Chilantaisaurus ("Jilantai Salt Lake lizard") is an extinct genus of large theropod dinosaur that lived in present-day China during the Late Cretaceous period. It was described by Chinese paleontologist Hu Show-Yung in 1964. The genus contains a single valid species, C. tashuikouensis, though several other species have been assigned to the genus. C. tashuikouensis is known from a single, incomplete postcranial skeleton, the holotype specimen. This specimen was found by a joint Sino-Soviet expedition to Inner Mongolia in rock layers coming from the Ulansuhai Formation. These rock layers are thought to date to the Santonian or Campanian stages of the Cretaceous period, around 85.7 to 72.2 million years ago. However, the age of the Ulansuhai Formation is debated.
Chilantaisaurus was around in length and weighed . This makes it among the largest known theropod genera, comparable to Tyrannosaurus. The forelimbs of Chilantaisaurus are only known from a humerus and a manual ungual (hand claw); this humerus is one of the largest humeri known from a theropod dinosaur. It measures in length and has a greatly expanded . The ratio between the humerus and femur length is very high, in contrast to those of carcharodontosaurids like Mapusaurus but comparable to those of spinosaurids like Suchomimus. The hindlimbs were long with a robust metatarsus, the largest known of any purported megaraptoran or Neovenator itself.
The classification of Chilantaisaurus has been in flux since its original description. Hu believed it was a member of Carnosauria; later studies have challenged this notion. While some studies suggested it was a member of Spinosauridae, others have proposed that Chilantaisaurus was a coelurosaur, possibly related to Megaraptora, or a carcharodontosaurian related to genera like Neovenator. The Ulansuhai Formation is dominated by red mudstone and siltstone, indicating a floodplain environment defined by meandering rivers. Other dinosaurs known from this site include the ornithomimosaur Sinornithomimus and the pachycephalosaur Sinocephale.
Discovery and species Fossils assigned to Chilantaisaurus were first discovered in 1960 by members of a Sino-Soviet expedition at the Tashuikou site, north of Jilantai Salt Lake 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, and were cataloged as IVPP V.2884.1 at the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing. The strata of the Tashuikou site are made up of red mudstones and siltstones which belong to the Ulansuhai Formation of the Dashuigou Group. The age of the Ulansuhai Formation is not constrained with confidence, though it is definitely younger than (Turonian age); Evans and colleagues (2021) suggested that the formation is likely dated to the Santonian or Campanian ages or older. On the other hand, Benson and Xing (2008) stated it dates to the Aptian or Albian ages.The material was found disarticulated and consists of a right , an , a fragment from the left , both , a complete right and incomplete left , an incomplete left , right 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 Salt Lake near where the fossils were found) and the Greek word sauros meaning "lizard", a common suffix for dinosaur names. The specific name tashuikouensis refers to the Tashuikou site where the fossils were found. Hu (1964) did not select a lectotype or holotype specimen, though Benson and Xing (2008) chose the right humerus as the lectotype and the rest of the material were designated as paralectotypes. In his description, Hu also assigned an isolated tooth, mid , 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. 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. Since its description, C. tashuikouensis' classification has been uncertain, and few studies of its remains have been published.
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 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. The same study noted that it was difficult to rule out the possibility that Chilantaisaurus was the same taxon as 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) at the time. However, few phylogenetic analyses have found the two to be synonymous or even closely related and Shaochilong is regarded as distinct. Chilantaisaurus sibiricus is a combination of Allosaurus sibiricus made by Molnar and colleagues (1990) based on their geographic similarities and age. 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. However, it is now considered a nomen dubium and indeterminate beyond Theropoda. 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. 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. In a phylogenetic analysis by Hartman and colleagues (2019), it was found in polytomy with the basal therizinosaur genera Alxasaurus, Enigmosaurus, in addition to the family Therizinosauridae. Nanshiungosaurus was suggested to be a synonym of "C". zheziangensis by Kirkland and Wolfe (2001); however this has not seen widespread use in literature.
Description
Chilantaisaurus was a large theropod, measuring long and weighing . While Brusatte and colleagues (2010) estimated that Chilantaisaurus might have weighed about based on femur length similar to that of Tyrannosaurus, Persons and colleagues (2020) argued that greater femoral circumference indicates a greater capacity to withstand greater locomotor loads, not greater body mass. Chilantaisaurus was estimated to be long in a 2013 study by American paleontologists Lindsay E. Zanno and Peter J. Makovicky assuming that it was a megaraptoran. This indicates it was comparable in size to the other giant possible megaraptoran Siats, which was around long according to the same study.
The humerus is massive, measuring in length, making it the largest known humerus of any non-coelurosaurian theropod. For comparison, it is about double the size of Allosaurus, a theropod which measures about in length. It is around half the length of the femur, which measures in length. This humerus-to-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 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. On the distal (away from body) end of the humerus are expanded and , with a well-developed process on the outer surface of the radial condyle. 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 elongate, well-defined vascular grooves that run along the length of the bone.
The ilium fragment is extremely thin relative to its size, measuring a mere 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 bears a recurved process, Chilantaisaurus lacks this. Both of the femora are known, but have many breaks and extensive wear. The femoral head is oriented medially (towards the midline) and is angled slightly proximally (towards the body), like in other large theropods. The 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. In total, the is in length and an estimated in width, larger than in Neovenator or any megaraptoran known. The metatarsal IV is very robust, in contrast to the gracile nature of Australovenator's, and is trapezoidal in cross-section.
Classification
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. Later, Harris (1998) placed Chilantaisaurus within Allosauroidea, possibly as a sister taxon to Carcharodontosaurus or closely related to Neovenator and Acrocanthosaurus. Several studies have classified Chilantaisaurus as a spinosaurid or spinosauroid, although Chilantaisaurus has not been classified as a spinosaurid in many studies since. In a 2003 phylogenetic analysis, German paleontologist Oliver Rauhut found Chilantaisaurus to be a basal member of Spinosauroidea or an indeterminate megalosauroid. In 2012, Allain and colleagues classified the genus in Spinosauridae because of its high humeral to femoral length ratio, straight humeral shaft in lateral view, enlarged manual ungual, and a robust longitudinal ridge near the astragalar facet, all traits similar to or found in spinosaurids. If Chilantaisaurus were a spinosaurid, it would be the youngest member of the clade and one of the few spinosaurids unearthed in Asia. Allain and colleagues' phylogenetic analysis recovered Chilantaisaurus formed a polytomy with Spinosaurus, Irritator, Ichthyovenator, and the spinosaurid subfamily Baryonychinae.