Megaraptora

Megaraptorans Temporal range: Early Cretaceous - Late Cretaceous, 130–83 Ma PreЄ Є O S D C P T J K Pg N
Diagram showing the skull and skeleton of Murusraptor
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	Saurischia
Suborder:	Theropoda
Clade:	Avetheropoda
Clade:	†Megaraptora Benson, Carrano & Brusatte, 2010
Subgroups
†Aoniraptor? †Bahariasaurus?[1] †Fukuiraptor †Rapator †Megaraptoridae †Aerosteon †Australovenator[2] †Megaraptor †Murusraptor †Orkoraptor †Tratayenia

Megaraptora is a group of carnivorous theropod dinosaurs with controversial relations to other theropods.

Description

Megaraptorans are medium to large-sized theropods, ranging from about 4.2 meter (14 feet) long Fukuiraptor to the 9 meter (30 feet) long Aerosteon, and the 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.

They likely had long and low snouts, with very long nares and flat nasals, as evidenced by the long anterodorsal process of Murusraptor's lacrimal as well as the remains a juvenile Megaraptor specimen. Their lower jaws were also long and graceful according to dentaries found in the holotype of Australovenator.^[3][4]

Megaraptoran teeth show much variety between genera, although they are generally small compared to the snout with minimal enamel ornamentation. Some megaraptorans, such as Orkoraptor and Megaraptor, had teeth which were 8-shaped in cross section and completely unserrated from the front (similar to dromaeosaurids and compsognathids)^[3] while Murusraptor had anterior serrations only at the tip of its teeth and Australovenator had extensive serrations on both sides.^[4] Fukuiraptor had very laterally compressed and blade-like teeth (similar to carcharodontosaurs) with both anterior and posterior serrations.

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.^[4]

The slender leg bones and long metatarsals of several species indicate that this group likely had cursorial habits.

The most characteristic trait of this group is the enlarged 1st and 2nd fingers of the hand. These fingers, particularly the 1st, bear massive grooved claws which were flattened from the side. However, the 3rd claw of the hand was much smaller than those of most theropods. The first megaraptoran remain discovered, the 1st claw of Megaraptor, was initially believed to belong to the foot of a giant dromaeosaurid, hence why it was given the name 'Megaraptor'.^[5]

The most advanced South American members of this clade belong to the family Megaraptoridae, although the family name is sometimes used to refer to the whole clade of Megaraptora.

Evolution and origin

The origins of megaraptorans have recently been determined. Studies by paleontologists Phil Bell, Steve Salisbury et al. of a newly discovered, as-yet-unnamed megaraptorid (referred to by the public media as "Lightning Claw," and possibly synonymous with Rapator) from opal fields southwest of Lightning Ridge, Australia, dating back 110 million years ago reveals that megaraptorans likely evolved in Australia, then spread to the rest of Gondwana in an episode of evolutionary radiation. The specimen also allowed for alternative phylogenetic testing as to the placement of megaraptorans as either tyrannosauroids or carcharodontosaurids.^[6][7]

Classification

Megaraptora has historically been a group with highly controversial relationships.

Early phylogenetic studies of the group's relationships conducted by Benson, Carrano and Brusatte in 2010 and Carrano, Benson and Sampson in 2012 recovered the group as a branch of the allosauroids (specifically within the family Neovenatoridae), part of a large group of theropods that also includes the metriacanthosaurids, carcharodontosaurids, and allosaurids. This would make megaraptorans the last surviving allosauroids; at least a few megaraptorans, including Orkoraptor, lived near the end of the Mesozoic era, dating to the late Santonian stage of the late Cretaceous period, about 84 million years ago.^[8][9] Another study published later in 2010 found the Australian theropod Rapator to be a megaraptoran extremely similar to Australovenator.^[10]

On the other hand, Novas et al. published a study in 2012 which, while confirming that Neovenator was closely related to carcharodontosaurids, also found Megaraptor and its relatives to be coelurosaurs closely related to tyrannosaurids.^[11] Study of the skull anatomy of a juvenile specimen of Megaraptor also suggested to Novas and his colleagues that it was a tyrannosauroid.^[3] Analysis of the theropod Gualicho published by Apesteguía and colleagues in 2016 recovered megaraptorans as either allosauroids or basal coelurosaurs, and depended on whether Gualicho's anatomy was plugged into Carrion's analysis or Novas' analysis. This suggested that the controversial placement of megaraptorans was more of a consequence of incomplete analyses than to their actual anatomy.^[2]

Later in 2016, Novas and colleagues published a study of megaraptoran hand anatomy, in an attempt to help settle the question of their classification. They found that megaraptorans lacked most of the key features in the hands of derived coelurosaurs including Guanlong and Deinonychus. Instead, their hands retain a number of primitive characteristics seen in basal tetanurans such as Allosaurus. Nevertheless, there are still a number of traits that support megaraptorans as members of the Coelurosauria.^[5]

The cladogram below follows Coria & Currie (2016), based on the Carrano et al. (2012) dataset.^[4][12]

The cladogram below follows the strict consensus (average result) of the 12 most parsimonious trees (the simplest evolutionary paths, in terms of the total amount of sampled features evolved or lost between sampled taxa) found by Porfiri et al. (2018)'s phylogenetic analysis.^[13] This analysis found that megaraptorans were basal coelurosaurs, outside of Tyrannosauroidea. Although the results are different, the methodology analysis was practically identical to that of Apesteguia et al. (2016), only differing in the fact that it incorporated Tratayenia and Murusraptor, two megaraptorans not sampled in the analysis of Apesteguia et al.^[2]

References