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

CANALE, JUAN IGNACIO; NOVAS, FERNANDO EMILIO; SCANFERLA, CARLOS AGUSTÍN

San Juan

Jornada; XXII Jornadas Argentinas de Paleontología de Vertebrados; 2006

A joint exploration of the Museo “Ernesto Bachmann” of Villa El Chocón  (Neuquén) and Museo Argentino de Ciencias Naturales (Buenos Aires) resulted in the discovery of beautifully articulated skeleton of an apparently new abelisaurid theropod. This material constitute the best preserved of the oldest known abelisaurids from Gondwana. The skeleton was excavated from levels of 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-PV 49) measuares around 6 meters long, and exhibit abelisaurid features. The hypertrophied cervical epipophyses of the new abelisauri 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 Majungatholus. 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 Ilokelesia aguadagrandensis, which distinguishes from the new abelisaurid in the more primitive condition of the postorbital bone (which is slightly expanded distally), the pooly defined diapopostzygapophysial laminae of cervical vertebrae, as well as in the laterally directed transverse process of the middle caudal vertebrae, different from the lateroposteriorly directed transverse process in the new material. The underlying Candeleros Fm. has yielded the remains of the basal abelisaurid Ekrixinatosaurus novasi, 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 indicates that basal abelisaurids were abundant and diverse during Turonian to Santonian times, playing the role of medium-sized predators alolgside with the big carcharodontosaurids, which occupied the role of top predators.