July 22 , 2016

by Sarah Gibson

South America has proven very lucrative in the last few decades when it comes to new theropod dinosaur discoveries from the Late Cretaceous. The list is impressive, with genera such as Megaraptor [1], Orkoraptor [2], Aerosteon (published in PLOS ONE in 2008) [3], and even Gualicho, published last week in PLOS ONE [4]. Gualicho aside, the other theropods listed above all belong to a clade known as Megaraptora, which also includes a member from Australia, Australovenator, published in 2009 in PLOS ONE [5] and Fukuiraptor from Japan [6]. Megaraptora are notable for their large size, characteristically large claws, and air-filled, birdlike, pneumatized bones. These critters have spurred many debates as to the evolutionary relationships of theropods. And now there is a new megaraptoran dinosaur on the block, Murusraptor barrosaensis from the Upper Cretaceous Sierra Barrosa Formation of Argentina, that could potentially impact previous hypotheses of relationships concerning these toothy titans.

The paper, published today in PLOS ONE by Rodolfa A. Coria from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina, and Phillip Currie from the University of Alberta, Canada, describes this new megaraptoran dinosaur, first discovered in a canyon wall in 2001 (hence its name Murusraptor, meaning “Wall Raptor”). This site has produced a number of other terrestrial vertebrates, including turtles, crocodiles, saurischian dinosaurs, mammals, as well as some ichnofossils, such as bird footprints.

The specimen represents one of the most complete megaraptorans found, and was discovered partially articulated. Based on the position of the bones, it was lying on its right side when it died and was subsequently buried. It also showed some pathologic damage. In life, Murusraptor was approximately 6.5 meters long, and is notable for a long skull with small teeth. It’s bones were highly pneumatized. In fact, its sacral ribs were hollow and tubelike, a characteristic not seen in any other megaraptorans. The authors of this study suggest that this specimen was likely immature, but that it likely obtained a size larger than that of Megaraptor, more comparable to Aerosteon and Orkoraptor.

As I mentioned earlier, this part of the evolutionary tree of life is contested between different researchers. The position of megaraptorids among theropods falls into two schools of thought: some recover them as derived allosauroid neovenatorids, while others recovered them as tyrannosauroid coelurosaurs. Each hypothesis of relationships differs with regard to origin and ancestry of the group. Though the subject of this new study was not to resolve the phylogeny of the clade, Coria and Currie (2016) did examine the phylogenetic placement of Murusraptor by considering it within each of the competing phylogenetic hypothesis.

Based on the results of the analyses examining Murusraptor in each of the competing phylogenetic hypotheses of evolutionary relationships, Murusraptor is unequivocally recovered within Megaraptoridae in both analyses, and also likely belonging to a group that represents a distinct diversification of South American megaraptorans, which includes Megaraptor, Aerosteon, and Orkoraptor, from the other members of the Megaraptoridae family, namely Australovenator from Australia, and Fukuiraptor from Japan. However, the study couldn’t clear up the debate regarding megaraptorid relationships to other theropods, so the debate rages on.

As the authors note, “Eventually, further discoveries of…specimens in older rocks from both South America and Australia will [give us] a better understanding of the early evolution of the clade, and, therefore, clarify its basal phylogenetic afiinities.”

References:

[1] Novas FE (1998) Megaraptor namunhuaiquii, gen. et sp. nov., a large-clawed, Late Cretaceous theropod from Patagonia. Journal of Vertebrate Paleontology 18(1): 4–9.

[2] Novas FE, Ezcurra MD, Lacuna A (2008) Orkoraptor burkei nov. gen. et sp., a large theropod from the Maas- trichtian Pari Aike Formation, southern Patagonia, Argentina. Cretaceous Research 29: 468–80.

[3] Sereno PC, Martinez RN, Wilson JA, Varricchio DJ, Alcober OA, et al. (2008) Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina. PLoS ONE 3(9): e3303. doi:10.1371/journal.pone.0003303

[4] Apesteguía S, Smith ND, Juárez Valieri R, Makovicky PJ (2016) An Unusual New Theropod with a Didactyl Manus from the Upper Cretaceous of Patagonia, Argentina. PLoS ONE 11(7): e0157793. doi:10.1371/journal.pone.0157793

[5] Hocknull SA, White MA, Tischler TR, Cook AG, Calleja ND, Sloan T, et al. (2009) New Mid-Cretaceous (Latest Albian) Dinosaurs from Winton, Queensland, Australia. PLoS ONE 4(7): e6190. doi:10.1371/journal.pone.0006190

[6] Azuma Y, Currie PJ (2000). A new carnosaur (Dinosauria: Theropoda) from the Lower Cretaceous of Japan. Canadian Journal of Earth Sciences 37(12): 1735–53. doi: 10.1139/e00-064

[7] Coria RA, Currie PJ (2016) A New Megaraptoran Dinosaur (Dinosauria, Theropoda, Megaraptoridae) from the Late Cretaceous of Patagonia. PLoS ONE 11(7): e0157973. doi:10.1371/ journal.pone.0157973