A new species of megaraptorid dinosaur discovered in Patagonia may help discern the evolutionary origins of the megaraptorid clade, according to a study published July 20, 2016 in the open-access journal PLOS ONE by Rodolfo Coria from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina, and Phillip Currie from the University of Alberta, Canada.
The Patagonian region of Argentina has previously proven to be rich in fossils from the Late Cretaceous epoch, including a number of megaraptorids, a clade whose carnivorous diet gave rise to their name meaning 'giant thieves'. These medium-sized theropod dinosaurs, including South American genera Megaraptor, Orkoraptor, and Aerosteon as well as genera from Australia and Japan, have characteristically large claws and air-filled, birdlike bones.
The fossilized partial skeleton of a megaraptorid dinosaur analyzed in this study was discovered in Sierra Barrosa, in northwest Patagonia and represents one of the most complete megaraptorids found, with an unusually intact braincase. With unique skull features, the dinosaur, which they named Murusraptor barrosaensis, is a new species in the megaraptorid clade. This specimen appears to be immature, but the authors suggest that the species is larger and slenderer than Megaraptor and comparable in size with Aerosteon and Orkoraptor. While sharing many features with the other species, Musuraptor has distinctive facial features not previously seen amongst megaraptorids, as well as unusually shaped hip bones.
While phylogenetic analysis could not clearly determine evolutionary relationships, the authors note that these fossils provide new anatomical information which might help to resolve current debates as to whether the megaraptorids are a clade of the allosauroid or the coelurosaurid theropods.
The dentary of Australovenator wintonensis (Theropoda, Megaraptoridae); implications for megaraptorid dentition
Authors:
White et al
Abstract:
Megaraptorid theropods were an enigmatic group of medium-sized predatory dinosaurs, infamous for the hypertrophied claw on the first manual digit. Megaraptorid dentition is largely restricted to isolated teeth found in association with skeletal parts; however, the in situ maxillary dentition of Megaraptor was recently described. A newly discovered right dentary pertaining to the Australovenator holotype preserves in situ dentition, permitting unambiguous characterisation of the dentary tooth morphology. The new jaw is virtually complete, with an overall elongate, shallow profile, and fifteen visible in situ teeth at varying stages of eruption. In situ teeth confirm Australovenator exhibited modest pseudoheterodonty, recurved lateral teeth with a serrate distal carina and reduced mesial carina, similar to other megaraptorids. Australovenator also combines of figure-of-eight basal cross-section with a lanceolate shape due to the presence of labial and lingual depressions and the lingual twist of the distal carina. Computed tomography and three-dimensional imagery provided superior characterisation of the dentary morphology and enabled an accurate reconstruction to a pre-fossilised state. The newly established dental morphology also afforded re-evaluation of isolated theropod teeth discovered at the Australovenator holotype locality and from several additional Winton Formation localities. The isolated Winton teeth are qualitatively and quantitatively similar to the in situ dentary teeth of Australovenator, but are also morphometrically similar to Abelisauridae, Allosauridae, Coelophysoidea, Megalosauridae and basal Tyrannosauroidea. Qualitative characters, however, clearly distinguish the teeth of Australovenator and the isolated Winton teeth from all other theropods. Evidence from teeth suggests megaraptorids were the dominant predators in the Winton Formation, which contrasts with other penecontemporaneous Gondwanan ecosystems.
A large-clawed theropod (Dinosauria: Tetanurae) from the Lower Cretaceous of Australia and the Gondwanan origin of megaraptorid theropods
Authors:
Bell et al
Abstract:
Megaraptoridae comprises a clade of enigmatic Gondwanan theropods with characteristic hypertrophied claws on the first and second manual digits. The majority of megaraptorids are known from South America, although a single genus (Australovenator) plus additional indeterminate material is also known from Australia. This clade has a controversial placement among theropods, and recently has been interpreted alternatively as a carcharodontosaurian or a tyrannosauroid lineage. We describe new fragmentary but associated postcranial remains from the opal fields of Lightning Ridge (middle-Albian, Griman Creek Formation) in north-central New South Wales. The new unnamed taxon exhibits a number of unusual features that suggest the presence of a hitherto unrecognized Australian megaraptorid. From an Australian perspective, the Lightning Ridge taxon predates Australovenator by c.10Ma and is minimally coeval with megaraptoran material reported from the Eumeralla Formation of Victoria (but potentially 6.1–9.5 Ma younger). It is also notable as the largest predatory dinosaur yet identified from Australia and is only the second theropod known from more than a single element. A Bayesian phylogenetic approach integrating morphological, stratigraphic and palaeogeographic information tested both the carcharodontosaurian and tyrannosauroid placements for Megaraptora. Regardless of the preferred placement among Tetanurae, rigorous palaeobiogeographic analyses support an Asian origin of Megaraptora in the latest Jurassic (about 150-135 Ma), an Early Cretaceous (about 130-121 Ma) divergence of the Gondwanan lineage leading to Megaraptoridae, and an Australian root for megaraptorid radiation. These results indicate that Australia’s Cretaceous dinosaur fauna did not comprise simply of immigrant taxa but was a source for complex two-way interchange between Australia-Antarctica-South America leading to the evolution of at least one group of apex predatory dinosaurs in Gondwana.
Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation
Authors:
Porfiri et al
Abstract:
Megaraptorids are a group of predatory dinosaurs that inhabited Gondwana from Cenomanian to Santonian times (Late Cretaceous). Phylogenetic relationships of megaraptorids have been matter of recent debate, being alternatively interpreted as basal coelurosaurs, carcharodontosaurian allosauroids, megalosauroids, and basal tyrannosauroids. One of the main reasons for such different interpretations is the incomplete nature of most available megaraptorid skeletons and, in particular, the scarce information about their cranial anatomy. Here we describe a partially preserved skeleton of a juvenile specimen of Megaraptor namunhuaiquii that provides substantial new information about the cranial morphology of this Patagonian taxon. The specimen comes from the Upper Cretaceous (Turonian–Coniacian) of the Portezuelo Formation, northwestern Patagonia, Argentina. The anatomy of the new specimen bolsters the recently proposed hypothesis that megaraptorids are nested within Coelurosauria, and possibly within Tyrannosauroidea. The most relevant features that megaraptorans share with tyrannosauroids include several foramina on the premaxillary body, extremely long and straight prenarial process of the premaxilla, incisiviform premaxillary teeth with a D-shaped cross-section, and cranially expanded supratemporal fossae separated from each other by a sharp sagittal median crest on frontals, which was presumably extended caudally above the parietals (not preserved). Information gathered from the present specimen allows to make for the first time a reconstruction of the skull of Megaraptor and hypothesize about evolutionary trends within Tyrannosauroidea.
