Megaraptor

''Megaraptor'' () is a genus of large theropod dinosaur that lived during the Turonian and Coniacian ages of the Late Cretaceous. Its fossils have been discovered in the Patagonian Portezuelo Formation of Argentina, South America. 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. Subsequent discoveries revealed that ''Megaraptor'''s large claw actually came from its hand. The taxonomic position of it and related genera (collectively known as megaraptorans) is unknown, though it is generally regarded as either a neovenatorid allosauroid or a coelurosaur, possibly related to 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 in length, and weighed around . Its skull was long, low, and slender. Similar to tyrannosaurs, it had small, conical teeth at the front of its jaws, and longer, more curved teeth near the back. The hand claws of ''Megaraptor'' were very long and strongly curved, with the claw of the first finger measuring . The claw of the third finger was the smallest, only in length.

History of discovery

In January 1996, Argentine palaeontologist Fernando E. Novas recovered the fragmentary remains of a large theropod, consisting of a right ulna, left manual phalanx, part of a right metatarsal, and a very large ungual phalanx (claw). It was discovered in strata belonging to the Portezuelo Formation, part of the Río Neuquén Subgroup in Neuquén, northwestern Patagonia. The specimen, catalogued as MCF-PVPH 79, was transported to the Museo Carmen Funes, a palaeontological collection in Plaza Huincul. In December 1997, Novas presented a cast of the ungual, which he believed to come from the second digit of the foot, to the Houston Museum of National Sciences. The next year, he described it in a paper published in the ''Journal of Vertebrate Paleontology''. Believing that the new taxon was somewhat related to dromaeosaurids and troodontids, (though noting that a more basal position was possible), Novas gave it the binomial name ''Megaraptor namunhuaiquii''. The genus name derives from the Greek ''mega'' (large) and the Latin ''raptor'' (thief), while the species name derives from the Mapuche ''namun'' (foot) and ''huaiqui'' (lance). In 2004, a second ''Megaraptor'' specimen (MUCPv 341), was described in a paper helmed by Jorge O. Calvo. Consisting of a right ulna, radius and manus found in association with a sauropod femur, it demonstrated that the large ungual belonged to the first digit of the manus, as opposed to the second digit of the foot. In 2014, a third ''Megaraptor'' specimen (MUCPv 595), consisting of the partial skeleton of a juvenile, was described by a team led by Juan D. Porfiri. A fourth (MUCPv 278), consisting of a humerus, was described in 2025.

Description

South American megaraptorids were generally very large, exceeding in length and in mass. The holotype of ''Megaraptor'' was estimated by Fernando Novas to measure around in length. In 2010, Gregory S. Paul estimated its length at , its weight at .

Skull

The skull of ''Megaraptor'' is known from a single juvenile specimen, which preserves both premaxillae and maxillae, nasals, a left frontal, and a partial braincase. The premaxilla is fairly small and bears several large foramina, as in many tyrannosauroids. The maxilla is long and subtriangular, and has a combination of coelurosaur traits, such as the lengthening of the anterior (front) maxillary ramus, and allosauroid traits, such as the straightness of the dorsal (upper) margin. At the same time, there are traits that match neither group, such as the overall morphology of the maxillary ramus. The nasals were paired and unfused, though this may be attributed to the juvenile nature of the known skull. Similarly, the nasal rugosities present in other many other basal tetanurans are absent, though this may again be due to the specimen's age upon death. The frontal is quadrangular, with a wide supratemporal fossa, bound by a strong ridge which contacted that of the opposite frontal, very similar to the condition seen in tyrannosauroids and unlike that of allosauroids. The braincase, too, has a mosaic of coelurosaur (particularly tyrannosauroid) and allosauroid traits. At least four teeth were present in each premaxilla, and presumably fifteen were present in each maxilla. ''Megaraptor'''s teeth were heterodont, meaning that two different tooth shapes were present: the premaxillary teeth were short and conical, whereas the maxillary teeth were larger and more recurved. This resembles the pattern seen in tyrannosauroids.

Postcranial skeleton

''Megaraptor'''s neck contained ten cervical vertebrae. The neural spine of the axis (the second cervical vertebra) was tall and narrow, with a convex dorsal margin, resembling basal coelurosaurs like ''Scipionyx'', while differing from allosauroids and derived tyrannosauroids. All of the cervical vertebrae were opisthocoelous, meaning that they were convex anteriorly and concave posteriorly (towards the back). The back contained twelve dorsal vertebrae. The first four neural spines were short dorsoventrally (top-to-bottom) and anteroposteriorly (front-to-back), while those further along the vertebral column were longer and taller. Eight gastralia, bones which support the abdominal organs and serve as muscle attachment points, are known. They resemble those of carcharodontosaurids and tyrannosaurids, while differing from those of most non-tyrannosaur coelurosaurs. The exception is '' Sinocalliopteryx'', which has very similar gastralia. The gastralia were large, being slightly shorter than the dorsal ribs, similar to the condition seen in other large theropods. The sacrum, a mass of fused vertebrae which sat at the hips, contained five vertebrae. Only two caudal (tail) vertebrae are known from ''Megaraptor'', both possessing blade-like neural spines as deep as those of ''Allosaurus''.The pectoral girdle of ''Megaraptor'' differed from that of carcharodontosaurids in that its scapula was slenderer, and the acromial process was shallower, though resembled that of ''Allosaurus''. The coracoid was almost flat, and had a slightly developed biceps tuber. Characteristic of megaraptorans, there was a deep depression behind the glenoid. Though not known from ''Megaraptor'' specifically, megaraptorans overall had large, robust deltopectoral crests. Megaraptorans as a whole had attachment sites for strong forelimb flexor and extensor muscles. ''Megaraptor'''s forearm, measured from the proximal (close to the body) end of the radius to the distal (far from the body) end of the first digit, measured , with the manus alone comprising around of that. Though fragmentary, the carpals of ''Megaraptor'' are known to have been semi-lunate (crescent-shaped), similar to the related '' Australovenator'' and to certain coelurosaurs. The hands were unusually elongate.Calvo, J.O., Porfiri, J.D., González-Riga, B.J., and Kellner, A.W. (2007) "A new Cretaceous terrestrial ecosystem from Gondwana with the description of a new sauropod dinosaur". ''Anais da Academia Brasileira de Ciências'', 79(3): 529–4

/ref> The first ungual (claw) was very large, and, if measured along its curve, was around in length. The second ungual was smaller , while the third was the smallest . Each of ''Megaraptor'''s unguals was strongly curved, more so than in Spinosauridae, spinosaurids.

Classification

In his paper describing ''Megaraptor'', Fernando Novas expressed uncertainty about its taxonomic position. While he tentatively placed it within Coelurosauria, and named it based on perceived similarities to dromaeosaurids and troodontids, he noted similarities to more basal theropod clades. In their reappraisal of the genus, Calvo et al. discounted the hypothesis that it was a coelurosaur, and suggested that it was instead a basal tetanuran, belonging to a clade that predated the split between allosauroids and coelurosaurs. Roger B. J. Benson, Matthew T. Carrano and Stephen L. Brusatte erected the clade Megaraptora to encompass ''Megaraptor'' and its closest relatives. Megaraptorans were placed within the family Neovenatoridae, and were therefore considered carcharodontosaurs. The 2014 paper describing a juvenile ''Megaraptor'' suggested, based on a phylogenetic analysis, that megaraptorans were nested within Tyrannosauroidea.

The cladogram shown below follows an analysis by Porfiri ''et al.'', 2014.

In the 2022 description of '' Maip'', Rolando ''et al.'' noted the presence of two distinct megaraptoran clades: a more inclusive clade, comprising all megaraptorids except '' Fukuiraptor'' and ''Australovenator'', (shown below as "Clade A"), and a more exclusive clade of larger, entirely South American megaraptorids (shown below as "Clade B"). Like some previous analyses by other authors, Megaraptora is nested within Coelurosauria, as the sister taxon to Tyrannosauroidea. The cladogram below displays the megaraptoran results of the phylogenetic analyses by Rolando ''et al'' (2022).

Paleoecology

''Megaraptor'' is known from the Late Turonian to Early Coniacian-dated Portezuelo Formation of Argentina. Other named dinosaurs known from the formation include the titanosaurian sauropods '' Futalognkosaurus'', '' Baalsaurus'', and '' Malarguesaurus'', and several other theropod taxa including the dromaeosaurids '' Neuquenraptor'', ''Unenlagia'', and '' Pamparaptor'', the alvarezsaurid ''Patagonykus'', and the abelisaurid '' Elemgasem''. Indeterminate remains belonging to an unnamed megaraptorid, a possible noasaurid, and ornithopods have also been recovered from the formation. Fossils of teleost fish ('' Leufuichthys''), turtles ('' Portezueloemys'' and a species of ''Prochelidella''), birds, and pterosaurs ('' Argentinadraco'') are also known.

References

External links

Science Daily

{{Portal bar, Dinosaurs, Paleontology

Megaraptora
Dinosaur genera
Turonian dinosaurs
Portezuelo Formation
Fossil taxa described in 1998
Taxa named by Fernando Novas
Dinosaurs of Argentina