Megaraptor

Megaraptor (lit. 'large thief') is a genus of large theropod dinosaur, the type genus and namesake of the clade Megaraptora and family Megaraptoridae. Its fossils have been discovered in the Patagonian Portezuelo Formation of Argentina, South America, dating to the Turonian and Coniacian ages of the Late Cretaceous, roughly 90–88 million years ago. One species of Megaraptor, M. namunhuaiquii, has thus been named, known from seven partial or fragmentary skeletons, with only two including skull elements.

The type specimen of Megaraptor consists of a fragmentary assemblage of limb bones, discovered in 1996 by Argentine palaeontologist Fernando E. Novas. Believing that a large claw found at the site came from the animal's foot, he determined that it was probably a coelurosaur related to dromaeosaurs and troodontids, and named it accordingly. While Novas never stated that Megaraptor belonged to either family, contemporary depictions often portrayed it as a giant dromaeosaurid. The discovery of another specimen revealed that Megaraptor's large claw actually came from its hand, and it was surmised in 2004 to belong to a novel lineage predating the carnosaur–coelurosaur split. Subsequent analyses recovered Megaraptor and related genera (collectively known as megaraptorans) as relatives of the allosauroid Neovenator, though since the description of a relatively complete juvenile skull in 2014, it has been recovered primarily within Coelurosauria, and may have been a close relative of tyrannosaurs.

No complete skeletons of Megaraptor are known, so its anatomy has been pieced together over the years through only a few fragmentary specimens. It has been estimated that Megaraptor measured 8 m (26 ft) in length, and weighed around 1 t (2,200 lb). Its skull, the anatomy of which is known primarily from a juvenile specimen, was long, low, and slender, though in adults it was likely deeper and more robust, with smaller eye sockets and more robust frontal bones. Similar to tyrannosaurs, it had small, conical teeth at the front of its jaws, and longer, more curved teeth near the back, a condition known as heterodonty. Megaraptor had very large deltopectoral crests on its upper arm bones, and various other muscle attachment sites suggest that its arms were strong. The hand claws of Megaraptor were very long and strongly curved, with the claw of the first finger measuring 35 cm (14 in) in total length; the claw of the third finger was the smallest, only 6.5 cm (2.6 in) in length. Similar to dromaeosaurids and birds of prey, these claws may have been lengthened considerably by a sheath of keratin.

The depositional environment of the Portezuelo Formation was a fluvial (river) deposit, with a humid climate. Megaraptor would have shared its ecosystem with various other non-avian archosaurs, including the abelisaurid Elemgasem, the titanosaurian sauropod Futalogknosaurus, the unenlagiine theropod Unenlagia, and the azhdarchoid pterosaur Argentinadraco. It was initially suggested that Megaraptor may have used its hand claws to open carcasses, though a role in active predation is now generally favoured. Due to the relatively gracile construction of its skull compared to that of other theropods, Megaraptor, along with other megaraptorans such as Australovenator, may have relied on its large hand claws to grab prey items, pulling them towards its chest so that they could be dispatched by its jaws.