Megaraptora is a clade of carnivorous theropod dinosaurs. Its derived members, the Megaraptoridae are noted for their large hand claws and powerfully-built forelimbs, which are usually reduced in size in other large theropods.
Megaraptorans are incompletely known, and no complete megaraptoran skeleton has been found. However, they still possessed a number of unique features. Their forelimbs were large and strongly built, and the ulna bone had a unique shape in members of the family Megaraptoridae, a subset of megaraptorans which excludes Fukuiraptor and Phuwiangvenator. The first two fingers were elongated, with massive curved claws, while the third finger was small. Megaraptoran skull material is very incomplete, but a juvenile Megaraptor described in 2014 preserved a portion of the snout, which was long and slender. Leg bones referred to megaraptorans were also quite slender and similar to those of coelurosaurs adapted for running. Although megaraptorans were thick-bodied theropods, their bones were heavily pneumatized, or filled with air pockets. The vertebrae, ribs, and the ilium bone of the hip were pneumatized to an extent which was very rare among theropods, only seen elsewhere in taxa such as Neovenator. Other characteristic features include opisthocoelous neck vertebrae and compsognathid-like teeth.
Although undoubtedly members of the clade Tetanurae, their relationships to others members of this group have been subject to dispute for over a decade. Originally, megaraptorans were placed as basal tetanurans as part of the family Neovenatoridae within the allosauroid clade Carcharodontosauria. By the early 2020s, while a handful of studies still supported an allosauroid classification, many studies have recovered megaraptorans as members of Coelurosauria. However, their exact position within this clade is uncertain, with some studies recovering them as members of Tyrannosauroidea.
Megaraptorans were most diverse in the early Late Cretaceous period of South America, particularly Patagonia. However, they had a widespread distribution. Phuwiangvenator and Fukuiraptor, the most basal and second most basal known members of the group, lived in Thailand and Japan, respectively. Megaraptoran material is also common in Australia, and the largest known predatory dinosaur from the continent, Australovenator, was a megaraptoran.
Description
Megaraptorans were medium to large-sized theropods, ranging from Fukuiraptor, which was about in length, to the long Orkoraptor, the long Maip and the long Bahariasaurus, if it is a member. Most megaraptorans are known from very fragmentary remains, although certain characteristics can be identified in multiple members of the clade. At least some megaraptorans, such as Murusraptor and Aerosteon, had extensively pneumatic bones (most noticeably the ilia and ribs), which likely housed sinuses connected to the lungs, similar to modern birds. The slender leg bones and long metatarsals of several species indicate that members of this group likely had cursorial habits. Most megaraptorans are part of the family Megaraptoridae, which was named by Fernando Novas and his colleagues in 2013. This family is united by several adaptations of the ulna and claws which are not present in the basal megaraptoran Fukuiraptor.
Skull and teeth
No megaraptoran fossil is known to preserve a complete skull, although skull material is known for several taxa. Aerosteon, Megaraptor,Orkoraptor, and Murusraptor preserve several bones of the rear part of the skull, lower jaws are known from Australovenator, and a juvenile specimen of Megaraptor described in 2014 preserved much of the snout as well as parietal fragments. Teeth have been found in many genera. Collectively, megaraptorans can be reconstructed as having a long, lightly built skull with many relatively small teeth.
Based on Megaraptor, the premaxillary bone at the tip of the snout is small, with a long and rod-like branch of bone which extends above the external nares (nostril holes). The nares themselves were very large and elongated, akin to some early tyrannosauroids (Dilong, Proceratosaurus, etc.). The snout also had some similarities to carcharodontosaurids, namely the straight upper edge of the maxilla and rectangular nasal bones. The parietal bones at the top of the skull, behind the eyes, had a strongly developed sagittal crest, as in tyrannosauroids. Otherwise, the rear part of the skull is rather simple, without any pronounced crests or bosses, although the lacrimal and postorbital bones did have rugose patches in some genera. Aerosteon and Murusraptor possessed a pneumatic quadrate, as in a few allosauroids (Sinraptor, Mapusaurus) and tyrannosauroids. The dentary, which is only known in Australovenator, is long and graceful, with the first tooth smaller than the rest (as in tyrannosauroids). The mandible as a whole has only a single meckelian foramen, as in carcharodontosaurians, tyrannosaurids, and ornithomimids. However, the rear part of the mandible (as seen in Murusraptor) was significantly more lightly built than that of tyrannosauroids. Preserved braincase material has similarities to both carcharodontosaurians and tyrannosauroids.
The premaxillary teeth of Megaraptor were variably similar to those of tyrannosauroids, being small, incisiform (chisel-like) and D-shaped in cross section. However, Murusraptor's premaxillary teeth were fang-like, as in non-tyrannosauroid theropods. Megaraptoran maxillary teeth show much variety between genera, although they were generally small compared to the snout with minimal enamel ornamentation. Some megaraptorans, such as Orkoraptor, Australovenator, and Megaraptor, had teeth which were 8-shaped in cross section and completely unserrated from the front (similar to dromaeosaurids and compsognathids), while Murusraptor had anterior serrations only at the tip of its teeth. Fukuiraptor had very laterally compressed and blade-like teeth (similar to carcharodontosaurs) with both anterior and posterior serrations.
Vertebrae and ribs
The cervical (neck) vertebrae of megaraptorans were nearly unique among theropods in the fact that they were strongly opisthocoelous. This means that they were convex from the front and concave from behind. Opisthocelous vertebrae are also characteristic of Allosaurus and sauropods, and they may facilitate high flexibility without sacrificing defense against shear forces. Otherwise, the cervicals were similar to those of carcharodontosaurians, with short neural spines, transverse processes (projecting rib facets) located around mid-length on the centra, and a pair of large lateral pits known as pleurocoels. In fact, one or more pleurocoels were present in most megaraptoran vertebrae, and they connected to a complex system of numerous small air pockets within the vertebrae. This web-like internal structure of megaraptoran vertebrae (and that of a few other theropods) has been described as "camellate".
The proximal caudals (vertebrae at the base of the tail) had a longitudinal ridge running along their lower surface, similar to the case in Neovenator but unlike tyrannosauroids. They also had a pair of lateral ridges which stretched downwards from the transverse processes to the centra. These ridges, known as centrodiapophyseal laminae, defined a large depression (infradiapophyseal fossa) under the transverse processes. Although these ridges were also present in dorsal (back) vertebrae and have been found in other theropods, megaraptorans were practically unique in the fact that their centrodiapophyseal laminae were well-developed at the base of the tail, sometimes even more so than the dorsal vertebrae. Only spinosaurids share this feature. The strong development of these ridges may indicate that the tail was deep and muscular.
The dorsal ribs were thick and curved yet hollow and pierced by a hole near their connection to the vertebrae. The gastralia (belly ribs) were wide and strongly built paddle-shaped structures, with the left and right sides fused at the midline of the chest. These features signified that megaraptorans were wide-bodied theropods, akin to the condition in tyrannosaurids.
Forelimbs Megaraptorans have a sigmoid (S-shaped) humerus (upper arm bone), similar to that of both basal allosauroids and basal coelurosaurs. Most megaraptorans had large, robust humeri akin to those of Allosaurus, but the basal-most member Fukuiraptor has a much more slender humerus. The distal part of the humerus (near the elbow) has a well-developed system of condyles and grooves similar to that of coelurosaurs, particularly the dromaeosaurids.The ulna of megaraptorids is characteristic in several regards. The olecranon process is well-developed, though it is thin, blade-like, and extends as a crest longitudinally down the shaft of the ulna. In addition, megaraptorids have acquired another long, crest-like structure on the ulna called the lateral tuberosity, which is perpendicular to the blade of the olecranon. As a result, the ulna of megaraptorids is T-shaped in cross section, with three prongs formed by the forward-projection anterior process, the outwards-projecting lateral tuberosity, and the backwards-projecting olecranon process. These adaptations are absent in the most basal megaraptoran, Fukuiraptor. The radius is not unusual compared to other theropods.
Megaraptorans also had very characteristic hands. The first two fingers were large and slender, but the third one was small. These relative differences in finger length are somewhat similar to the case in tyrannosauroids and various other basal coelurosaurs, but the megaraptoran trend of forearm and finger enlargement is opposite to the trend of forearm diminishment which characterizes the lineage towards more advanced tyrannosauroids. Megaraptor retained a vestigial fourth metacarpal, the hand bone that would have connected to the fourth finger in early dinosaurs. This was a primitive feature lost by most other tetanurans. The first two fingers had absurdly large unguals (claws); in Megaraptor the first claw was larger than the entire ulna. Unlike the large unguals of many other theropods (megalosauroids, for example), megaraptoran claws were thin and oval-shaped in cross-section. These claws also had asymmetrically positioned grooves on their flat faces and a sharp ridge on their lower edge in megaraptorids (non-Fukuiraptor megaraptorans). The carpus (wrist) of megaraptorans incorporated a semilunate (crescent-shaped) carpal similar to that of maniraptorans.
Examinations of the forelimbs of megaraptorans by Rolando, Novas, and Porfiri et al., that were published in January 2023 show that the megaraptorans' forelimb bones are remarkably well-developed; powered by strong pectoral and front limb muscle that were functionally significant and important to the paleobiology of this group of theropods. Their data also suggests these muscle attachments became increasingly pronounced through megaraptoran evolutionary history, being substantially better developed in derived taxa such as Australovenator and especially Megaraptor itself than in earlier genera such as Fukuiraptor. Their results further suggest that the highly specialized forelimbs were capable of highly complex movements, such as great extension and flexion, particularly in the highly derived hands, as well as enhanced humeral protraction; attributes that likely aided in prey capture.
Hindlimbs