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DinoFan83 · 04-06-2021, 07:50 AM

Megaraptor ("giant thief") is a genus of unusual theropod dinosaur that lived in the Turonian to Coniacian ages of the Late Cretaceous. Its fossils have been discovered in the rocks of the Portezuelo Formation in what is now Neuquén Province, Argentina. The holotype specimen was thought to be a giant dromaeosaurid at the time of naming in 1998 due to a very large claw interpreted as belonging to the second toe (as in dromaeosaurids). A more complete specimen described in 2004, however, showed the claw to not belong to the second toe at all but rather to the first digit of the manus, nullifying the previously proposed dromaeosaurid affinities.
The manus itself was found to be quite distinct from other known theropods at the time of its discovery, so it was not initially clear whether Megaraptor belonged to any known family of Gondwanan theropods or to a new family of theropods altogether. Early phylogenetic analyses placed Megaraptor as a basal tetanuran (both as an allosauroid and a spinosauroid), with the allosauroid theory becoming more widely accepted when other close relatives of Megaraptor (like Aerosteon and Orkoraptor) were discovered and found to form the family Megaraptora with it.
New juvenile material of Megaraptor published in 2014 has established Megaraptor (and all other megaraptorans) were tyrannosauroids instead of allosauroids or spinosauroids, and that has been upheld very well, with almost all phylogenetic analyses since then recovering them as such. But, while bearing much morphological resemblance to the primitive tyrannosauroid family Proceratosauridae, megaraptorans were unlike the well-known tyrannosaurids in that they underwent forelimb enlargement instead of reduction and had very different ecology as a result of it.
Megaraptor itself was no exception to this. It would have had large, powerfully-built forelimbs bearing three-fingered hands, with an enlarged claw on the first digit. The humerus was long and robust with a prominent deltopectoral crest, and the ulna had a hypertrophied, blade-like olecranon process, indicating the arm muscles (like the triceps and deltoids) were very well-developed for use in prey capture. The manus was very elongated to facilitate grasping of prey items, as was the first digit's claw, with the largest claws found so far being well over 35 centimeters long and quite sharp to boot. In fact, current estimates suggest the forelimbs of Megaraptor were both longer and more robust (therefore almost certainly more powerful) relative to the animal's size than the forelimbs of carnivorous mammals like big cats or bears, strongly indicating it had a need just as great as - if not greater than - said carnivorous mammals for forelimb use in prey acquisition.
The rest of the animal showed similarities to various other tyrannosauroids. Cranial material shows the skull was long-snouted with many small and sharp teeth (similar to the primitive tyrannosauroids Dilong and Xiongguanlong), and the preserved portions of the axial skeleton show the torso would have been deep and wide (akin to derived tyrannosaurids). The hindlimbs were very distally elongated with particularly long, gracile metatarsals, no different to the nearly universal cursorial adaptations in almost all of Tyrannosauroidea, suggesting Megaraptor was probably a good runner.
Megaraptor was a large theropod. Known adult specimens were at least 6.5 meters long, and have been estimated to weigh 1000 kg by Gregory S. Paul. Owing to its size, it may have been the apex predator of the Portezuelo, dominant over the much smaller Unenlagia and unnamed abelisaurids.

DinoFan83 · 04-06-2021, 08:40 AM

Megaraptor claw by Duffymeg on Wikimedia Commons.