Maip macrothorax, the shadow of the death

Maip macrothorax. Image credit: Agustín Ozán

Patagonia has yielded the most comprehensive fossil record of Cretaceous theropods from Gondwana, including Megaraptora, a clade of medium-sized and highly pneumatized theropods characterized by their elongate skulls, and the formidable development of their manual claws on digits I and II. The enigmatic nature of this group has been a matter of discussion since the description of Megaraptor namunhaiquii in 1990s . Other representatives of the clade are Aoniraptor libertatem, Aerosteon riocoloradensis, Australovenator wintonensis, Murusraptor barrosaensis, Tratayenia rosalesi and Orkoraptor burkei. The phylogenetic position of Megaraptora is still controversial. But despite the lack of consensus, megaraptorans themselves remain a well-supported clade. Now, a new megaraptoran theropod dinosaur from the Upper Cretaceous of the Santa Cruz Province, Argentina, sheds light on on these enigmatic predators and their evolutionary radiation.

Maip macrothorax is a large-bodied megaraptorid from lower Maastrichtian Chorrillo Formation in Santa Cruz Province, Argentina. The holotype (MPM 21,545) includes the axis (only lacking both prezygapophyses and its right postzygapophysis), several dorsal and caudal vertebrae, three incomplete cervical ribs, numerous incomplete or fragmentary dorsal ribs, numerous gastral elements, left coracoid, distal end of a second metatarsal, and fragments of the scapula. The generic name, Maip, is derived from an evil entity in Aonikenk mythology that represents “the shadow of the death”. The specific name, macro, derives from the Greek word makrós (meaning long), and the Latin word thorax (meaning chest) in reference to its wide thoracic cavity (which has, approximately, more than 1.20 m width).

Axis of Maip in lateral (A, A´), anterior (B, B´), posterior (C, C´), dorsal (D, D´) and ventral (E, E´). Scale bar: 5 cm. From Aranciaga et al., 2022

The new specimen was discovered in 2019, but due the outbreak of the COVID-19 pandemic in early 2020 the dig was temporarily interrupted. The most striking feature of Maip is its large size. Maip macrothorax was between nine and 10 meters (30-33 feet) and weight about 5 tons. Several vertebrae and ribs of Maip show striations or rugosities interpreted as the attachment sites for the costovertebral and costotransversarium ligaments, a condition not commonly observed in other theropods.

The new study, lead by Aranciaga Rolando, recovered two new clades comprising some derived megaraptorids from South America. The first one, Clade A, comprises Megaraptor, Murusraptor, most of the Cenomanian–Turonian Patagonian forms with 6 or 7 m in length. The second one, Clade B, includes Orkoraptor, Tratayenia, Aerosteon and Maip, most of the Santonian through Maastrichtian megaraptorids from South America, with 8 or 10 m in length. This clade is supported by two synapomorphies: dorsal vertebrae with a bifurcated lamina anterior to the transverse process and forming an accessory fossa, and round and large articular facets of pre- and postzygapophyses of proximal caudal vertebrae. Additionally, the work suggests that after the Turonian, megaraptorids showed an increase in the body size and (with other theropod groups) replaced carcharodontosaurids in the role of apex predators within the Southern continents in the course of the Late Cretaceous.

 

References:

Aranciaga Rolando, A.M., Motta, M.J., Agnolín, F.L. et al. A large Megaraptoridae (Theropoda: Coelurosauria) from Upper Cretaceous (Maastrichtian) of Patagonia, Argentina. Sci Rep 12, 6318 (2022). https://doi.org/10.1038/s41598-022-09272-z

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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