Megaraptoran

Megaraptora is a clade of carnivorous tetanuran theropod dinosaurs with controversial relations to other theropods. Its derived members, the Megaraptoridae are noted for their elongated hand claws and proportionally large arms, 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.

The clade was originally named in 2010 as a subset of the family Neovenatoridae, a group of lightly-built allosauroids related to the massive carcharodontosaurids such as '' Giganotosaurus'' and '' Carcharodontosaurus''. A 2013 phylogenetic analysis by Fernando Novas and his colleagues disagreed with this classification scheme, and instead argued that the megaraptorans evolved deep within Tyrannosauroidea, a superfamily of basal coelurosaurs including the famous '' Tyrannosaurus''. Subsequent refinements to Novas's data and methodologies have supported a third position for the group, at the base of Coelurosauria among other controversial theropods such as '' Gualicho'', but not within the Tyrannosauroidea. Regardless of their position, it is clear that megaraptorans experienced a large amount of convergent evolution with either ''Neovenator''-like allosauroids or basal coelurosaurs.

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 4.2 meters (13.8 feet) in length, to the 9 meter (30 feet) long '' Aerosteon'', the 9 to 10 meter (30 to 33 feet) long '' Maip'' and the 12.8 meter (42 foot) 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 (forearm 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

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