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Hindlimb Osteology and Distribution of Basal Dinosauromorphs from the Late Triassic of North America

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Hindlimb Osteology and Distribution of Basal Dinosauromorphs from the Late Triassic of North America Journal of Vertebrate Paleontology 29(2):498–516, June 2009 # 2009 by the Society of Vertebrate Paleontology ARTICLE HINDLIMB OSTEOLOGY AND DISTRIBUTION OF BASAL DINOSAUROMORPHS FROM THE LATE TRIASSIC OF NORTH AMERICA { STERLING J. NESBITT,*,1,2 RANDALL B. IRMIS,3, WILLIAM G. PARKER,4 NATHAN D. SMITH,5,6 { ALAN H. TURNER,1,2, and TIMOTHY ROWE7 1Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA, [email protected]; 2Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, USA; 3Museum of Paleontology and Department of Integrative Biology, 1101 Valley Life Sciences Building, University of California, Berkeley, CA 94720-4780, USA; 4Division of Resource Management, Petrified Forest National Park, P.O. Box 2217, Petrified Forest, AZ 86028, USA; 5Committee on Evolutionary Biology, University of Chicago, 402 Culver Hall, 1025 E. 57th Street, Chicago, IL 60637, USA; 6Department of Geology, The Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, IL 60605, USA; 7Department of Geological Sciences and Vertebrate Paleontology Laboratory, University of Texas, Austin, TX 78712, USA ABSTRACT—The recent discovery of early dinosauromorphs from North America demonstrates that they were con- temporaries with dinosaurs and other basal archosaurs during a substantial portion of the Late Triassic Period. Hindlimb material (femora, tibiae, a fibula, astragalocalcanea, and phalanges) of Dromomeron romeri, a non-dinosauriform dino- sauromorph from the Petrified Forest Member of the Chinle Formation from north-central New Mexico, is described. A new species of Dromomeron from the lower portion of the Chinle Formation (eastern Arizona) and Dockum Group (northern Texas) is also described, based on several disarticulated femora and tibiae. D. romeri, Lagerpeton, and the new taxon form the sister group to all other dinosauromorphs and demonstrate that this clade, Lagerpetidae, persisted well into the Norian. Lagerpetidae is supported by several synapomorphies: femoral head hook-shaped in medial and lateral views; ventral emargination on the anterolateral side of the femoral head; an enlarged posteromedial tuber of the proximal end of the femur; femoral crista tibiofibularis larger than the medial condyle; anteromedial corner of the distal end of the femur forms 90o or acute (>90o) angle; and a posterior ascending process of the astragalus. An ontogenetic series of the femur of Dromomeron indicates that some character states previously used in phylogenetic analyses of early dinosaurs may be ontogenetically variable. INTRODUCTION Lagerpeton chanarensis (estimated at 70 cm in length from the femur) is known from the complete hindlimb, pelvis, and partial- By the beginning of the Middle Triassic, the clade Archo- ly articulated series of dorsal and caudal vertebrae from several sauria had diversified into two lineages, Pseudosuchia and specimens (Romer, 1971, 1972; Arcucci, 1986; Sereno and Ornithodira (Gauthier et al., 1988; Sereno, 1991; Gower and Arcucci, 1994b). Because of its phylogenetic placement, Lager- Sennikov, 2000; Nesbitt, 2003). Pseudosuchian archosaurs in- peton is critical for understanding the interrelationships among clude the phytosaurs, aetosaurs, “rauisuchians,” and crocodylo- dinosauromorphs and basal ornithodirans, and has been used morphs, whereas the Ornithodira includes the pterosaurs, a repeatedly to polarize character states among dinosaurs (Novas, variety of basal dinosauromorphs and dinosauriforms, and dino- 1989, 1992, 1996; Sereno and Arcucci, 1994a, 1994b; Arcucci, saurs (including birds) (Benton and Clark, 1988; Benton, 2004). 1997). All of the aforementioned authors agree that Lagerpeton is Throughout the Triassic Period, the pseudosuchians dominated the basal-most member of Dinosauromorpha, and the sister-taxon ornithodirans in terms of size and diversity (Benton, 2004). By to Dinosauriformes. Though well-preserved, known material of the end of the Triassic, all pseudosuchian clades were extinct Lagerpeton lacks certain diagnostic dinosauriform character except for Crocodylomorpha, and ornithodirans became taxo- states (e.g., the presence of an anterior trochanter), thus support- nomically diverse and globally distributed. The early history of ing its basal position. Pseudosuchia is well documented throughout Gondwana and Our understanding of the early evolution of Ornithodira is Laurasia, but the early history of Ornithodira is only well-known further hampered by the uncertainty of the phylogenetic posi- from the Middle Triassic of Argentina. This material includes tion of pterosaurs (Benton, 1985; Wellnhofer, 1991; Bennett, the incomplete skeletons of the earliest known basal dinosauro- 1996; Unwin, 1999; Peters, 2000; Hone and Benton, 2007). Be- morphs Lagerpeton Romer, 1971, Marasuchus Sereno and cause pterosaurs appear in the fossil record as derived flying Arcucci, 1994a, and Pseudolagosuchus Arcucci, 1987. archosaurs, they share few unambiguous synapomorphies with other archosaurs that would elucidate their ancestry. Addition- ally, the controversial Scleromochlus is poorly preserved and *Corresponding author. many morphological features are open to interpretation, making { Current address: Utah Museum of Natural History and Department comparisons difficult (Padian, 1984; Sereno, 1991; Bennett, 1996; of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112 Benton, 1999). { Current address: Department of Anatomical Sciences, Stony Brook Uni- Irmis et al. (2007) reported the first occurrence of a Late versity, Health Sciences Center T-8 (040), Stony Brook, NY 11794 USA. Triassic non-dinosauriform dinosauromorph. This new taxon, 498 NESBITT ET AL.—NORTH AMERICAN DINOSAUROMORPHS 499 Dromomeron romeri, from the Chinle Formation of New Mex- end of the femur that forms an angle near or less than 90o, and 6) ico clearly indicates that non-dinosauriform dinosauromorphs an astragalus with a posteriorly situated ascending process. were contemporaneous with dinosaurs and pseudosuchians in North America during the Late Triassic and had a broader geo- DROMOMERON Irmis et al., 2007 graphic distribution than previously believed. Here, we fully describe the hindlimb of Dromomeron romeri, name and de- Diagnosis—Differs from all other dinosauromorphs in posses- scribe a new second species of Dromomeron, and demonstrate sing the following synapmorphies: 1) a concave posterolateral that non-dinosauriform dinosauromorphs had an extensive evo- surface of the crista tibiofibularis of the distal end of the femur; lutionary history in the Late Triassic of North America. Further- 2) distinct scar on the anterior surface of the distal end of the more, a growth series of femora from the new taxon provides a femur; and 3) posterolateral condyle of the proximal portion of better understanding of the ontogeny of character states in non- the tibia is ventrally deflected or “hooked.” dinosauriform dinosauromorphs. Type Species—Dromomeron romeri Irmis et al., 2007. Institutional Abbreviations—AMNH, American Museum of Natural History, New York; BMNH, The Natural History Muse- DROMOMERON ROMERI Irmis et al., 2007 um, London, United Kingdom; GR, Ruth Hall Museum of Pale- ontology, Ghost Ranch, NM; MB, Museum fu¨ r Naturku¨ nde, Holotype—Complete left femur, GR 218 (Fig. 1). Humboldt Universita¨t, Berlin, Germany; MCP, Museu de Cieˆn- Paratypes—A right femur, GR 219, and a left tibia, GR 220, cias e Tecnologia, Pontificia Universidade Cato´lica do Rio may belong to the same individual as the holotype. Additional Grande do Sul, Porto Alegre, Brazil; MOTT VPL, Museum at material includes GR 221, a partial left femur; GR 234, a com- Texas Tech, Vertebrate Paleontology Locality; NMMNH, New plete right femur; GR 222, a complete left tibia; and GR 223, a Mexico Museum of Natural History and Science, Albuquerque, complete astragalocalcaneum. NM; PVL, Instituto Miguel Lillo, Tucuma´n, Argentina; SMNS, Referred Material—GR 238, partial articulated skeleton; GR Staatliches Museum fu¨ r Naturkunde, Stuttgart, Germany; TMM, 239, isolated right tibia (cnemial crest crushed); NMMNH P- Vertebrate Paleontology Laboratory, Texas Natural Science 35379, a complete astragalocalcaneum; AMNH FR 2721, distal Center, Austin, TX; TTUP Texas Tech University Paleontology portion of a femur; AMNH FR 30648, distal portion of a right Collections, Lubbock, TX; UCMP, University of California Mu- tibia; AMNH FR 30649, distal portion of a right tibia. seum of Paleontology, Berkeley, CA; UNLR, Museo de Paleon- Diagnosis—Differs from Dromomeron gregorii (see below) tologia, Universidad Nacional de La Rioja, La Rioja, Argentina; and all other basal dinosauromorphs in possessing the following ZPAL, Instytut Paleobiologii PAN, Warsaw, Poland. autapomorphies: 1) absence of a distinct ridge for the attach- th Anatomical Abbreviations—a. articulates with; aap, anterior ment of the M. caudifemoralis longus (=4 trochanter); 2) pres- ascending process; ald, anterolateral depression of the astraga- ence of a sharp ridge on the anteromedial edge of the distal end lus; am, anteromedial process of the distal portion of the tibia; of the femur; 3) presence of a lateral tuberosity on the antero- amp, anteromedial process; at, anterior trochanter; as, astra- lateral edge of the distal end of the femur; and 4) a large crest on galus; cal, calcaneum; c-a, calcaneum–astragalus suture; cc, the anteromedial edge of the astragalus and associated antero- cnemial crest; cr,
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