Australovenator (meaning "southern hunter") is a genus of megaraptoran theropod dinosaur that lived during the Late Cretaceous period in what is now Australia. The holotype (name-bearing) specimen consists of a partial skeleton that was discovered in rocks of the Winton Formation in near Elderslie Station in 2006. Its remains, including elements of the forelimbs, hindlimbs, pelvis, and lower jaws, were found intermingled with those of the sauropod dinosaur Diamantinasaurus. These rock layers derive from the Cenomanian stage of the Cretaceous period, about 95 million years ago. Australovenator was described by Australian paleontologist Scott Hocknull and colleagues in 2009. The genus contains a single species, A. wintonensis, though fragmentary remains from the Eumeralla and Wonthaggi Formations may belong to Australovenator as well.
Australovenator was around long and weighed . This makes it smaller than most South American megaraptorans, but among the largest carnivorous dinosaurs known from Australia. Although a skull is not known, it likely had an elongated, slender head like other megaraptorans. Its dentaries (lower jaws) are lightly-built and have 19 teeth each. The forelimbs are massive, with bowed, S-shaped humeri (upper arm bones) and terminate in giant, sharp claws. Its hindlimbs are relatively gracile, with a thin metatarsus in contrast to the robust metatarsi of theropods like Chilantaisaurus. Most phylogenetic analyses recover Australovenator as a basal member of Megaraptoridae, a family of long-armed, carnivorous tetanurans that occupied Australia, South America, Asia and possibly Africa and North America during the Cretaceous period. However, the higher relationships of Megaraptora are uncertain. The Winton Formation is dominated by sandstone, indicating an oxbow lake or similar lacustrine environment. Other dinosaurs known from this site include the sauropods Diamantinasaurus and Wintonotitan, an unnamed ankylosaur, and a larger, possibly distinct, megaraptoran.
History of discovery
Initial description
Prior to the discovery of Australovenator, Australian theropods were only represented by taxa known from fragmentary and/or isolated material.Long, J.A. (1998). Dinosaurs of Australia and New Zealand and other animals of the Mesozoic Era. Harvard University Press, UNSW Press, pp 94–96. Beginning in 2006, the Australian Age of Dinosaurs Museum of Natural History (AODF) and the Queensland Museum began a series of expeditions in the "Matilda Site" at Elderslie Station. Fossils at the Station were first identified by the owners of property, with the first discovery being sauropod bone fragments. This locale is located north-west of Winton, central Queensland, Australia. In 2006, an incomplete skeleton of a large theropod dinosaur was discovered at the "Matilda Site" in a layer of the Winton Formation, which dates to the Cenomanian stage of the Late Cretaceous period, around 95 million years ago. Strata containing the theropod fossils was made up of black soils that lacked recognizable facies and plant fossil-rich claystones, though all are known to derive from the Winton Formation. This skeleton was found intermingled with the remains of a sauropod, later named Diamantinasaurus matildae. After an initial three years of excavation from 2006 to 2009, the theropod skeleton was deposited at the AODF under specimen number AODF 604. Due to the material being encased in a phosphatic concretion, many fossils remained unidentified for years. The first wave of preparation uncovered nine teeth, a left (lower jaw bone), several ribs, an incomplete (upper pelvic bone), parts of both hindlimbs and forelimbs, and some (toe bones) and (finger bones). This included a giant, recurved manual ungual (hand claw), a trait characteristic of megaraptorans, a clade of theropods. In 2009, Australian paleontologist Scott Hocknull and colleagues published a paper in the journal PLOS One in which they described several recently discovered specimens from the Winton Formation. Hocknull and colleagues scientifically described AODF 604 as the holotype (the specimen that forms the basis and bears the name of a species) of a new genus and species of theropod, Australovenator wintonensis. The generic name Australovenator derives from the Latin australis "southern", in reference to being unearthed in the Southern Hemisphere in Australia, and venator "hunter", in reference to its carnivorous diet. The specific name witonensis derives from Winton, the township the holotype was collected in. In the same paper, Hocknull and colleagues named Wintonotitan and Diamantinasaurus, two sauropods that were found in the "Matilda site".
Although the holotype was first discovered in 2006 and first described in 2009, the processing of excavating the "Matilda site" is still ongoing. This included the preparation of additional fossils from the concretion containing the holotype, including elements of the arms and hands (which were described in 2012), hindlimbs (which were described in 2013), and the right dentary (which was described in 2015). In total, the holotype is represented by both dentaries, many teeth, three dorsal ribs, some rib shaft pieces, (upper arm bones), (upper forearm bones), (lower forearm bones), (a carpal bone), both first , incomplete (hands), an ilium fragment, left (thigh bone), (shin bones), , left , incomplete (feet), and nine fragments.
Rapator and assigned material
In 1932, German paleontologist Friedrich von Huene named Rapator ornitholestoides, a species now recognized to be a megaraptoran, on the basis of a single metacarpal. This fossil was found in Lightning Ridge, an outcrop of the Cenomanian-aged Griman Creek Formation. Following the discovery of Australovenator, comparisons were made of the two. While Hocknull and colleagues (2009) noted some differences in the anatomy based on a poorly preserved metacarpal I from Australovenator, their relationships were indeterminate. In 2010, Argentine researcher Federico Agnolín and colleagues classified Rapator as a megaraptoran, possibly even Australovenator's sister taxon (closest relative). They followed up by stating that Megaraptor was less similar to Rapator than Australovenator, and that no definite differences between the latter two could be identified. In 2013, Australian researcher Matt White and colleagues stated that the two were both morphologically and temporally different from one another, as the Griman Creek Formation is 10 million years older than the Winton Formation. This suggests that the two are not synonyms (the same taxon).
In 1981, Australian paleontologist Ralph E. Molnar described an isolated left astragalus (NMVP 150070) that was discovered in an outcrop of the Aptian-aged Wonthaggi Formation in Victoria, Australia. He assigned the fossil to a species of pygmy Allosaurus, calling it Allosaurus sp. Although some studies later considered it an allosauroid, allosaurid, or abelisauroid, the consensus is that it belongs to a megaraptoran. In a 2019 paper, Australian paleontologist Stephen Poropat and colleagues described several isolated megaraptoran fossils from the upper Aptian to lower Albian-aged Eumeralla Formation of Victoria, Australia. This material included two isolated teeth, two right manual unguals (hand claws), a right astragalus, and fragments possibly from a right pedal phalanx. The authors provisionally assigned the fossils to cf. Australovenator. In 2020, White and colleagues described a heavily eroded specimen from the "Marilyn" Site of the Winton Formation. This specimen, comprising two vertebrae, three incomplete metatarsals, and a pedal phalanx fragment, showed size and morphological differences from Australovenator. The authors went on to state that this could be because of ontogenetic changes, intraspecific variation, or the presence of a second, distinct megaraptorid in the Winton Formation.
Description Megaraptorans were bipedal, carnivorous theropods with elongated snouts, large arms, and relatively enlarged claws. Australovenator was a relatively lightweight predator, leading Hocknull to coin it the "cheetah of its time". South American megaraptorans attained very large sizes, exceeding in length in genera like Megaraptor. In contrast, in a 2009 interview Hocknull stated that Australovenator was tall at the hip and in length. In 2016, American paleontologist Gregory S. Paul placed Australovenator at long and weighed . However, a 2014 study by American researcher Roger Benson and colleagues claimed its body mass was about . Supporting Information Fossils of a megaraptoran, possibly Australovenator itself, from the Eumeralla Formation come from a slightly smaller individual than the holotype. However, morphologically they are very similar.
Dentary and teeth
Overall, the dentary is gracile and long, measuring in length as preserved. The end of the (the area in which the dentaries meet) is rounded and lacks a "chin", unlike those of allosauroids like Allosaurus. Each dentary bears 19 (tooth sockets), all of which touch . These plates connect on their anterior (front) and posterior (back) ends, creating an elongated interdental ridge that trails across the whole tooth row. On the medial (inner) face of the dentary is a shallow paradental groove which runs below the alveolar margin and makes up the ventral (bottom) side of a protruding band of bone. This band is also known from Neovenator, but is absent in Eotyrannus. Although (small pits of bone) on the lateral (outside) side of the dentary were described by Hocknull and colleagues (2009), later analysis has determined that Australovenator lacked these foramina. The absence of this feature may be an autapomorphy (characteristic unique to a species) of Australovenator.
Its teeth are recurved with fine serrations, though the location of the serrations differs between the anterior and posterior teeth. Those with circular to quadrangular bases and two series of serrations, one on the lingual (facing towards the back of the jaw) (cutting edges) and the other on the distal carina, come from the anteriormost section of the dentary or . In contrast, teeth that are thinner with serrations on the mesial (towards the front midline of the jaw) margin and the distal carina come from the posterior region of the dentary. Unlike the wrinkly teeth of carcharodontosaurids and Neovenator, Australovenator's and Fukuiraptor's, a megaraptoran from Japan, are smooth at the base. Uniquely compared to most theropods, the dentition of megaraptorans like Australovenator displays some heterodonty, as the anatomy of the mesial and distal teeth are different. This suggests a functional difference between the two kinds of teeth.
Postcrania