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Fossil crocodylians from the Eocene Devil's Graveyard and Canoe Formations, Brewster County, Texas.

Abstract. -- Fragmentary crocodylian remains from the Lower Eocene of west Texas represent at least three taxa; these are the extinct generalized taxon Borealosuchus, the terrestrial Pristichampsus and a derived alligatoroid closely resembling forms given the name Allognathosuchus. These remains are significant, as they preserve the youngest known occurrence of Borealosuchus and may be the southernmost known occurrence of Allognathosuchus.

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Although most vertebrate paleontological work in the Eocene sequence of western Texas has focused on mammals (e.g., Wilson 1967; 1971; 1974; 1977; Wilson & Schiebout 1981; Gustafson 1986; Runkel 1988), nonmammalian tetrapods are known. These include crocodylians, turtles, squamates and lissamphibians (Wilson 1986; Runkel 1988). However, only one nonmammalian vertebrate has been described from the Eocene of this region, Pristichampsus, which may have been a terrestrial rather than semiaquatic predator (Busbey 1986). Material described herein derives from the Devil's Graveyard and Canoe Formations, both of which are exposed in Brewster County, Texas. The Devil's Graveyard Formation is a sequence of fluviolacustrine sandstones and mudstones interbedded with tuffs spanning the Uintan and Chadronian North American Land Mammal Ages (NALMAs; Stevens et al. 1984; Runkel 1988). The Canoe Formation is predominantly sandstone, and is correlative with the lower part of the Devil's Graveyard Formation (Runkel 1988). All localities from which crocodylians have been collected are regarded as early Uintan in age based on associated large mammals (Stevens et al. 1984; Wilson 1977; 1984; 1986; Wilson & Schiebout 1981) and K-Ar dates obtained from underlying and overlying volcanic deposits in the Devil's Graveyard Formation (Henry & McDowell 1986; Schucker & Nelson 1988).

Specimens described by Busbey (1986) and some of those described herein are derived from the Whistler's Squat Local Fauna, which refers to several localities in the Agua Fria region of Brewster County northwest of Big Bend National Park (Wilson 1986). Others are from assemblages and local faunas from elsewhere in Brewster County contemporaneous with Whistler's Squat Local Fauna (Runkel 1988).

The purpose of this report is to briefly describe the crocodylian taxa diagnosable from this unit based on material deposited in the Vertebrate Paleontology Laboratory, Texas Memorial Museum, Austin, Texas (TMM). One, Pristichampsus, has already been described from this area, representing a stratigraphic extension into the Uintan (Busbey 1986). A partial skeleton from the Canoe Formation indicates an extension into the Uintan for Borealosuchus, and a small alligatoroid of uncertain affinities (but closely resembling several blunt-snouted taxa from the North American Tertiary) can also be recognized.

SYSTEMATIC PALEONTOLOGY

EUUSUCHIA Huxley 1875

CROCODYLIA Gmelin 1789

Borealosuchus Brochu 1997

Referred material. -- TMM 40146-6, associated vertebral centra, osteoderms, phalanges, teeth.

Occurrence. -- Canoe Formation, Canoe A Local Fauna (Runkel 1988), White Amphitheater, Big Bend National Park, Brewster County, Texas.

Discussion. -- The material catalogued under TMM 40146-6 was found in association and probably represents a single individual. The vertebral column is represented by procoelous mid-dorsal centra from which the neural arches have completely separated.

Among the osteoderms (Figure 1f) is the anterior ossification from a bipartite ventral element. Bipartite ventral osteoderms such as these are known in three separate crocodylian lineages: extant caimans, the European alligatoroid Diplocynodon, and Borealosuchus, a North American lineage including several taxa formerly classified as Leidyosuchus by Huxley (1859), Buscalioni et al. (1992) and Brochu (1997). By itself, the isolated ventral element does not allow one to distinguish among these three lineages, but the remaining osteoderms, which include dorsal elements (e.g., Figure 1e), are unkeeled, arguing against affinities with both Diplocynodon and caimans. Given the absence of Diplocynodon in North America during the Tertiary, but the ubiquity of Borealosuchus in Lower Tertiary deposits of this continent, including the underlying Black Peaks Formation in Big Bend (Brochu 2000), identity with Borealosuchus appears most likely.

[FIGURE 1 OMITTED]

The anterior ventral element associated with TMM 40146-6 bears a single row of large pits immediately posterior to a broad, flat region that would have passed dorsal to the osteoderm immediately in front of it. In caimans and Diplocynodon, the anterior ossification usually bears small pits posterior to the imbrication zone, and these pits are not arranged in a single discrete row (e.g., Ludwig 1877: plate 14). However, a single row of large pits is characteristic of anterior ossifications in Borealosuchus wilsoni from the Lower Eocene of western North America. The nature of pits in Diplocynodon varies, and so caution must be observed when using this feature to diagnose ventral ossifications in crocodylians.

If these remains are from Borealosuchus, then they represent a stratigraphic extension of the taxon. The youngest previous occurrence of Borealosuchus is B. wilsoni, which ranges across the Wasatchian and Bridgerian NALMAs of the Bridger, Bighorn, and Green River Basins (Mook 1959; Bartels 1980; 1983; Brochu 1997). Borealosuchus has not been reported from units younger than the Bridgerian north of the Big Bend area.

PRISTICHAMPSINAE Kuhn 1968

Pristichampsus Gervais 1853

Referred specimens. -- TMM 42952-113 (in part) teeth; TMM 42953-11, tooth. Many more dental, cranial, and mandibular specimens were listed by Busbey (1986).

Occurrence. -- TMM 42952-113: Devil's Graveyard Formation, in an assemblage correlative with the Whistler's Squat Local Fauna (Runkel 1988), Dogie Mountain, Brewster County, Texas. TMM 42953-11: Canoe Formation, Canoe A Local Fauna (Runkel 1988), Crusher Big Yellow, Big Bend National Park, Brewster County, Texas. The specimens listed by Busbey (1986) were all from the Whistler's Squat Local Fauna.

Discussion. -- The material described by Busbey (1986) includes cranial and mandibular remains as well as isolated teeth. This is important, as the flattened, serrated teeth characteristic of Pristichampsus and its presumed close relatives appeared independently in several other crocodyliform lineages (Benton & Clark 1988). The additional specimens listed here merely add to the number of formations and local faunas from which Pristichampsus has been found in the Uintan of Big Bend.

These remains are among the youngest reported for Pristichampsus in North America, which is otherwise best known from the Wasatchian and Bridgerian NALMAs of North America (Troxell 1925; Langston 1975; Golz & Lillegraven 1977; Bartels 1980; Gingerich 1989) and units correlative with the Bridgerian in Europe (Berg 1966; Kuhn 1938; Rauhe & Rossmann 1995; Efimov 1993; Rossmann 1998). Uintan pristichampsines have also been collected from California (Bramble & Hutchison 1971).

BREVIROSTRES von Zittel 1890

ALLIGATOROIDEA Gray 1844

GLOBIDONTA Brochu 1999

Referred material. -- TMM 41576-7, fragments of left dentary and splenial; TMM 42952-113 (in part), isolated teeth.

Occurrance. -- TMM 41576-7: Devil's Graveyard Formation, Whistler's Squat Local Fauna, Wax Camp, Brewster County, Texas; TMM 42952-113: Devil's Graveyard Formation, in an assemblage correlative with the Whistler's Squat Local Fauna (Runkel 1988), Dogie Mountain, Brewster County, Texas.

Discussion. -- The rounded dorsal profile and stoutness of the symphysis (Figure 2a) suggests a placement deep within Alligatoroidea. The dentary bore a large fourth alveolus, but a small third. Alveoli posterior to the fourth are covered with matrix, but a natural mold of one of these alveoli, visible in cross section on the posterior surface of the specimen, indicates that they were very small. These are features consistent with members of Globidonta, which includes the crown-group Alligatoridae and a few of its closest extinct relatives such as Brachychampsa and Stangerochampsa; (refer to Brochu 1999). The relative sizes of the anterior alveoli are consistent with several extinct members of this assemblage, but are different from those of the horned alligatorid Ceratosuchus, in which the fourth alveolus is not much larger than the third (Bartels 1984).

The dentary symphysis extended back to approximately the same posterior extent as the sixth or seventh alveolus. The splenials met at the midline, and a distinct anterior foramen intermandibularis oralis is preserved (Figure 2a). Presence of a splenial symphysis rules out identity with the large alligatorid from the Paleocene Black Peaks Formation of Big Bend National Park (Brochu 1996), in which the splenials did not meet at the midline.

The second mandibular fragment represents a posterior segment of the left dentary (Figure 2b). The dorsal surface slopes anteriorly, and three alveoli are preserved. The splenial is not preserved, but the space for its attachment along the medial face of the dentary can be seen; the splenial nearly bordered the posterior most of the preserved alveoli, and the dorsal splenial-dentary suture would have projected ventrally toward the symphysis. This fragment corresponds closely with the region of the dentary bearing the tenth through twelfth alveoli in Allognathosuchus polyodon, A. wartheni, A. mooki and Wannaganosuchus brachymanus (cf. Mook 1961; Case 1925; Simpson 1930; Erickson 1982); it is less similar with those of Procaimanoidea (cf. Gilmore 1946; Mook 1941; Wassersug & Hecht 1967) or basal members of Alligator, in which this region of the jaw is more elongate.

[FIGURE 2 OMITTED]

TMM 42952-113 comprises several isolated crocodylian teeth and osteoderms. Some of the teeth are flattened and serrated, and may be referred to Pristichampsus (see above), but two of them are dorsoventrally flattened and globular, closely resembling the enlarged teeth found in the back of the dentary and maxillary toothrows of several extinct alligatorids, such as Allognathosuchus, Procaimanoidea, Hassiacosuchus, Ceratosuchus and some extinct Alligator. Indeed, globular posterior teeth are also characteristic of the immediate sister taxa to Alligatoridae (Norell et al. 1994; Wu et al. 1996) and are very likely plesiomorphic at the level of the crown group (Brochu 1999).

These remains are most consistent with any of several taxa given the name Allognathosuchus. However, phylogenetic analysis of fossil alligatorids (Brochu 1999) suggests the nonmonophyly of Allognathosuchus as currently used in the literature. As such, assigning this jaw to Allognathosuchus is premature; some taxa currently named Allognathosuchus (e.g., Allognathosuchus mooki) may be renamed in the future, and it is not currently known to which Allognathosuchus species the Texas alligatoroid is most closely related. Indeed, there are no characters in this specimen unambiguously placing it within the crown-group Alligatoridae. For these reasons, this specimen is not identified beyond the level of Globidonta. However, this material is biogeographically significant in representing the southernmost known occurrence of a blunt-toothed alligatoroid in the Tertiary of North America.

ACKNOWLEDGMENTS

The basis for this note is an extensive Eocene vertebrate collection developed over four decades by J. A. Wilson, J. Schiebout, J. and M. Stevens, A. Runkel, and their associates. I thank M. Norell (American Museum of Natural History, New York), R. Purdy (U.S. National Museum, Smithsonian Institution, Washington, DC), B. Erickson (Science Museum of Minnesota, St. Paul), G. Buckley (Field Museum of Natural History, Chicago), and P. Holroyd (University of California Museum of Paleontology, Berkeley) for access to specimens that proved invaluable for comparative purposes, and J. Sankey for providing information on relevant material in the Louisiana State University collections. M. Winans provided curatorial assistance. Funding was provided by NSF Dissertation Improvement Grant DEB-9423428 (to T. Rowe), the Roosevelt Fund of the American Museum of Natural History, the Paleontological Society, and the University of Texas Geology Foundation.

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Christopher A. Brochu

Department of Geological Sciences

University of Texas at Austin

Austin, Texas 78712

Present Address: Department of Geology Field Museum, 1400 S. Lake Shore Drive

Chicago, Illinois 60605

CAB at: cbrochu@fmppr.fmnh.org
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Date:Feb 1, 2000
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