lacking axial projection of the telson over the spine. Description.?Carapace small, longer than wide, domed trans- versely in axial region; flairing anterolateral and lateral margins. Frontal margin gently convex forward. Anterolateral corner pro- duced into acute spine, anterolateral margin with fine, forward- directed branchiostegal spines. Lateral margins gently arcuate, ra- dius of curvature increasing posteriorly. Posteroventral corner rounded tightly, curving into concave posterior margin. Marginal furrow extends from anterolateral corner around perimeter of car- apace defining a broad ventral ridge, not well expressed on mold of interior, and narrow posterior ridge. Dorsal midline occupied anteriorly by prominent medial crest, becoming narrower poste- riorly, disappearing at cervical groove or continuing faintly nearly to posterior border; pronounced anterior definition of medial crest may be base of falciform rostrum. Gastric spines well defined, directed anteriorly, situated on broadly inflated gastric regions; remainder of carapace unadorned. Cervical groove well devel- oped, originates at level of gastric spines, curves around lateral margin of gastric regions and continues as convex-forward arc to dorsal midline; intersects midline at approximately 20 degree an- gle at distance about two-fifths total carapace length from front margin. Antennules with at least two basal elements and two fla- gella. Antennae with at least four basal elements, a single long flagellum, and well-developed, acuminate antennal scale extend- ing beyond distal-most basal element. Thoracic somites well differentiated, covered completely by carapace but no dorsal fusion with carapace apparent. Somites VII-XIII bear equal-sized, narrow, biramous pereiopods with pro- topods directed anterolaterally. Abdomen of six somites and telson, broadly arched. First so- mite shorter than others. Somites 2-6 subequal in length; width of somites 2-5 subequal, somite 6 slightly narrower. Terga smooth, with pair of slit-like setal pits on either side of midline FIGURE 3?Mamayocaris serendipitous n. sp. 1, Partial remains of the cephalic region with antennules, antennae, and antennal scale, paratype (USNM 486026; 2, holotype (USNM 486025), nearly complete cephalothorax; 3, partially exposed cephalothorax with well-preserved gastric spines and anterolateral spines, paratype (USNM 486027); 4, partial thoracic and abdominal somites with proximal elements of pereiopods, paratype (USNM 486028); J, portion of abdomen, somites 3-6, and well-developed pleura, on same sample Figure 3.1, paratype (USNM 486026); 6, latex cast of complete abdomen and telson with numerous fecal pellets surrounding, paratype (USNM 486029). Bar scale equals 1 cm. HOTTON ET AL?PERMIAN CRUSTACEA FROM TEXAS 489 490 JOURNAL OF PALEONTOLOGY, V. 76, NO. 3, 2002 TABLE 1?Measurements (in mm) of Mamayocuris serendipitous n. sp. and Paulocaris schrami n. sp. from the Petrolia Formation. Abbreviations: Cl, carapace lengtli; Cw, carapace widtli; Abl, length of abdominal somites; Tel, axial length of telson, including spine. Taxon USNM no. Cl Cw Abl Tel M. serendipitous 486025 15.8 11.8 ? ? 486030 >10.2 8.9 ? ? 486027 >10.7 10.2 ? ? 486031 7.6 6.2 ? ? 486032 16.1 12.4 ? ? 486029 ? ? 7.5 3.5 486033 ? ? >4.2 2.0 486034 ? ? 10.6 ? P. schrami 486035 7.1 7.0 ? ? 486036 6.1 6.1 ? ? 486037 11.5 12.2 ? ? and between pleuron and tergum on somites 3-6; pleura 2-5 small, triangular, terminate in sharp apex directed toward poste- rior; pleuron of somite 6 more bluntly pointed, posterior margin of pleuron 6 with convex curve toward telson. Pleopods of somite 6 expanded into well-developed uropods with smooth, flattened endite and exite, both apparently with smoothly rounded margins. Telson smooth, straight-sided to about midlength, slightly wider posteriorly, becoming narrow posteriorly in two abrupt steps; an- teriormost step most prominent, serving as point of attachment of flabellate furcal lobes; posteriomost step less well developed; pos- terior margin of telson a straight-sided concavity, in which is sit- uated a prominent, elongate, articulating telson spine. At least three pairs of slit-like setal pits situated along margin of telson. Etymology.?^The trivial name alludes to the fortuitous discov- ery of the first specimen, a very small chip of material in a very small deposit. Types.?^Holotype, USNM 486025, a nearly complete cepha- lothorax (Fig. 3.2) from locality 5; paratypes, USNM 486026- 486034 from locality 2 and locality 5 (Fig. 1). Measurements.?Measurements are given in Table 1. The total length of the specimen (Cl) may be less than the true carapace length because the anterior margin is not exposed on any speci- men. In most specimens, the carapace width (Cw) cannot be taken as the true width of the carapace because of post-mortem crush- ing. "Abl" is the length, measured along the axis, of abdominal somites 1-6; if fewer somites are preserved, the somites measured are indicated in parentheses. The axial length of the telson (Tel) includes the telson spine. Occurrence.?Specimens were collected from the Petrolia For- mation, Wichita Group, Leonardian Series, Lower Permian, at lo- calities 1 (UTM [Universal Transverse Mercator coordinate] 14SNN04782132, Dundee 7.5' topographic quadrangle. Archer County), 2 (UTM 14SNN01951984, Dundee SW 7.5' topographic quadrangle, Baylor County), 3 (UTM 14SNN08542603, Dundee 7.5', Archer County), 4 (UTM 14SNN00562121, Dundee 7.5' to- pographic quadrangle, Baylor County), 5 (UTM 14SNN00542128, Dundee 7.5' topographic quadrangle, Baylor County), 6 (UTM 14SNN00142115, Dundee 7.5' topographic quadrangle, Baylor County), and 7 (UTM 14SNN03843164, Dundee 7.5' topographic quadrangle, Baylor County). Discussion.?The morphologic terminology used herein is con- sistent with that of Brooks (1969). Although many of the terms refer to biological structures, this usage does not imply direct analogy with so-named structures in higher malacostracans be- cause too little is known about Mamayocaris serendipitous. Two species previously have been assigned to this genus. The type species, Mamayocaris jepseni Brooks, 1962, tends to be smaller than M. serendipitous and exhibits several features in the region of the telson that serve to exclude the new species. The basis of the uropod on the type species has a convex distal margin whereas that of Mamayocaris serendipitous is scalloped. Similar- ly, the lateral margins of the telson of the type species is smoothly tapered posterior to the articulation of the furcae whereas that margin on the new species narrows in a step-like manner Ma- mayocaris jepseni has been reported from the Leonardian Series (Permian) in the middle Clear Fork Group near Lawn, Texas, and from correlative rocks of the Opeche Formation near Rapid City, South Dakota (Brooks, 1962, p. 192-193). The specimens were preserved as compressed, articulated individuals, making it diffi- cult to compare aspects of convexity of the carapace of the two species. Schram (1974) named Mamayocaris jaskoskii based upon nu- merous dorsoventrally compressed, articulated specimens from the Middle Pennsylvanian Essex fauna of Mazon Creek, Illinois. This extended the range of the genus downward into the Penn- sylvanian. Several features of this animal distinguish it from M. serendipitous. The cervical groove on M. jaskoskii is nearly straight across the carapace laterally and becomes very faint and nearly parallel to the midline axially. That groove is convex for- ward laterally and convergent and strongly developed axially on M. serendipitous. The telson on M. jaskoskii bears caudal furcae that do not extend beyond the telson spine, and the telson spine is overlain axially by a projection of the telson. By contrast, the telson of Mamayocaris serendipitous bears furcae that extend well beyond the tip of the telson spine, and the axial termination of the telson does not exhibit a projection over the spine. Over 100 specimens of M. serendipitous exhibit part or all of the dorsal carapace, the abdomen and telson, or the thoracic ster- nites with some attached pereiopods. Many of the specimens, par- ticularly the carapaces, have been crushed and distorted. Cuticle is present in only one specimen (USNM 486035, the obverse side of which preserves the holotype of a second new species de- scribed below), which is dorsoventrally compressed so that the lateral margins have been rotated into the plane of the dorsum. Because the integument appears to have been uniformly very thin and fragile, interior molds of the cuticle may be regarded as ac- curate expressions of surface morphology. Comparison of the fea- tures exhibited on these interior molds with those on the surface of the carapace indicates that surficial detail is expressed on the interior of the carapace. Many of the interior molds exhibit evi- dence of post-mortem distortion prior to preservation and solution of the carapace. Although remains of the cephalothorax, abdomen, and telson are preserved, none of the specimens is articulated. Thus, it is probable that most of the specimens represent molted individuals. There is no evidence that corpses are present in the sample. Although Schram (1974) reconstructed Mamayocaris jaskoskii as having a cylindrical carapace in cross section, all specimens of M. serendipitous indicate that the anterolateral margins of the carapace were turned outward into the horizontal plane. Exami- nation of the surfaces of the specimens gives no indication that they have been broken or distorted. This observation confirms the configuration of the carapace in the type species as described by Brooks (1962). Family NOTOCARIDIDAE Brooks, 1962 Discussion.?^Brooks (1962) included Paulocaris Clarke, 1920, and Notocaris Broom, 1931, in this family. The cladistic analysis of Taylor et al. (1998) does not support this arrangement; how- ever, this part of their tree is poorly resolved. They noted (p. 830) that it would require better understanding of the southern hemi- sphere forms to clarify these relationships. Because a restudy of HOTTON ET AL?PERMIAN CRUSTACEA FROM TEXAS 491 FIGURE 4?Paulocaris schrami n. sp. 1, Cephalothorax (holotype, USNM 486035); 2, cephalothorax (paratype, USNM 486036). Bar scale equals 1 cm. these taxa is beyond the scope of the present work, the classifi- cation of Brooks (1962) is used to avoid unnecessary confusion. Genus PAULOCARIS Clarke, 1920 Type species.?Paulocaris pachoecoi Clarke, 1920, p. 135, pi. 2, figs. 1-12, pi. 3, figs. 1-8. Diagnosis.?Notocaridid with carapace as wide as long, prom- inent dorsal keel (Brooks, 1969). Discussion.?The type species is represented by several spec- imens from the Irati Shale, Lower Permian, of Guarehy, Sao Pau- lo, Brazil. The most useful morphological features serving to associate the specimen with Paulocaris include the length/width relationship which mimics that of the brachyurans, the vaulted dorsal surface, and development of the medial ridge and cervical groove. Al- though there is no evidence that the ventrolateral margins are recurved in the Texas specimens, the other characters so closely resemble those of Paulocaris that placement is relatively certain. Pinto and Adami-Rodrigues (1996) reviewed the pygocephal- omorps from Brazil and South Africa and concluded that three species referred to Paulocaris, and all from the Irati Formation, were not referable to that genus. Both Paulocaris clarkei Beurlen, 1953, and P. marianoi Beurlen, 1953, were said to lack the key characters of the genus. We concur. The third taxon, Paulocaris brasiliensis Beurlen, 1934, noted by Mezzalira (1971) was ap- parently a reference to a specimen that is neither Pygaspis bras- iliensis Beurlen, 1934, nor a species of Paulocaris (Pinto and Adami-Rodrigues, 1996, p. 46, pi. 3, fig.7). Thus, the genus Pau- locaris includes only the type species and the new species de- scribed herein. PAULOCARIS SCHRAMI new species Figure 4 Diagnosis.?Paulocaris with lateral margins becoming more convex anteriorly, strongly developed medial ridge, and lacking thickening of posterior carapace margin. Description.?Carapace small, quadrate, maximum width slightly greater than length; vaulted with highest region along midline in posterior third of carapace. Lateral margins gently con- vex, increasing in convexity near the posterolateral corners. Pos- terior margin smoothly and deeply concave. Anterolateral margins inclined obliquely toward frontal region, which is broken. Dorsal surface divisible into four regions by medial ridge and cervical groove. Medial ridge narrow and prominent in cephalic region, becoming less well defined near junction with cervical groove, terminating just in advance of posterior margin. Cervical groove deeply impressed, originating at midline near center of carapace, intercepting midline at approximately 35 degree angle, curving anterolaterally to point about three-fifths the distance from mid- line to lateral border, where it curves abruptly toward the anterior. Gastric region with anteriorly directed, nodose ridge. Branchial regions generally smooth, lateral margins of branchial regions folded tightly ventrally. Abdominal region and appendages unknown. Etymology.?^In honor of Dr. Frederick Schram, who has con- tributed much to our understanding of North American malacos- tracans. Types.?^Holotype, USNM 486035, partial cephalothorax (Fig. 4.1) from locality 5 (UTM 14SNN00542128), Dundee 7.5' to- pographic quadrangle; paratypes, USNM 486036 (Fig. 4.2) and 486037, partial cephalothoraxes from locality 2 (UTM 14SNN01951984), Dundee SW 7.5' topographic quadrangle. Measurements.?Measurements are given in Table 1. Carapace length (Cl) is less than the total length because the anterior of the carapace is not well preserved in any specimen. Occurrence.?Specimens were collected from the Petrolia Formation, Wichita Group, Leonardian Series, Lower Permian at locality 2 (UTM 14SNN01951984, Dundee SW 7.5' topo- graphic quadrangle, Baylor County), and locality 5 (UTM 14SNN00542128, Dundee 7.5' topographic quadrangle, Baylor County). Discussion.?The material referred to this taxon consists of one inflated and one crushed mold of the interior of the dorsal cara- pace and several fragmentary specimens of the cephalothorax. Although the anterior margin is broken away or absent, the gen- eral outline and the convexity of the carapace are clearly shown on the inflated specimen. There is some evidence of distortion, particularly on the left lateral region, but the lateral compression of the region can clearly be identified when compared to the un- distorted right side of the specimen. Because none of the speci- mens is articulated and the remains are limited to cephalothoraxes, it is probable that the specimens are from molted individuals. Paulocaris has not been reported previously from deposits out- side Brazil. Although the Texas specimens are few and incom- plete, major features of the carapace are shared with those of the type species, P. pachoecoi. These include the general shape, vaulted dorsal surface, and development of the medial ridge and cervical groove. The recurved ventrolateral margins of P. schrami are not evident in P. pachoecoi. Paulocaris pachoecoi is larger than P. schrami and has a more uniformly convex lateral margin and a smoother dorsal surface. The medial ridge of Paulocaris pachoecoi tends to be more strongly developed than that of P. schrami. The posterior margin of the type species is reported to be thickened and, occasionally, dissociated from the carapace (Clarke, 1920, pi. 3, figs. 6 and 7). No similar thickening is observed on Paulocaris schrami. There is little doubt that the two forms are distinct from one another but that, given the present classification of Paleozoic malacostra- cans, they are best assigned to the same genus. ASSOCIATED BIOTA Plants.?^Plant assemblages of low diversity, ranging from four to 10 species, are preserved with the pygocephalomorph remains. Although conifer remains are common as three-dimensional im- pressions on the surface of the fossiliferous limestones, preser- vation is much better and floral diversity is greater in associated claystones that were excavated for plant collections at localities 2, 3, and 7 (Fig. 1, Table 2). At these localities, plant fossils are fragmentary and include both vegetative remains and reproductive organs. Conifers are the most commonly encountered fossils, with 492 JOURNAL OF PALEONTOLOGY, V. 76, NO. 3, 2002 TABLE 2?Plant taxa in rank-order abundance found in association witli py- gocephalomorplis of the Petrolia Formation at localities 2, 3, and 7; USNM collection numbers, total number of specimens per locality, and number of specimens per identification indicated. Locality 2 (USNM 40032, 40033), 161 specimens total Ernestiodendron sp. Pecopteris cf. P. cyathea Calamite stem Unidentified flabellate leaf Locality 3 (USNM 40020), 264 specimens total Ernestiodendron sp. Walchia cf. schneiderii Autunia conferta Odontopteris sp. Unidentified flabellate leaves Taeniopteris sp. Compsopteris sp. Gigantopteridium americanum Walchia pinniformis Calamite stem Locality 7 (USNM 40626, 40627), 70 specimens total Unidentified conifer with long, broad, lax leaves Ernestiodendron sp. cf. Culmitschia speciosa Walchia cf. W. schneiderii Calamite stems Sphenopteridium sp. Gigantopteridium americanum Taeniopteris sp. Walchia hypnoides 65 2 1 1 106 67 46 8 5 2 2 2 1 1 12 11 11 only minor amounts of non-coniferous foliage. Fish scales or whole fish, spirorbids, myalinid pelecypods, and conchostracans are intermixed with plant remains at all three sites. Minor amounts of fusain also occur in association with the fossils. Of the conifers, Ernestiodendron sp. occurs at all three sites and was the most common element at two of the localities. Other conifers include forms with long, lax leaves, one resembling Wal- chia schniederii; an unattributed, rare specimen similar in form to W. pinniformis and W. hypnoides; and a conifer with foliage resembling Culmitschia speciosa. Calamite stems are rare at all three sites. Other plants that occur at two sites include Taeniop- teris sp. and Gigantopteridium americanum. Sphenopteridium sp., Autunia conferta, Odontopteris sp., Compsopteris sp., and Pecop- teris sp., and two kinds of unidentifiable flabellate foliage occur at one site only. A diversity of seeds, cone scales, and bracts occurred in each deposit, often in abundance. Invertebrates.?Spirorbid worm tubes are preserved with the pygocephalomorphs in gray claystones at localities 2 and 3 (Fig. 1). Conchostracans occur in association with the pygocephalo- morphs at localities 1, 2, 3, 6, and 7. A few poorly preserved conchostracans are found as impressions on the limestone bed. By contrast, abundant, well-preserved conchostracans occur in dark gray claystones that are associated with fossiliferous lime- stone beds at localities 2, 3, and 7, which were excavated for plant remains. Myalinid pelecypods also occur at these three lo- calities. Vertebrates.?Fish, amphibian, and reptile remains found as- sociated with the pygocephalomorph-bearing beds reflect part of the diversity of vertebrate assemblages known from the Petrolia Formation (Table 3). Most of the specimens from localities 2, 3, and 7 were found in fossil-plant excavations of dark gray clay- stones associated with the fossiliferous limestone. The remaining localities were not excavated and their collections represent sur- face samples. The acanthodian Acanthodes, the palaeoniscoid Platysomus [= TABLE 3?Vertebrate taxa found in association with pygocephalomorphs of the Petrolia Formation. Numbers in parentheses indicate localities shown on Figure 1. Chondrichtyes Undetermined hybodont spine (1) Acanthodii Acanthodes (3) Osteichthyes Platysomus sp. (2) Sagenodus sp. (1) Spermatodus pustulosus (1) Undetermined small fusiform palaeoniscoids (1, 3, 7) Amphibia Diplocaulus sp. (1) Eryops megacephalus (1) Trimerorhachis insignis (1, 4, 5) Reptilia Ctenospondylus sp. (1) Dimetrodon sp. (1, 2, 5) Schaefferichthys; see Zidek, 1992], and the fusiform palaeonis- coids are represented by small (<5 cm), nearly complete individ- uals. The hybodont spine, the lungfish Sagenodus, the coelacanth Spermatodus, the amphibians Diplocaulus and Eryops, and the pelycosaur Ctenospondylus are recorded by isolated elements found at the surface. Remains of the perennibranchiate amphibian Trimerorhachis and the pelycosaur Dimetrodon are preserved in the pygocephalomorph-bearing limestone at locality 5. Two aspects of this vertebrate record are anomalous. First, the complete absence of xenacanth sharks is unusual because their remains, particularly teeth, are common to abundant in other channel-fill deposits within the Petrolia Formation in the field area. Second, the occurrence of the amphibian Diplocaulus is un- expected; although this genus is the most common amphibian found in the overlying Clear Fork Group, it is almost unknown in the Wichita Group (Hook, 1989). The identification of this taxon at locality 1 is based upon a highly distinctive tabular horn and diagnostic trunk vertebrae. PALEOENVIRONMENTAL CONDITIONS The sedimentologic aspects of the pygocephalomorph-bearing beds of the Petrolia Formation indicate that these deposits formed in standing-water conditions within abandoned segments of high- sinuosity, suspended-load, fluvial channels. The fossiliferous limestones and dark gray claystones record a range of paleoen- vironmental conditions. Abundant invertebrate fecal pellets pre- served in the limestone indicate that the cutoff lakes teemed with invertebrate life. The three-dimensional preservation of pellets and fossil remains in the limestones suggests that the carbonate muds were rapidly lithified, and that the limestones record mini- mal time averaging. In contrast, fossils in the claystones are flat- tened into two dimensions and occur in several bedding planes. Some bedding surfaces contain abundant conchostracan carapac- es, which suggests intermittent anoxic conditions. A lack of in situ pedogenic features in the fossiliferous first storey at locality 2 (Fig. 2) indicates that water depth in the small cutoff lake precluded plant growth. The actual channel depth can- not be determined because lateral-accretion beds of the second storey scoured down into the mudstones of the lower storey. Sim- ilar conditions existed at localities 4, 5, and 7, thus indicating the recurrence of small, permanent (for periods >100 yr; see Olson, 1977; Behrensmeyer and Hook, 1992) floodplain lakes through an interval of approximately 100 m. Previous occurrences of Mamayocaris and Paulocaris have HOTTON ET AL?PERMIAN CRUSTACEA FROM TEXAS 493 been attributed variously to marine, brackish, or freshwater set- tings without much site-specific sedimentologic data. Brooks (1962) strongly favored a lagoonal environment for M. jepseni and Paulocaris and regarded them as euryhaline. Schram (1981) considered Paulocaris to be a brackish to freshwater organism and Mamayocaris to be nearshore marine. Mamayocaris jaskoskii occurs in the Late Carboniferous marine to brackish water Essex assemblage of Mazon Creek, Illinois. The Essex assemblage, however, does include well-preserved fully terrestrial remains, no- tably the dissorophoid amphibian Amphibamus. In our collections, several fish taxa found in association with the pygocephalomorphs also are known from brackish as well as freshwater deposits. However, well-preserved terrestrial plants, amphibious to terres- trial vertebrate remains, and sedimentologic and stratigraphic data indicate that the Petrolia occurrences represent freshwater assem- blages. EVOLUTIONARY AND PALEOECOLOGICAL IMPLICATIONS The biota and paleoenvironment of the pygocephalomorph- bearing deposits in the Petrolia Formation indicate that Lower Permian freshwater invertebrates of North-Central Texas mirror evolutionary and paleoecologic patterns found in coeval plant and vertebrate records. The earliest occurrence of Mamayocaris is in coal-bearing Upper Carboniferous rocks. Nearly all the plants and vertebrates found in association with the pygocephalomorphs in the Petrolia Formation likewise have generic or familial represen- tation in coal-bearing Upper Carboniferous rocks of Texas, the Midcontinent, the Southwest, or the Dunkard Basin of the Ap- palachians. Vertebrate paleontologists have long recognized the relictual nature of assemblages from the Wichita Group (Romer, 1928, 1935; Olson, 1952, 1985), and ongoing paleobotanical stud- ies have identified a similar pattern of Carboniferous-aspect plants in parts of the Wichita Group (DiMichele et al., 1991). In North-Central Texas, the depositional context of these relic- tual plant and animal assemblages invariably is some variety of a local claystone-rich channel fill. These standing-water floodplain lakes probably represent the only preserved record of Early Perm- ian environments that are somewhat comparable edaphically to the more extensive, poorly drained, peat-forming environments of the Late Carboniferous. The Early Permian plant record of the region suggests that the floodplain lakes and surrounding settings were evolutionary refugia during a time of floristic change in better-drained, nearby environments. In this regard, the new crus- tacean data from the Petrolia Formation corroborate the obser- vations of Schram (1981, p. 132), who noted that Early Permian pygocephalomorph genera of his brackish water community, which included Paulocaris, persisted, "even at a time when the Paleozoic crustacean fauna was undergoing some cataclysmic changes." The youngest record of Mamayocaris is from the Clear Fork Formation of Taylor County, Texas; the sedimentologic con- text of this occurrence, which is remarkably similar to that of the fossiliferous claystones reported above, was interpreted as a small lake within a paralic setting by Olson and Mead (1982). CONCLUSIONS The discovery of abundant Early Permian pygocephalomorphs in association with well-preserved plants, conchostracans, fishes, amphibians, and reptiles in recurring claystone-dominated channel fills of the continental Petrolia Formation provide new informa- tion on the paleoenvironmental, paleoclimatic, and paleoecologic setting of the Petrolia Formation. The deposits originated as aban- doned-channel segments that developed into floodplain lakes. These lakes supported a fairly diverse aquatic fauna and were flanked by streamside terrestrial plant and animal communities. Because the pygocephalomorphs, along with nearly all of the ver- tebrate and some of the plant taxa, have direct phylogenetic ties to forms known from coal-bearing Upper Carboniferous rocks, these assemblages are regarded as relictual. Their occurrence in restricted, poorly drained floodplain settings indicates that they were ecologic refugia that resembled the edaphic conditions of earlier Permo-Carboniferous times. ACKNOWLEDGMENTS We are indebted to Cowan and Sons, the Waggoner Estate, and the late Kenneth and David Williams for property access. The late Ruth O. Hotton discovered some of the localities noted in this paper and was an invaluable co-worker in the field. A. D. Lewis and D. S. Chaney assisted in field work. C. E. Schweitzer and W. J. Nelson read the manuscript and provided numerous points of clarification. Thoughtful reviews by F R. Schram and D. S Ber- man substantially improved the final manuscript. 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