Chilantaisaurus

Fossils assigned to Chilantaisaurus were first discovered in 1960 by members of a Sino-Soviet expedition in at the Tashuikou site, 60 kilometres (37 mi) north of Jilantai Salt Lake [zh] in the eastern Alashan Desert in Inner Mongolia, China. This expedition unearthed fossils of ornithopods, sauropods, ankylosaurs, and theropods, as well as turtles, plants, and invertebrates. The remains of Chilantaisaurus found consisted of fragmentary forelimb and hindlimb bones likely from the same individual,^[3] and were cataloged as IVPP V.2884.1 at the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing.^[2][3][4] The strata of the Tashuikou site is made up of red mudstones and siltstones which belong to the Ulansuhai Formation of the Dashuigou Group, though the age of this formation is debated.^[3] The age of the Ulansuhai Formation is not constrained with confidence, though it is definitely younger than 92 million years ago (Turonian age); Evans et al. (2021) suggested that the formation is likely dated to the Santonian-Campanian ages or older.^[5] On the other hand, Benson and Xing (2008) stated it dates to the Aptian or Albian ages.^[3]

The material was found disarticulated and consists of: a right humerus, an ungual phalanx, a fragment from the left ilium, both femora, a complete right and incomplete left tibiae, an incomplete left fibula, right metatarsals II-IV, and left metatarsals III-IV. In 1964, Chinese paleontologist Hu Show-Yung scientifically described the remains and assigned them to a new genus and species of carnosaurian theropod, which they named Chilantaisaurus tashuikouensis. The generic name is a combination of Chilantai (in reference to the Chilantai Salk Lake near where the fossils were found) and the Greek word saurus meaning "lizard", a common suffix for dinosaur names. The specific name tashuikouensis is refers to the Tashuikou site where the fossils were found.^[2] Hu (1964) did not select a lectotype or holotype specimen,^[2] though Benson and Xing (2008) chose the right humerus as the lectotype and the rest of the material was made paralectotypes.^[3] In his description, Hu also assigned an isolated tooth, mid caudal vertebra, and distal (away from body) caudal vertebra to C. tashuikouensis, however none of these remains have overlap with the lectotype or paralectotypes and were not associated with those specimens, leaving their statuses in question.^[3][6] Benson and Xing (2008) stated the tooth is Theropoda indet., while the mid caudal belongs to Sauropoda indet. and the distal caudal is Dinosauria indet.^[3] Since its description, C. tahsuikouensis' classification has been uncertain, and few studies of its remains have been published.^[3][4][7]

Species

• Chilantaisaurus maortuensis was named by Hu in 1964 in the same paper describing C. tashuikouensis. It was named on the basis of an incomplete skull, including the braincase, an incomplete axis, and six caudal vertebrae. These fossils were found at the nearby Maortu site which comes from the Miaogou Formation, which likely dates to the Aptian-Albian or Barremian-Albian^[8] ages of the Cretaceous period. It was assigned to Chilantaisaurus despite the lack of overlap between the C. maortuensis fossils and those of C. tashuikouensis. In 2000, American paleontologist Daniel Chure assigned it to its own genus, "Alashansaurus", in a thesis, however American researcher Steve Brusatte and colleagues (2009) placed it in the new genus Shaochilong, which they classified as a carcharodontosaurid.^[7][9]
• Chilantaisaurus sibiricus is a combination of Allosaurus sibiricus made by Molnar and collagues (1990) based on their geographic similarities and age.^[10] A. sibiricus is known from an isolated fourth metatarsal, now missing, that was unearthed from an Early Cretaceous deposit in Buryatia, Russia and was named by Russian paleontologist Anatoly Riabinin in 1914.^[11][6] However, it is now considered a nomen dubium and indeterminate beyond Theropoda.^[6]
• Chilantaisaurus zheziangensis was named by Chinese paleontologist Dong Zhiming in 1979 on the basis of an incomplete right tibia and complete pes that had been found in 1972 in an outcrop of the Upper Cretaceous-aged Tangshang Formation in Zhejiang Province, China.^[12] Dong believed it was a species of Chilantaisaurus based on perceived similarities in their unguals, however recent studies have concluded that it likely comes from an indeterminate therizinosaur.^[3][13][14] In a phylogenetic analysis by Hartman and colleagues (2019), it was found in polytomy with the other therizinosaurs Alxasaurus, Enigmosaurus, and Therizinosauridae.^[15] Nanshiungosaurus was suggested to be a synonym of "C". zheziangensis by Kirkland and Wolfe (2001),^[16] however this has not seen widespread use in literature.^[17][18]

Chilantaisaurus was a large theropod, measuring 11 metres (36 ft) long and weighing 2.5–4 metric tons (2.8–4.4 short tons).^[4][19][20] While Brusatte et al. (2010) estimated that Chilantaisaurus might have weighed about 6 metric tons (6.6 short tons) based on femur length similar to that of Tyrannosaurus,^[7] Persons et al. (2020) argued that greater femoral circumference indicates the greater capacity to withstand greater locomotor loads, not greater body mass.^[21]

The humerus is massive, measuring 580 millimetres (23 in) in length, making it the largest known humerus of any non-coelurosaurian theropod. It is around half the length of the femur, which measures 1,190 millimetres (3.90 ft) in length. This humerus:femur length ratio, which is 0.49 in the genus, is comparable to the proportions of spinosaurids like Suchomimus (0.54), but much higher than that of carcharodontosaurids like Acrocanthosaurus (0.29) and Mapusaurus (0.23). However, the overall length of the humerus is shorter than that of Deinocheirus and Gigantoraptor. The humerus has a prominent deltopectoral crest which has a large, anteriorly (front) protruding flange. On the anterior surface of the crest is a crescent-shaped, pitted muscle scar that is unique to the species among theropods.^[3] On the distal (away from body) end of the humerus are expanded radial and ulnar condyles, with a well-developed process on the outer surface of the radial condyle.^[2] The manual ungual is giant, elongate, and three times as long as it is high. Spinosaurids and basal coelurosaurs like Sinosauropteryx have a similar condition. On the lateral faces of the ungual are an elongate, well-defined vascular grooves that runs along the length of the bone. The ilium fragment is extremely thin relative to its size, measuring a mere 6–7 millimetres (0.24–0.28 in) in transverse diameter. In contrast, large theropods like Afrovenator, Allosaurus, Giganotosaurus, and Piatnitzkysaurus have relatively robust ilia. While the anterior end of the ilia of Tyrannosaurus, Albertosaurus, and Allosaurus bear a recurved process, Chilantaisaurus lacks this.^[2] Both of the femora are known, but have many breaks and extensive wear. The femoral head is oriented medially (towards right) and is slightly proximally (towards body) angled, like in other large theropods. The fourth trochanter is greatly reduced and is flanked by a small depression, unlike the prominent fourth trochanters observed in basal theropods. This condition is observed in carcharodontosaurids like Giganotosaurus, however not to the degree found in Chilantaisaurus.^[3]

In 1964, Hu regarded Chilantaisaurus as a member of Carnosauria somewhat related to Allosaurus. At the time, Carnosauria was a wastebasket group that included all large theropods. However, in the 1980s and 1990s this term changed definition.^[22][23]

Some subsequent studies suggested that it may be a spinosauroid, possibly a primitive member of the spinosaurid family (Sereno, 1998; Chure, 2000; Rauhut, 2001) because it had large claws on the forelimbs thought to be unique to that group. Other studies suggested that it could be a member of an alternate offshoot of neotetanuran theropods, with some similarities to allosauroids, spinosauroids, and coelurosaurians.^[3]

A 2009 study noted that it was difficult to rule out the possibility that Chilantaisaurus was the same animal as the carnosaur Shaochilong, which was thought to be from the same geological formation (a later study indicated that the latter genus actually derives from the Early Cretaceous Miaogou Formation^[24]). However, they did note an enormous size difference between the two.^[9]

In a 2010 study, American paleontologists Roger Benson, Matthew Carrano, and Steve Brusatte found Chilantaisaurus to be a member of a monophyletic clade named Neovenatoridae, along with many genera thought to be megaraptoran theropods. Chilantaisaurus and Neovenator were basal members of the clade, while derived genera like Australovenator and Megaraptor formed the new clade Megaraptora. Neovenatorid taxa were united by several features, such as the shortness of the scapula and the pneumaticity of the ilia. Benson and collagues thought that Neovenatoridae originated in the early-mid Cretaceous and lasted until the uppermost Cretaceous, spreading to Europe, Asia, South America, and even Australia. A similar result was found in 2016 by Argentine paleontologist Sebastián Apesteguía and colleagues, who found the same result but with Deltadromeus and Gualicho as basal neovenatorids as well.^[25] However, many phylogenetic analyses have found these two genera as members of other groups instead.^[26][27] The cladogram below follows the 2010 analysis by Benson, Carrano and Brusatte:^[4]

Neovenatoridae	Neovenator unnamed Chilantaisaurus Megaraptora unnamed Australovenator Fukuiraptor Orkoraptor? Aerosteon Megaraptor

The cladogram below follows a 2016 analysis by Sebastián Apesteguía, Nathan D. Smith, Rubén Juarez Valieri, and Peter J. Makovicky based on the dataset of Carrano et al. (2012).^[25] Phylogenetic analysis published by Porfiri et al. in 2018 recovered Chilantaisaurus as a primitive coelurosaurian.^[28]

The Uluanshi Formation is dominated by red mudstone and siltstone, indicating a floodplain environment defined by meandering rivers. However, some geologic features such as calcrete indicate drier components of the ecosystem. The formation was deposited during a period of transition for the Gobi from wet, fluvial ecosystems towards the desertic dune-dominated ecosystems of later Cretaceous deposits. This is one of several dinosaur-bearing rock formations in Inner Mongolia, showing shifts in the Cretaceous dinosaur fauna of Asia.^[29] Other dinosaurs from the formation include the ornithomimosaur Sinornithomimus and the pachycephalosaur Sinocephale.^[5]