| Sibirotitan | |
|---|---|
 | |
| Sibirotitan astrosacralis metatarsal I, PM TGU 16/0−8, in multiple views | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Macronaria |
| Clade: | †Somphospondyli |
| Genus: | †Sibirotitan Averianovet al., 2018 |
| Type species | |
| Sibirotitan astrosacralis Averianovet al., 2018 | |
Sibirotitan ("Siberiantitan") is a genus ofsomphospondylsauropod from theEarly Cretaceous (Barremian age)Ilek Formation ofRussia. Thetype and only species isS. astrosacralis.[1]
The material assigned toSibirotitan was found in the Shestakovo 1 locality of theIlek Formation, deposited in a cliff on theKiya River's right bank, nearShestakovo Village inKemerovo Province, WestSiberia,Russia. Vertebrate remains were first recovered in 1953, and larger dinosaur fossils were found later in the sixties. Definitive sauropod remains were first discovered during expeditions in 1994 and 1995.[1] In 2002, a reasonably complete foot would be described by Russianvertebrate paleontologist Alexander Averianov and colleagues; there was insufficient material to name the taxon, but they identified it as a member ofTitanosauriformes, noting teeth from the locality indicated possiblebrachiosaur identity, but that acaudal vertebra from a nearby locality indicated the presence of atitanosaur as well.[2]
Specimens would continue to turn up, and in 2018, Averianovet al. would name those which could be confidently referred to as the same taxon asS. astrosacralis; this included teeth, a sacrum, assorted vertebrae, and the previously described foot. This made it the second sauropod species named from the country, afterTengrisaurus, named earlier in the same year; the genusArkharavia, named in 2010, although originally described as a sauropod, was later reassigned toHadrosauridae.[1] Earlier, in 2015, it had been informally nicknamed "Sibirosaurus" in the press.[3][4] The final name ofSibirotitan was derived fromSiberia, where it was found, and theGreek Τιτάν (titan), the second group of divine beings inGreek Mythology, preceding theOlympians. The specific nameS. astrosacralis is derived from the Greek ἄστρον, meaning "star", andLatinos sacrum, meaning "sacred bone". This refers to the star-like way in which the sacral ribs radiate in dorsal view.[1]
The authors noted thatSibirotitan was one of the oldest titanosauriform species discovered in Asia.[1] More primitive known species were found in other parts of the world, such asEurope,North America, orGondwana.Sibirotitan, alongsideFukuititan, are the first to appear in the Asian fossil record, both thought to have lived in theBarremian age. Later relatives are well known from Asia afterwards, from theAptian through to theSantonian, and it is proposed that one of these later taxa could have given rise to theLithostrotia, a titanosaur group whose earliest representatives are Asian genera such asTengrisaurus (from the Barremian or Aptian of Russia) andJiangshanosaurus (from the Albian ofChina). The latter was noted to have similardorsal vertebrae to the much olderSibirotitan.[1]
Most of the referred specimens, although dis-articulated, are thought to have belonged to a single individual; it was suggested the large remains found in the 1960s could have originally been from the same skeleton as later remains, some found as recently as 2011. A singularcervicalcentrum was noted to be juvenile, and from a different individual, as the rest of the remains belonged to an adult animal. Among these fossils, PM TGU 120/10-Sh1-22, a dorsal vertebra, was selected as theholotype specimen.[1]
An axis vertebra from the type locality was later assigned toSibirotitan astrosacralis as well.[5]
Two characters were found to distinguish it from its close relatives. The hyposphene ridge on itsdorsal vertebrae was particularly high, unlike other members ofTitanosauriformes; this is the onlyautapomorphy of the taxon. Its possession of only fivesacral vertebrae distinguishes it from all othersomphospondyls, as possession of six is asynapomorphy of the clade. It was noted that juveniles have fewer vertebrae, but given the adult age of the individual examined, the trait was instead seen as an evolutionary reversal.[1]
Using one of the cervical vertebrae, a size estimate was attempted. They found it roughly corresponded in size to that of thediplodocoid sauropodApatosaurus, and suggested a similar size of 20 tonnes might have been possible.[1]
Several characters were noted among the remains, four of which allowed classification in the groupTitanosauriformes. These include the fashion in which thetooth crowns align in the jaw, presacral bone texture, and both the elongation of and the concave nature of certain parts of thecervical centra. An additional character, relating to the size of the arch of the cervical vertebrae, was used to assign it more specifically as a member ofSomphospondyli. Characters in its vertebrae and ribs were found shared withEpachthosaurus andEuhelopus.[1]
Aphylogenetic analysis was performed. It was noted that many differing and conflicting matrices exist for analyzing thesystematics of Titanosauriformes, due to many taxa being poorly known. Because it is suited well for basal Titanosauriformes in particular, the matrix of Mannionet al. (2013) was chosen. In this analysis, it was found to be a relatively advanced non-titanosaurian somphosondyl. The followingphylogenetic tree was produced:[1]
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