A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs

Abstract

Abelisaurids are a clade of large, bizarre predatory dinosaurs, most notable for their high, short skulls and extremely reduced forelimbs. They were common in Gondwana during the Cretaceous, but exceedingly rare in the Northern Hemisphere. The oldest definitive abelisaurids so far come from the late Early Cretaceous of South America and Africa, and the early evolutionary history of the clade is still poorly known. Here, we report a new abelisaurid from the Middle Jurassic of Patagonia, Eoabelisaurus mefi gen. et sp. nov., which predates the so far oldest known secure member of this lineage by more than 40 Myr. The almost complete skeleton reveals the earliest evolutionary stages of the distinctive features of abelisaurids, such as the modification of the forelimb, which started with a reduction of the distal elements. The find underlines the explosive radiation of theropod dinosaurs in the Middle Jurassic and indicates an unexpected diversity of ceratosaurs at that time. The apparent endemism of abelisauroids to southern Gondwana during Pangean times might be due to the presence of a large, central Gondwanan desert. This indicates that, apart from continent-scale geography, aspects such as regional geography and climate are important to reconstruct the biogeographical history of Mesozoic vertebrates.

Neovenatorid theropods are apex predators in the Late Cretaceous of North America

“…Although, the identification of a derived neoventatorid (that is, Megaraptora), and a derived carcharodontosaurid (Acrocanthosaurus 30 ) in mid-Cretaceous ecosystems of North America (Fig. 5b) is consistent with growing evidence suggesting that the majority of theropod clades had global or near global distributions until the latest Cretaceous 11,33,44 it is also consistent with biogeographic hypotheses involving continental scale dispersal across portions of Laurasia during this interval. Phylogenetically informed biodiversity data from a range of terrestrial vertebrates inhabiting western North America have spurred hypotheses of local extirpation and dispersal that may correlate with subaerial exposure of an Asian-North American land bridge in the Aptian/Albian 18,32,45-47 -a hypothesis dubbed EKLInE 32 .…”

“…With regard to Carcharodontosauria, basal most and/or oldest members of both its constituent subclades (for example, Neovenator, Chilantaisaurus, Siats, Concavenator, Acrocanthosaurus, Shaochilong, Veterupristisaurus and Neovenatoridae indet from Australia) are distributed across both Laurasian and Gondwanan landmasses 33,43 (Fig. 5) supporting hypotheses of near global distribution of theropod clades until the latest Cretaceous that stem from Jurassic origins 11,43,44 . Although, the identification of a derived neoventatorid (that is, Megaraptora), and a derived carcharodontosaurid (Acrocanthosaurus 30 ) in mid-Cretaceous ecosystems of North America (Fig.…”

“…However, unlike microfaunal components, for which we have rich fossil data 18,49 51 , which are known from more complete skeletons, are more compelling, yet fewer in number. Although Siats fills a critical gap in the biogeographic history of Early to mid-Cretaceous theropods, the poor fossil record of the group globally 11,29,34,43,44 , various possible resolutions of Megaraptora in our analyses, and long ghost lineages subtended by most megaraptoran species continue to permit both global distribution and interchange biogeographic scenarios with equal likelihood. Increased resolution of neovenatorid relationships and increased sampling of Early Cretaceous theropods around the globe will be necessary to test ancestral geographic origins for Siats and other carcharodontosaurians.…”

An Early Jurassic Sauropod Tooth From Patagonia (Cañadón Asfalto Formation): Implications for Sauropod Diversity

“…These hypotheses, however, are debated, given that some authors considered wide tracks cannot be unequivocally assigned to a certain clade (Henderson, 2006) and that the Middle Jurassic taxa mentioned above have been placed outside neosauropods in other phylogenetic studies (e.g., Wilson, 2002;Wilson and Upchurch, 2009 The presence of a narrow-crowned tooth of putative neosauropod affinities in the latest Early-Middle Jurassic Cañadón Asfalto Formation (Cúneo et al, 2013) adds new data to this debate and would set an even earlier time of origin for the modern lineages of neosauropods. Interestingly, a similar pattern of undetected early diversification has been recently proposed for other dinosaur groups based on discoveries from the Cañadón Asfalto Formation (Pol and Rauhut, 2012), which could be related with worldwide scarcity of dinosaur assemblages from the latest Early and Middle Jurassic (Mannion et al, 2011). (Bonaparte, 1986;Coria, 1994;Rauhut et al, 2001;Pol et al, 2009) (Bonaparte, 1986).…”

Theropod teeth from the Middle-Upper Jurassic Shishugou Formation of northwest Xinjiang, China

“…Each clade was selected at the family and superfamily levels (Coelophysoidea, Noasauridae, Abelisauridae, Megalosauridae, Spinosauridae, Allosauridae, Neovenatoridae, Carcharodontosauridae, Tyrannosauridae, Dromaeosauridae, Troodontidae) following the phylogenies obtained by Pol and Rauhut (2012) for Ceratosauria, Carrano et al (2012) for Coelophysoidea and non-coelurosaur Tetanurae, Brusatte et al (2010b) for Tyrannosauroidea, and Turner et al (2012) for Deinonychosauria (see SOM 3). Only a few groupings are paraphyletic (i.e., non-neotheropod Theropoda, non-abelisauroid Ceratosauria, and non-tyrannosaurid Tyrannosauroidea), but members of each paraphyletic group share similar dentition (CH personal observations).…”