Australovenator

''Australovenator'' (meaning "southern hunter") is a genus of megaraptoran theropod dinosaur from Cenomanian (Late Cretaceous)-age Winton Formation (dated to 95 million years ago) of Australia. Some specimens from the Albian-aged Eumeralla Formation and the Wonthaggi Formation may belong to ''Australovenator''. It is known from partial cranial and postcranial remains, which were described in 2009 by Scott Hocknull and colleagues, although additional descriptions and analyses continue to be published. It is the most complete predatory dinosaur discovered in Australia. It has been suggested that ''Australovenator'' is a sister taxon to '' Fukuiraptor'', although some phylogenetic analyses find it to be a more derived member of the Megaraptora, possibly being part of the main Megaraptoridae family itself.

History of discovery

''Australovenator'' is based on a theropod specimen ( AODF 604), affectionately nicknamed "Banjo" after Banjo Paterson, which was found intermingled with the remains of the sauropod '' Diamantinasaurus matildae'' at the "Matilda site" ( AODL 85). The parts of the holotype as it was initially described, which are held at the Australian Age of Dinosaurs Museum of Natural History, consists of a left dentary, teeth, partial forelimbs and hindlimbs, a partial right ilium, ribs, and gastralia. ''Australovenator'' was described in 2009 by paleontologist Scott Hocknull of the Queensland Museum and colleagues. The type species is ''A. wintonensis'', in reference to nearby Winton. Although the holotype was first discovered in 2006 and first described in 2009, the process of excavating the "Matilda site" is still ongoing, and papers describing new elements of the holotype are still being published.
Additional arm elements of the holotype were described in 2012, more leg elements were described in 2013, and a right dentary was described in 2015.

For some time, Australovenator was considered to be within the same genus as another Australian Megaraptorid called '' Rapator''. ''Rapator'' is based on a metacarpal first described by Friedrich von Huene during the early 1900s and prior to the recognition of Megaraptora, it was considered to be either an alvarezsaurid or an intermediate theropod. After the discovery of Australovenator, some scientists noted the similarity between ''Rapator'' and ''Australovenator''. While Hocknull ''et al.'' (2009) identified a few distinguishing characters between the two taxa, based on a poorly preserved metacarpal I from the holotype of ''Australovenator''. Agnolin ''et al.'' (2010) reclassified ''Rapator'', instead finding it a megaraptoran, potentially a sister taxon to ''Australovenator''. They mentioned that ''Megaraptor'', the only other taxon also preserving metacarpal I, was less similar to ''Rapator'' than ''Australovenator''. However, there were no clear differences between the two latter taxa. The metacarpals of both taxa were redescribed in White ''et al.'' (2014), who determined that they were not synonymous, adding multiple features to the potential characters identified by Hocknull ''et al.'' and Angolan ''et al.''

In 2019, material from the Eumeralla Formation (Albian age) was referred to cf. ''Australovenator.'' In 2020, a heavily eroded specimen was described, belonging to an indeterminate megaraptoran found near the type locality. The fossil material comprises "two fragmentary vertebrae, three partial metatarsals and the distal end of a pedal phalanx" as well as other indeterminate bone fragments. The animal was slightly larger than the holotype individual of ''Australovenator''.

Description

According to Gregory S. Paul, it was estimated at long, with a body mass of . A 2014 study estimated its body mass to be around . Because it was a relatively lightweight predator, Hocknull coined it as the "cheetah of its time". Like other megaraptorans, ''Australovenator'' would have been a bipedal carnivore.

Classification

A phylogenetic analysis found ''Australovenator'' to be an allosauroid carnosaurian, with similarities to '' Fukuiraptor'' and carcharodontosaurids. In the initial analysis, it was shown to be the sister taxon of the Carcharodontosauridae. More detailed studies found that it formed a clade with several other carcharodontosaurid-like allosaurs, the Neovenatoridae. Recent phylogenetic analysis suggests ''Australovenator'' is a tyrannosauroid, like with all other megaraptorans. A phylogenetic analysis in 2016 focusing on the new neovenatorid ''Gualicho'' found that ''Australovenator'' and other megaraptorids were either allosauroids or basal coelurosaurs as opposed to being tyrannosauroids.

The ankles of ''Australovenator'' and ''Fukuiraptor'' are similar to the Australian talus bone known as NMVP 150070 that had previously been identified as belonging to ''Allosaurus'' sp., and this bone likely represents ''Australovenator'' or a close relative of it. Alternatively, this bone could belong to an abelisaur.

The cladogram below follows the 2010 analysis by Benson, Carrano, and Brusatte. Another study published later in 2010 also found the Australian theropod '' Rapator'' to be a megaraptoran extremely similar to ''Australovenator''.

The cladogram below follows the 2014 analysis by Porfiri ''et al''. that finds megaraptorans to be tyrannosauroids.

Palaeobiology

With very comprehensive and well-preserved hand and foot remains, ''Australovenator'' has been made a topic of various research papers studying the dynamics of theropod appendages.

A 2015 study tested the range of motion of ''Australovenator's'' arms using computer models and found that it had flexible arms, with the forearms capable of making an angle of 144 to 66 degrees with the humerus, an elbow range of motion similar to that of maniraptoriforms. Unusually, its radius could slide independently of the ulna when its arm was flexed, similar to that of birds but unlike most non-avian dinosaurs. However, the study also found that ''Australovenator's'' fingers were capable of extension far beyond those of any other sampled theropod, with only ''Dilophosaurus'' having capabilities even near it. This study concluded that ''Australovenator's'' flexibility, facilitated by a combination of traits in both primitive and advanced theropods, played a role in prey capture, giving it the ability to grasp prey towards its chest to make it easier for its weak jaws to disembowel food. The gracile morphology of the skull also concludes that this genus had a specialisation towards prey capture using its arms and hands.

A 2016 study used CT scans of an emu foot to digitally reconstruct the musculature and soft tissue of an ''Australovenator'' foot, as well as determine how soft tissue affects flexibility. The study determined that muscular range of motion is often overestimated when not accounting for soft tissue and that soft tissue reconstruction is vital for making future analyses of theropod flexibility more accurate. A review of hindlimb elements described in 2013 re-identified several phalanges which were initially positioned incorrectly. In addition, it noted that ''Australovenator's'' phalanx II-3 was splayed, a pathology that may have resulted from the impacts of kicking motions. Some modern birds, such as the cassowary, are known to use their second toe as weapons in defensive or territorial fights.

A 2017 follow-up to the 2016 study used a 3-D printed model of the reconstructed foot to make footprints in a matrix of clay and sand in an effort to understand the creation of dinosaur footprints. The study specifically was designed to clarify the identity of particular controversial footprints from Lark Quarry, which may have been left from either a large theropod (like ''Australovenator'') or an ornithopod (like ''Muttaburrasaurus''). The study found that the artificial ''Australovenator'' footprints were similar to those at Lark Quarry, concluding that the trackways in question were likely those of a theropod. The writers of the study expressed interest in creating a reconstruction of a ''Muttaburrasaurus'' foot as an extension of the study, although no ''Muttaburrasaurus'' pedal material is known.

Palaeoecology

AODL 604 was found about northwest of Winton, near Elderslie Station. It was recovered from the lower part of the Winton Formation, dated to the late Cenomanian. AODL 604 was found in a clay layer between sandstone layers, interpreted as an oxbow lake, or billabong, deposit. Also found at the site were the type specimen of the sauropod '' Diamantinasaurus'', bivalves, fish, turtles, crocodilians, and plant fossils. The Winton Formation had a faunal assemblage including bivalves, gastropods, insects, the lungfish ''Metaceratodus'', turtles, the crocodilian ''Isisfordia'', pterosaurs, and several types of dinosaurs, such as the sauropods '' Diamantinasaurus'' and ''Wintonotitan'', and unnamed ankylosaurians and hypsilophodonts. Plants known from the formation include ferns, ginkgoes, gymnosperms, and angiosperms.

References

External links

{{Taxonbar, from=Q131278

Megaraptora
Dinosaur genera
Cenomanian dinosaurs
Winton Formation
Fossil taxa described in 2009
Taxa named by Scott Hocknull
Dinosaurs of Australia