Describer

Sues, Nesbitt, Berman & Henrici 2011

Time

Triassic Late Rhaetian?

Classification

Saurischia Theropoda

Diet

Carnivore

Fossilsite

Chinle Formation, ‘Siltstone member’ , Coelophysis Quarry, Ghost Ranch, 20 km northwest of Abiquiu´ , Rio Arriba County, New Mexico, US. Geographical coordinates: latitude 368200 N, longitude 10682703000 E.

Info

Skull

Abstract

The oldest theropod dinosaurs are known from the Carnian of Argentina and Brazil. However, the evolutionary diversification of this group after its initial radiation but prior to the Triassic–Jurassic boundary is still poorly understood because of a sparse fossil record near that boundary. Here, we report on a new basal theropod, Daemonosaurus chauliodus gen. et sp. nov., from the latest Triassic ‘siltstone member’ of the Chinle Formation of the Coelophysis Quarry at Ghost Ranch, New Mexico.

Based on a comprehensive phylogenetic analysis, Daemonosaurus is more closely related to coeval neotheropods (e.g. Coelophysis bauri) than to Herrerasauridae and Eoraptor. The skeletal structure of Daemonosaurus and the recently discovered Tawa bridge a morphological gap between Eoraptor and Herrerasauridae on one hand and neotheropods on the other, providing additional support for the theropod affinities of both Eoraptor and Herrerasauridae and demonstrating that lineages from the initial radiation of Dinosauria persisted until the end of the Triassic.

Various features of the skull of Daemonosaurus, including the procumbent dentary and premaxillary teeth and greatly enlarged premaxillary and anterior maxillary teeth, clearly set this taxon apart from coeval neotheropods and demonstrate unexpected disparity in cranial shape among theropod dinosaurs just prior to the end of the Triassic.

Etymology

The generic nomen is derived from Greek daimon, evil spirit, and Greek sauros, reptile, in allusion to legends about evil spirits at Ghost Ranch, New Mexico. The specific epithet is derived from Greek chauliodous, with prominent teeth.

Holotype

CM (Carnegie Museum of Natural History) 76821, nearly complete but transversely crushed skull with mandible and associated anterior cervical vertebrae and ribs. It is possible that additional postcranial bones will be retrieved during further preparation of the large block C-4-81 in which CM 76821 was discovered in association with skeletal remains of C. bauri.

Diagnosis

Distinguished by the following unique combination of characters: skull proportionately deep and narrow, with short antorbital region; premaxillary and anterior maxillary teeth much enlarged relative to more posterior maxillary teeth; prefrontal large and occupies about 50 per cent of the dorsal margin of the orbit; ventral process of lacrimal with slender posterior projection extending along anterodorsal margin of jugal; dorsoventrally deep jugal with prominent lateral ridge; postorbital with anterolateral overhang over orbit; first two dentary teeth large and procumbent; alveolar margin of dentary downturned at symphysis; and third cervical vertebra with deep, rimmed, ovoid pleurocoel on the anterolateral surfaces of both centrum and neural arch.

Possible autapomorphies of Daemonosaurus include long posterior process of premaxilla that almost contacts anterior process of lacrimal and antorbital fenestra nearly the same size as external naris. Daemonosaurus differs from Herrerasaurus ischigualastensis in having amuch anteroposteriorly shorter antorbital fenestra, a posteroventral process of lacrimal that extends along the anterodorsal margin of the jugal, and much enlarged premaxillary teeth.

Daemonosaurus differs from Eodromaeus murphi in the absence of a distinct ridge on the lateral side of the maxilla, the proportionally much smaller antorbital fossa, presence of much enlarged premaxillary teeth, presence of a posteroventral process of the lacrimal that extends along the anterodorsal margin of the jugal, and greater dorsoventral expansion of the jugal.

Daemonosaurus differs from Eoraptor lunensis in the presence of much enlarged premaxillary and anterior maxillary teeth and a much more restricted antorbital fossa on the maxilla. Daemonosaurus differs from Tawa hallae and the neotheropod C. bauri especially in the presence of a dorsoventrally deep premaxilla, a slight subnarial gap and a proportionally larger prefrontal.

Daemonosaurus differs from Chindesaurus bryansmalli in the presence of an ovoid deep depression on the anterior portion of the centra of postaxial cervical vertebrae (postaxial cervical vertebrae are the only bones currently known for both taxa).

Ontogenetic age: It is difficult to assess the ontogenetic stage of CM 76821. To date, no postcranial bones other than a few cervicals for this specimen have been recovered; histological data from these elements are typically used to assess individual age. The proportionately large orbit, short snout and lack of fusion between the constituent elements of the braincase in CM 76821 are commonly considered indicators of somatic immaturity among theropod dinosaurs.

However, the neurocentral sutures between the centrum and neural arch on the axis and third cervical vertebra of CM 76821 are closed. The sequence of closure of these sutures (anterior to posterior versus posterior to anterior) in theropods is poorly understood, and both sequences of closure are present in that group.

In Crocodylia and other suchian archosaurs, closure of the neurocentral sutures proceeds from posterior to anterior. If the latter pattern was present in Daemonosaurus, CM 76281 might represent a skeletally more mature individual, with the apparently juvenile features being autapomorphies of this taxon.