A complete skeleton of an abelisaurid theropod from the Late Cretaceous of patagonia, Argentina

JUAN IGNACIO CANALE; FERNANDO NOVAS; CARLOS AGUSTÍN SCANFERLA

San Juan

Jornada; Jornadas Argentinas de Paleontología de Vertebrados; 2006

A joint exploration of the Museo "Ernesto Bachmann" of Villa El Chocon (Neuquen) and Museo Argentino de Ciencias Naturales (Buenos Aires) resulted in the discovery of beautifully articulated skeleton of an apparently new abelisaurid theropod. This material constitutes the best preserved of the oldest known abelisaurids from Gondwana. The skeleton was excavated from levels of the Huincul Formation (Turonian-Santonian) in only one big plaster jacket, weighting ap­proximately 6 tons. The skeleton, depicting an opistothonic position, preserves valuable anatomical details, especially re­garding with skull and jaws. The specimen (MMCH-FV 49) measures around 6 meters long, and exhibits abelisaurid fe­atures. The hypertrophied cervical epipophyses of the new abelisaurid lack the cranial projections that characterize Noasaurus, Aucasaurus and Carnotaurus. Furthermore, the jaw exhibits wide contacts between dentary and postdentary bones, a condition that is interpreted as less derived than that of Carnotaurus and Majungasaurus. The dorsal border of ilium is convex, not straight as in Carnotaurinii. Interestingly, the new abelisaurid lacks of frontal horns, retaining simple albeit thickned, supraorbital margins. Also recorded from beds of the Huincul Fm. is the basal abelisaurid llokelesia aguadagrandensis Coria y Salgado, which distinguishes from the new abelisaurid in the more primitive condition of the postorbital bone (which is slightly expanded distally), the poorly defined diapo-postzygapophysial laminae of cervical vertebrae, as well as in the laterally directed transverse process of the middle caudal vertebrae, different from the latero-posteriorly directed transverse process in the new material. The underlying Candeleros Fm. has yielded the remains of the basal abelisaurid Ekrixinatosaurus novasi Calvo el al., distinguishable from the new material by its longer and less in­clined contact between jugal and maxilla, and anterior trochanter of femur more cranially projected. Available evidence indicate that basal abelisaurids were abundant and diverse during Turonian to Santonian times, playing the role of me­dium-sized predators alongside with the big carcharodontosaurids which occupied the role of top predators.