Brief Introduction to Patagonian theropods

Skull of Giganotosaurus, MACN (Argentinean Museum of Natural Sciences)

The Cretaceous beds of Patagonia have yielded the most comprehensive record of Cretaceous non-avian theropods  from Southern Hemisphere, which includes at least five main theropod lineages: Abelisauroidea, Carcharodontosauridae, Megaraptora, Alvarezsauridae, and Unenlagiidae. These record facilitates the understanding of the origin, evolution, and radiation of theropods from Gondwana. The first remains of dinosaurs were found near Neuquen city by an officer army in 1882 and were sent to Florentino Ameghino, the “founder father” of Argentinian paleontology.

Genyodectes holotype. From Wikimedia Commons.

By the end of  the 1880s, Santiago Roth collected some dinosaur remains from Chubut and  sent them to Richard Lydekker and Arthur Smith Woodward. In 1901, A. Smith Woodward described Genyodectes, based on fragmentary skull bones, including portions of both maxillas, premaxillae,  parts of the supradentaries, and some teeth. Genyodectes remained as the most completely known  theropod from South American until the 1970s. In 2004, O. Rauhut concluded that Genyodectes is more closely related to Ceratosaurus than the more derived abelisaurs.

Skull of Carnotaurus sastrei, from MACN.

The  Abelisauroidea reached a great taxonomic diversity and their fossils have been recovered in Argentina, Brasil, Madagascar, India, Morocco, Lybia and France. The group has been divided in two main branches: the Noasauridae which includes the small-sized abelisauroids, and the Abelisauridae which comprises medium to large-sized animals, like the popular Carnotaurus sastrei.

The group exhibits strongly reduced forelimbs and hands, stout hindlimbs, with a proportionally robust and short femur and tibia. It has been suggested that from the Cenomanian to the Maastrichtian, most South American abelisaurids may have been isolated from other Gondwanan relatives.

The Carcharodontosauridae includes the largest land predators in the early and middle Cretaceous of Gondwana, like the popular, Giganotosaurus carolinii. The group evolved large skulls surpassing the length of the largest skull of Tyrannosaurus rex.  Another common trait is the fusion of cranial bones.

Bicentenaria Argentina. MACN

The Coelurosauria is also a diverse clade. Bicentenaria argentina is a very basal coelurosaur, medium sized with elongate and gracile hindlimb bones. Another basal coelurosaur is Aniksosaurus darwini.

Megaraptora  is a clade represented by Megaraptor, Orkoraptor and Aerosteon. It has been suggested that megaraptorans were basal coelurosaurs that shared the role of top predators with abelisauroids and carcharodontosauroids.

The Alvarezsauridae is a group of highly derived theropods. The group exhibits, among other features,   a lightly built skull bearing numerous small teeth restricted to the anterior portion of the snout, robust humerus with a proximally projected inner tubercle, a robust ulna and a hand with very robust digit I carrying a large and stout claw, and keeled sternum.

Unenlagia comahuensis by Nobu Tamura. From Wikimedia Commons

South American paravians are included within the clade Unenlagiidae: Buitreraptor gonzalezorum, Unenlagia comahuensis, Unenlagia paynemili and Austroraptor cabazai, all recovered from the Upper Cretaceous of Patagonia, Argentina.

The fossil record shows that the macroevolutionary patterns observed in Gondwana at the Late Cretaceous differ from the records from Laurasia, but both show a common  macroevolutionary pattern during post-Coniacian times.

References:

Novas, F.E., et al., Evolution of the carnivorous dinosaurs during the Cretaceous: The evidence from Patagonia, Cretaceous Research (2013), http://dx.doi.org/10.1016/j.cretres.2013.04.001

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