| Xuanhanosaurus | |
|---|---|
 | |
| An artist's restoration ofXuanhanosaurus qilixiaensis | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Genus: | †Xuanhanosaurus Dong, 1984 |
| Species: | †X. qilixiaensis |
| Binomial name | |
| †Xuanhanosaurus qilixiaensis Dong, 1984 | |
Xuanhanosaurus (meaning "Xuanhan lizard") is agenus oftheropoddinosaur that lived during theMiddle Jurassic (Bathonian) of theSichuan Basin,China, around 166 million years ago.[2] This taxon represents one of the various non-coelurosauriantetanuran taxa found on the Middle Jurassic of the region, uncovered in theLower Shaximiao Formation.[3] Although it has been known for more than 40 years, this taxon has been the subject of very few studies, although most seem to agree that it is a tetanuran, possibly a basalallosauroid, highlighting the fact that it has avestigial fourthmetacarpal.
Thetype speciesXuanhanosaurus qilixiaensis was named byDong Zhiming in 1984. The generic name refers toXuanhan County inSichuan, while thespecific name is derived from the town ofQilixia. The specimen was recovered in 1979, when the author was conducting a Dinosaurian Fossil survey in Sichuan Province, when was guided by Liu Yawen and the 137th team to examine the Qilixia section, where they recovered the holotype of this new taxon at the southern flank of the Qili Dorsal Slope.[3] Theholotype specimen,IVPP V.6729, was found inChina'sLower Shaximiao Formation. It consists of a partial skeleton without a skull, namely a scapula missing the posterior end, coracoid, a possible sternum humerus, radius, ulna, manus, as well a few fragmentary dorsal vertebrae.[3]
Xuanhanosaurus, originally thought to be around 6 m long, was approximately 4.8 meters (16 ft) in length, with a weight of 265 kilograms (585 lb).[5][6]Xuanhanosaurus had powerful forelimbs, over 65 cm long; this, along with the retention of the fourth metacarpal in the hand, led Dong to suggest thatXuanhanosaurus might have walked on all four legs. If so, it would be the only known four-legged meat-eater among dinosaurs.[3] Later paleontologists have not agreed with Dong's original assessment. They think this dinosaur walked on its hind legs as other theropods did,pronation of the lower arm being impossible. The strong arms could instead have been useful in catching prey.[7] The retention of the 4th metarcarpal is the most notorious feature of this taxon, yet the presence of a convex proximal surface on the distal carpal 2 has been used to argue it represents a second stage in the evolution of the "semilunate" carpal.[8] Some recent work have notified that beyond the 4th metarcarpal,Xuanhanosaurus also retains ceratosaurian-like short phalanges II-1, similar in length to the ones recovered onSaltriovenator.[9] The scapula was found to be bulkier than inAllosaurus. The distal humerus is enlarged laterally, forming a one-inch step.[3] The vertebrae are posteriorly concave and anteriorly protruding, with cervical centra having flat anterior articular surfaces, also seen inCondorraptor,Piatnitzkysaurus and"Szechuanosaurus" zigongensis, yet lacking posterior pneumatic foramen or fossa, which is seen on the first 2 genera.[10]
Originally,Xuanhanosaurus was assigned by Dong to theMegalosauridae, when this group was meant to include a larger amount of taxa, and was argued to be in the middle stage of the evolution of the Tetanurae due to its vestigial fourth metacarpals.[3] Latter it was found byRoger Benson in 2009 to belong to a primitive lineage of theMegalosauroidea.[11] A more recent study by Benson and colleagues in 2010 found that it was more likely to be the most primitive known member of theMetriacanthosauridae family.[4] In 2018, based on several conditions such as the length and retention of the vestigial 4th metacarpal, it was argued that this taxon and"Szechuanosaurus" zigongensis belong to early-diverging branches of the ceratosaurian-tetanuran node.[9] In 2019, Rauhut and Pol recoveredXuanhanosaurus outside of Metriacanthosauridae, as the basalmost member ofAllosauroidea.[7]
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