The Late Cretaceous Diversification of Abelisauridae (Theropoda: Ceratosauria): An Overview.

Abelisauridae is a mostly Gondwanan clade of large-bodied, basal theropods. The oldest abelisaurids were recovered from the Middle Jurassic deposits of South America, a continent that nowadays accounts more than half of named species of this clade. In the Lower Cretaceous, abelisaurids were relatively smaller and less abundant than coeval allosauroids and spinosaurids, a pattern that underwent dramatic variations during the Aptian-Cenomanian faunal turnover. By the Upper Cretaceous, in fact, abelisaurid became the most abundant lineage of large-bodied theropod in the southern hemisphere. In particular, combined decline of carcharodontosaurids and spinosaurids toward the end of the Cenomanian and major environmental changes throughout Gondwanan landmasses, may have driven abelisaurid diversification. Consequently, Latest Early Cretaceous extinction that affected all the basic components of the Gondwanan theropod fauna resulted in different starting conditions for their subsequent recovery. In contrast to carcharodontosaurids and spinosaurids, the diversity of abelisaurids in the earliest Santonian increased in comparison with the pre-extinction biota, as documented by expansion of two widespread abelisauroid families (Abelisauridae and Noasauridae). Nevertheless, the co-occurrence of the large-bodied Ekrixinatosaurus with carcharodontosaurids suggests that the evolution of larger size in abelisaurids started before giant tetanurans decline. Abelisaurid distribution during the Late Cretaceous and in particular the presence of a clade of Indo-Malagasy abelisaurids bracketed by South American taxa has often been linked to Gondwanan break-up, although the evidence supporting such scenario is weak. The survival of taxa long after geographic separation from the original ‘population’ is explained by early dispersal and following vicariance rather than through a late immigration.