NEW THEROPOD DINOSAURS FROM THE MIDDLE AND LATE JURASSIC OF CHUBUT, ARGENTINA

RAUHUT, O.W.; POL, D.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Recent phylogenetic analyses indicate that the Jurassic was a crucial time in the evolution of theropod dinosaurs, during which many groups originated or experienced their first major radiation. However, the Jurassic theropod fossil record is rather poor, especially in the Southern Hemisphere, from where only few and mainly fragmentary theropod specimens have been reported.Recent fieldwork in the Middle Jurassic (Aalenian-Bajocian) Cañadón Asfalto Formation of Chubut, Argentina, resulted in the discovery of two theropod skeletons, which represent the most complete theropods recovered from the Middle Jurassic of Gondwana. Both skeletons come from fine-grained sediments and were found partially articulated.An almost complete skeleton, missing only the anterior part of the skull, lower jaws, a few vertebrae, and distal phalanges, represents a new taxon of ceratosaur. Phylogenetic analysis places it at the base of the Abelisauridae, based on several characters, including a laterally hidden lacrimal antorbital fossa, a broad prespinal fossa in the cervical vertebrae, the presence of rudimentary centrodiapophyseal laminae in the anterior caudal vertebrae, and the presence of a flexor groove rather than a tubercle, in the pedal phalanges. Thus, the new taxon extends the fossil record of this clade from the Early Cretaceous into the early Middle Jurassic. This find indicates that ceratosaur radiation happened considerably earlier than currently recognized and that abelisauroids failed to reach the Northern Hemisphere during Pangean times, in contrast to other, globally distributed theropod lineages. The new taxon further demonstrates that forelimb reduction in ceratosaurs proceeded from the distal elements proximally.The second specimen represents a basal tetanuran, which is represented by an almost complete skull and lower jaws, complete presacral vertebral series, complete forelimbs, pubis, and partial hindlimbs. The taxon shows an interesting character combination in combining tetanuran plesiomorphies (amphi-platyan cervical vertebrae, lack of ventral keel in anterior dorsals) with megalosauroid synapomorphies (U-shaped cross-section of the postorbital jugal ramus, deep groove ventral to the occipital condyle on the basioccipital) and a derived, three- fingered hand. Due to this character combination, coding the new taxon into a recent large phylogenetic analysis of basal tetanurans fails to resolve the phylogenetic relationships of this new taxon.A fragmentary, but articulated theropod specimen from the Upper Jurassic Cañadón Calcáreo Formation represents the first diagnostic theropod remain from this epoch found in South America. The taxon is represented by caudal vertebrae, fragmentary scapulocoracoid and humerus, and fragments of the hindlimb. Phylogenetic analysis places it as a basal tetanuran, but an exact placement is made difficult by the very unusual character combination, in which a laterally facing scapula glenoid fossa is combined with an astragalus with a low, spike-like ascending process and a distally flared fibula. Thus, the new tetanurans from the Cañadón Asfalto and the Cañadón Calcáreo formations indicate that character evolution in basal tetanurans is more complex than currently recognized.