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A New Specimen of Hesperosuchus Agilis from the Upper Triassic of New Mexico and the Interrelationships of Basal Crocodylomorph Archosaurs

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A New Specimen of Hesperosuchus Agilis from the Upper Triassic of New Mexico and the Interrelationships of Basal Crocodylomorph Archosaurs Journal of Vertebrate Paleontology 20(4):683±704, December 2000 q 2000 by the Society of Vertebrate Paleontology A NEW SPECIMEN OF HESPEROSUCHUS AGILIS FROM THE UPPER TRIASSIC OF NEW MEXICO AND THE INTERRELATIONSHIPS OF BASAL CROCODYLOMORPH ARCHOSAURS JAMES M. CLARK1, HANS-DIETER SUES2, and DAVID S. BERMAN3 1Department of Biological Sciences, George Washington University, Washington, District of Columbia 20052; 2Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, and Department of Zoology, University of Toronto, Toronto, Ontario M5S 3G5, Canada; 3Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213 ABSTRACTÐAn articulated partial skeleton, including a nearly complete skull and left forelimb, of a crocodylomorph reptile from the Upper Triassic Chinle Formation of Ghost Ranch, New Mexico, is referable to Hesperosuchus agilis Colbert, 1952. It contributes much new information on the skeletal structure of this previously poorly known taxon. A large, circular palpebral bone is present in each orbit. The quadratojugal is particularly well preserved and suggests a reinterpretation of this element in related taxa including Sphenosuchus. Previous analyses of sphenosuchian relationships reached con¯icting results, and a critical review of the characters used in these analyses is presented. A phylogenetic analysis of basal crocodylomorph taxa provides weak support for sphenosuchian monophyly, placing Hesperosuchus with Saltoposuchus, Dibothrosuchus, and Sphenosuchus. INTRODUCTION and assess its phylogenetic relationships to other crocodylo- morph archosaurs. Ghost Ranch, located 20 km NW of Abiquiu in Rio Arriba Institutional AbbreviationsÐAMNH, American Museum County, New Mexico, is world-renowned for an occurrence of of Natural History, New York; BP, Bernard Price Institute for numerous well-preserved skeletons of the theropod dinosaur Palaeontological Research, University of the Witwatersrand, Jo- Coelophysis bauri (Cope, 1887) at the Whitaker or Coelophysis hannesburg; CM, Carnegie Museum of Natural History; CUP, quarry in strata of the Upper Triassic Chinle Formation (Chinle collections of the former Catholic University of Peking, now Group sensu Lucas, 1993) (Colbert, 1947, 1989; Schwartz and housed in The Field Museum, Chicago; IVPP, Institute of Ver- Gillette, 1994). However, skeletal remains of non-dinosaurian tebrate Paleontology and Paleoanthropology, Academia Sinica, vertebrates, including other archosaurian reptiles and ®shes Beijing; MCPUCRGS, Museu de CieÃncias, PontifõÂcia Univer- (Colbert, 1989), are relatively scarce at this site. sidade CatoÂlica do Rio Grande do Sul, Porto Alegre, Brazil; Initial excavation of the Whitaker quarry at Ghost Ranch by MCZ, Museum of Comparative Zoology, Harvard University; Edwin H. Colbert and his team from the American Museum of P, Department of Zoology, University College, London; PVL, Natural History (New York) in 1947 and 1948 yielded 13 FundacioÂn-Instituto Miguel Lillo, San Miguel de TucumaÂn, Ar- blocks of bone-bearing rock. Five of these blocks were kept in gentina; SAM, South African Museum, Cape Town; SMNS, New York for preparation and subsequent study, and the re- Staatliches Museum fuÈr Naturkunde, Stuttgart; UCMP, Uni- mainder was sent to other institutions (Colbert, 1989). Work at versity of California Museum of Paleontology, Berkeley; YPM, the site was then abandoned until the summers of 1981 and Peabody Museum of Natural History, Yale University. 1982, when the Whitaker quarry was reopened under the di- rection of and with principal funding from the Carnegie Mu- SYSTEMATIC PALEONTOLOGY seum of Natural History. The Museum of Northern Arizona (Flagstaff), the New Mexico Museum of Natural History (Al- ARCHOSAURIA Cope, 1869 buquerque), the Peabody Museum of Natural History at Yale CROCODYLOMORPHA Hay, 1930 emend. Walker, 1968 University (New Haven), Erindale College of the University of SPHENOSUCHIDAE Haughton, 1924 Toronto (Mississauga), and Ghost Ranch contributed personnel HESPEROSUCHUS AGILIS Colbert, 1952 and ®nancial support during various parts of the project. Sixteen HolotypeÐAMNH 6758, partial skeleton comprising a num- blocks of various sizes were removed from the quarry and, with ber of cranial bones, vertebrae, ribs, dorsal osteoderms, girdle the exception of one very large block distributed to each of the and limb-bones. museums in Albuquerque and Flagstaff, were shipped to Pitts- As ®rst pointed out by Bonaparte (1972), the holotype in- burgh. The Carnegie Museum subsequently sent one large cludes the skeletal remains of at least two individuals. The block, labeled C-3-82 (Colbert, 1989:®g. 6), to the National ``carpus'' illustrated by Colbert is actually a tarsus, but it is too Museum of Natural History (Washington, DC), where it served large to be associated with either of the two preserved feet. as the center piece of a temporary exhibit showing preparation This mixing of material may be explained by the manner in of dinosaurian bones. During the preparation of the many skel- which the fossils were collected. A letter dated April 22, 1930 etal remains of Coelophysis bauri preserved in this block, Mr. (on ®le at the American Museum of Natural History, New Alex Downs uncovered the skull and a considerable portion of York) from the collector, Barnum Brown, to Hubert Richardson, the largely articulated postcranial skeleton of a sphenosuchian the owner of a trading post near the type locality, is revealing: reptile. This ®nd constitutes one of the most complete and best ``That little reptile I collected up near Tanner Crossing is of preserved specimens of a basal crocodylomorph discovered to great interest and some of it lies buried under earth I threw out date. It has been accessioned into the collections of fossil ver- of another cut. I will have to screen over this material for frag- tebrates at the Carnegie Museum of Natural History as CM ments.'' 29894. We present here a description of this important specimen Horizon and LocalityÐLower part of Petri®ed Forest Mem- 683 684 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 4, 2000 ber (Chinle Formation); site on the north side of the Little Col- orado River, near Tanner Crossing, about 6 miles (9.6 km) SE of Cameron, Arizona. Age: Late Triassic (late Carnian). Newly Referred SpecimenÐCM 29894, skull and articulat- ed partial postcranial skeleton, described below. Horizon and Locality for CM 29894ÐUpper part of Pet- ri®ed Forest Member of Chinle Formation (Parrish, 1989); Whi- taker Quarry, Ghost Ranch, Rio Arriba County, New Mexico. Age: Late Triassic (late Carnian or early Norian). Revised DiagnosisÐDentary with deep symphyseal end. Other potentially diagnostic apomorphies include: single large palpebral bone in each orbit (condition unknown in related taxa); supratemporal fossa extending laterally to lateral edge of squamosal; parietal crests on medial edges of supratemporal fossae separated by narrow median sulcus. CommentsÐColbert (1952) initially classi®ed Hesperosu- chus as an ornithosuchid ``thecodont.'' Walker (1970) ®rst es- tablished its close relationship to Sphenosuchus, and this as- sessment has since been generally accepted. A partial braincase in the material of AMNH 6758 indicates that the posttemporal fenestrae may have been broader than in Dibothrosuchus. However, the squamosal is not preserved, and the opening could have been restricted by the latter bone with- out contacting the paroccipital process, as is the case in Sphen- osuchus (Walker, 1990). The quadrate lacks a deep depression as in Terrestrisuchus. Colbert could not identify a ``problem- atical'' bone associated with AMNH 6758, but Walker (1970) interpreted it as a coracoid. It is distinctive in the presence of an elongate, mediolaterally compressed posteroventral process and closely resembles the coracoids of Pseudhesperosuchus, Sphenosuchus, and Terrestrisuchus. Although no coracoid fo- ramen is evident, we accept Walker's identi®cation of the bone in question. Parrish (1991) described a partial mandible and postcranial skeleton of a crocodylomorph (UCMP 129740) from the upper part of the Petri®ed Forest Member of Petri®ed Forest National Park, Arizona. He tentatively identi®ed this specimen as cf. Sphenosuchus sp., citing the deep anterior end of the dentary and differences in the sculpturing on the paramedian dorsal os- teoderms between his specimen and the holotype of Hespero- suchus agilis, AMNH 6758. Sereno and Wild (1992) and Long and Murry (1995) reinterpreted UCMP 129470 as a larger in- dividual of H. agilis. The sculpturing is indeed less prominent on the dorsal osteoderms of AMNH 6758 (although not ``ab- sent,'' as claimed by Parrish [1993:210]). However, some of the osteoderms of AMNH 6758 are very similar in all other features to the element ®gured by Parrish (1991:®g. 7), and the weaker sculpturing probably re¯ects the small size and thus earlier on- togenetic stage of this specimen. The original diagnosis provided by Colbert (1952:567) for Hesperosuchus agilis no longer serves to distinguish this form from other archosaurian reptiles. CM 29894 shares with both AMNH 6758 and UCMP 129470 (Parrish, 1991:®g. 4A, B) the deep anterior end of the dentary, which differs from the con- FIGURE 1. Hesperosuchus agilis, CM 29894, skull and cervical re- dition in Dibothrosuchus, Sphenosuchus, and related taxa (Ser- gion in dorsal view. Abbreviations: an, angular; ar, articular; c, cani- eno and Wild, 1992). It further resembles AMNH 6758 in
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