Theropod remains from a new bone-bed of the Cañadón Asfalto Formation (Middle Jurassic), Chubut Province, Argentina

M.D. EZCURRA (1) and D. POL (2)

The Middle Jurassic theropod record is extremely scarce, being based on a fistful of taxa from Europe, China, Australia, and Argentina. Two basal tetanurans are known from the Argentinean MiddleJ u r a s s i c Ca ñ a d ó n As f a l t o F o rma t i o n : Piatnitzkysaurus Bonaparte and Condorraptor Rauhut. Since the summer of 2008, fieldwork has been undertaken at a new exceptionally rich bonebed of this formation (Cañadón Bagual). This locality bears several sauropod bones and less numerous turtle and theropod remains, housed at the MPEF collections. Several theropod shed teeth wererecovered, showing four different morphotypes: 1) with chisel-like mesial and distal denticles (frequency: 36%), 2) with chisel-like mesial and squared distal denticles (7%), 3) without mesial denticles (50%), and 4) with chisel-like and large mesial denticles and convex crown margins (7%). The last three morphotypes differ from Piatnitzkysaurus and Condorraptor, whereas morphotype 1 matches the dental morphology of these basal tetanurans, as well as an isolated dentary recovered at Cañadón Bagual. Nevertheless, this dentary differs from Piatnitzkysaurus in the absence of two lateral rows of foramina on its anterior end and the presence of a large and deep circular foramen directly ahead the tooth row. Otherwise, the dentary closely resembles Ceratosaurus Marsh in being a proportionally short bone, with a strongly dorsally curved alveolar margin, and apicobasally enlarged tooth crowns. Furthermore, a large-sized isolated ischium from this bone-bed also differs from Piatnitzkysaurus in the absence of a well-developed distal expansion. Accordingly, these preliminary observations reveal at least four different theropod taxa in the Bagual-site, widely increasing the South American Middle Jurassic theropod diversity.

(1) Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia". Buenos Aires, Argentina.
(2) Museo Paleontológico Egidio Feruglio. Trelew, Argentina.

XXIV Jornadas Argentinas de Paleontología de Vertebrados, San Rafael, Mendoza, Argentina. Mayo, 2009.