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XenotarsosaurusSK 7371.jpg

In 1986, Martinez et al., recognized the species Xenotarsosaurus bonapartei, based on appendicular and vertebral remains of a medium sized theropod, coming from the Bajo Barreal Formation (Senonian pre-Maastrichtian). These authors assigned it to the Family Abelisauridae due to similarities that it presented with Carnotaurus. Later, Novas (1989), although maintaining the family assignment, and diagnosing the species based on autapomorphies, recognized that the relationships of Xenotarsosaurus with the remaining Abelisauridae (Abelisaurus and Carnotaurus) are problematic. It proposed then, as a possible synapomorphy of Carnotaurus and Xenotarsosaurus, the presence of a relatively deep prespinal basin (1), although maintaining the possibility that this character is in fact a synapomorphy of Noasaurus + Abelisauridae. The discovery of a theropod in Neuquén, identified as PVPH-35, communicated in previous contributions (Coria et al., 1991) and that represents the sister group of Noasaurus + Abelisauridae (redefined in consequence to the [Neoceratosauria] (= Ceratosaurus + [Noasaurus + Abelisauridae]; sensu Novas 1989, 1992): a) confirms in part the hypothesis of Novas, since the character (1) is not exclusive to Abelisauridae but a synapomorphy of the group (PVPH-35 + [Noasaurus + Abelisauridae]); and b) it weakens the assignment of Xenotarsosaurus to Abelisauridae. It is proposed to preliminarily consider Xenotarsosaurus bonapartei as [Neoceratosauria] incertae sedis, until new materials are known in order to establish with more precision its affinities within the clade. The family Abelisauridae is in this way composed by Abelisaurus comahuensis and Carnotaurus sastrei, a group whose monophyly is strongly supported by characters, at the moment, exclusively cranial not present in PVPH-35 and unknown in Xenotarsosaurus.