REPORTS online material (SOM) (10)] including femora, A Complete Skeleton of a Late Triassic pelvis, and tail (GR 155) and cervical vertebrae (GR 243). A complete right femur (GR 244) is Saurischian and the Early from Hayden Quarry (HQ) site 3. Locality and horizon.Site2,HQ,GhostRanch,RioArriba Evolution of Dinosaurs County, New Mexico, USA. The HQ has been dated to ~215 to 213 million years ago (11)andis in the lower portion of the Petrified Forest Member Sterling J. Nesbitt,1,2*† Nathan D. Smith,3,4 Randall B. Irmis,5,6 12 7 8 1 of the Upper Triassic Chinle Formation ( ). Alan H. Turner, Alex Downs, Mark A. Norell Diagnosis. A theropod diagnosed by the fol- lowing combination of characters (autapomor- Characterizing the evolutionary history of early dinosaurs is central to understanding their rise phies are noted by an asterisk here and in Figs. and diversification in the Late Triassic. However, fossils from basal lineages are rare. A new 1 and 2): Prootics meet on the ventral midline of theropod dinosaur from New Mexico is a representative of the early North American diversification. the endocranial cavity; anterior tympanic recess Known from several nearly complete skeletons, it reveals a mosaic of plesiomorphic and derived greatly enlarged on the anterior surface of the features that clarify early saurischian dinosaur evolution and provide evidence for the antiquity basioccipital and extending onto prootic and of novel avian character systems including skeletal pneumaticity. The taxon further reveals parabasisphenoid; deep recess on the postero- latitudinal differences among saurischian assemblages during the Late Triassic, demonstrates dorsal base of paroccipital process*; sharp ridge that the theropod fauna from the Late Triassic of North America was not endemic, and suggests extending dorsoventrally on the middle of the that intercontinental dispersal was prevalent during this time. posterior face of the basal tuber*; incomplete ligamental sulcus on the posterior side of the y the Late Triassic (~230 million years Owen 1842 sensu Padian and May 1993. femoral head and semicircular muscle scar/ ago), Dinosauria had diversified into Theropoda Marsh 1881 sensu Gauthier 1986. excavation on the posterior face of the femoral Bthree major lineages: Sauropodomorpha, Tawa hallae, nov. taxa. head*; small semicircular excavation on the Theropoda, and Ornithischia (1, 2). In compari- Etymology. Tawa, Hopi name for the Pueb- posterior margin of the medial posterior condyle son to the later Mesozoic, fossils of Triassic early loan sun god; hallae, after Ruth Hall, who col- of the proximal end of the tibia*; “step” on ven- dinosaurs and their closest relatives are generally lected many of the specimens that formed the tral surface of the astragalus*; and metatarsal I rare, fragmentary, and incomplete (3, 4). Indeed, genesis of the Ghost Ranch Ruth Hall Museum similar in length to other metatarsals. See SOM on December 10, 2009 the record from the Ischigualasto Formation, which of Paleontology (GR) collections. Holotype.GR for differential diagnosis (10). provides some of the most detailed information 241. A nearly complete associated but disarticu- Description. The holotype material is a ju- on early dinosaur evolution (1, 2, 5), reveals that lated skull and postcranial skeleton. Paratypes. venile or subadult individual, based on compar- dinosaur specimens constitute less than 6% of the A nearly complete skeleton of a larger individual ison to the largest femur among the referred tetrapod assemblage (6). This depauperate fossil (GR 242) and at least six other individuals found material and the open braincase and neurocentral record has limited our understanding of early di- in the same area of the quarry [see supporting sutures. The premaxilla (Fig. 1) is similar to that nosaur interrelationships, diversification, and pa- f po so leobiogeography, and the origin of modern avian pf pa ls A la * www.sciencemag.org morphologies during a critical interval of Meso- n C op sq zoic climate change and faunal turnover (7–9). or pm pr dop Here we report on a new carnivorous dinosaur en af q atr * mx j ltf represented by two nearly complete skeletons qj ar bo and several other partial specimens collected in a pb su tightly associated small grouping at a single lo- mf an cality. Characterization of this taxon’s morphology d and phylogenetic history enables us to solidify basal saurischian dinosaur relationships and bears di- Downloaded from rectly on the evolution of early saurischian charac- B ter systems, paleobiogeography, and diversification. Systematic paleontology: Archosauria Cope 1869 sensu Gauthier and Padian 1985. Dinosauria 1Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA. 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA. 3Committee on Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA. 4Department of Geology, Field Museum of Natural History, Chicago, IL 60605, USA. T. hallae 5Utah Museum of Natural History, 1390 East Presidents Fig. 1. The skull of nov. taxa. (A) Reconstruction of the skull in lateral view. (B) The preserved Circle, Salt Lake City, UT 84112–0050, USA. 6Department skull elements of the holotype of T. hallae (GR 241) in left lateral view (j, qj, and the posterior portion of of Geology and Geophysics, University of Utah, Salt Lake the mandible were reversed, and the matrix was digitally erased from the pm, mx, and n. The processes of City, UT 84112–0102, USA. 7Department of Anatomical Sci- the maxilla are present but obscured by matrix in lateral view, so they are represented here in outline). (C) ences, Stony Brook University, Health Science Center T-8 (040), T. hallae 8 Braincase of in left lateral view (parabasisphenoid reversed). Abbreviations in the figure are as Stony Brook, NY 11794, USA. Ruth Hall Museum of Paleon- follows: antorbital fenestra (af), angular (an), articular (ar), anterior tympanic recess (atr), basioccipital tology, Ghost Ranch Conference Center, Abiquiu, NM 87510– 9601, USA. (bo), dentary (d), descending process of the opisthotic (dop), external naris (en), frontal (f), jugal (j), lacrimal (la), laterosphenoid (ls), lower temporal fenestra (ltf), maxilla (mx), mandibular fenestra (mf), *To whom correspondence should be addressed. E-mail: [email protected] nasal (n), opisthotic (op), orbit (or), parabasisphenoid (pb), prefrontal (pf), premaxilla (pm), postorbital †Present address: Jackson School of Geosciences, Univer- (po), prootic (pr), quadrate (q), quadratojugal (qj), supraocciptial (so), squamosal (sq), and surangular sity of Texas at Austin, Austin, TX 78712, USA. (su). Scale bar, 1 cm. Autapomorphies are noted by an asterisk. 1530 11 DECEMBER 2009 VOL 326 SCIENCE www.sciencemag.org REPORTS of coelophysids in possessing unserrated pre- recessed descending process of the opisthotic [the the accompanying phalanges are greatly reduced. maxillary teeth and a narial process of the pre- crista interfenestralis (10)] and a metotic strut. A Moreover, digit V is completely absent, as in maxilla that is elongate and forms a low angle weak parabasisphenoid recess is present on the Herrerasaurus and other basal theropods. The with the alveolar margin. It differs from neo- ventral surface of the braincase, similar to that in manus of Tawa retains a plesiomorphically small theropods by having a relatively tall maxillary Herrerasaurus, Eoraptor, and Neotheropoda. medial-most distal carpal. The hand of Tawa also process that extends dorsally beyond the poste- The vertebral column (Fig. 2) shares several retains nine carpals, similar to the basal ornithis- rior border of the naris as in Herrerasaurus.In apomorphic features with neotheropods. Cervical chian Heterodontosaurus,whereasHerrerasaurus dorsal view, the premaxilla-nasal suture is simple, vertebrae preserve anterior pneumatic pleurocoels has seven and C. bauri has five. lacking the W-shaped morphology present in (as rimmed fossae) and anterior and posterior in- The Tawa pelvis is generally plesiomorphic Neotheropoda. Like basal neotheropods (such as frazygapophyseal fossae; these features are present with respect to neotheropods. The preacetabular Coelophysis bauri and Dilophosaurus wetherilli), in all basal neotheropods. The diapophyses and process of the ilium (GR 241) does not extend a subnarial gap is expressed between the pre- parapophyses of the anterior to midcervical verte- anterior to the pubic peduncle. Additionally, the maxilla and maxilla, but unlike these taxa, Tawa brae are close together and nearly contact. Tawa anterior end is rounded, unlike the squared-off and Herrerasaurus lack an extensive antorbital shares with Herrerasaurus pronounced ventral morphology of neotheropods. The supraacetabular fossa on the lateral surface of the maxilla. A keels on the cervical vertebrae and elongate pre- crest projects laterally without any ventral deflection concave narial fossa is located on the anterolat- zygapophyses in the distal caudal vertebrae, as but is distally restricted in that it does not approach eral surface of the nasal. A lateral ridge on the in neotheropods. The dorsal vertebrae possess the articular facet of the pubic peduncle. The supra- nasal forms the dorsal border to the antorbital hyposphene-hypantra articulations and there ap- acetabular crest is continuous with the ventrolateral fossa, similar to Eoraptor lunensis and C. bauri. pear to be only two sacral vertebrae. edge of the postacetabular process, as in coelo- Unlike most