Systematic reinterpretation of the pigmy Allosaurus from the Lower Cretaceous of Victoria (Australia)

AGNOLIN, F. L.; EZCURRA, M. D.; PAIS, D. F.

Puerto Madryn

Encuentro; Reunión Anual de Comunicaciones de la Asociación Paleontologica Argentina; 2005

CENPAT

A wide revision of the Australian non-avian dinosaurian faunas, lead us to examine the remains of the Australian Allosaurus.This pigmy Allosaurus is one of the more emblematic members of the endemic Cretaceous fauna of Australia, since its original description based on an isolated and fragmentary left astragalus. Additionally, this taxon has been postulated as a Jurassic relic and its smaller size, in comparison with its North American co-generic (Allosaurus fragilis, Kimmeridgian-Titonian), was interpreted as consequence of the insular confinement. Albeit the original systematic interpretation of this material was challenged, the original authors lately supported their hypothesis More recently, the impossibility of diagnose this taxon beyond Allosauroidea has been claimed. The Australian astragalus was referred to Allosauridae due to: 1) restriction of the ascending process to the lateral portion of the bone, 2) medial condyle larger than the lateral one, and 3) a lower horizontal groove across the cranial surface of the astragalar body and to the genus Allosaurus on the bases of: 4) presence of a calcaneal notch, and. However, features 1 and 4 are more widely distributed among non-avian theropods; and traits 2 and 3 are also present in Abelisauroidea (e.g., Xenotarsosaurus, Masiakasaurus, Ekrixinatosaurus). Contrasting with the original interpretation, the current review of the astragalar morphology reveal common traits only shared by the of the Australian theropod and abelisauroids, including: 1) ascending process of the astragalus subrectangular in contour, 2) vertical groove onto the caudal surface of the ascending process, and probably 3) cranial vertical ridge onto the lateral edge of the ascending process. The systematic re-assignation of the Australian Allosaurus to Abelisauroidea is a more parsimonious explanation in the paleobiogeographic context of Gondwana, where the theropodian faunas are characterized by abelisauroid lineage.