| Siamotyrannus | |
|---|---|
 | |
| Illustration of the pelvic bones and tail vertebrae | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avetheropoda |
| Genus: | †Siamotyrannus Buffetaut, Suteethorn & Tong, 1996 |
| Species: | †S. isanensis |
| Binomial name | |
| †Siamotyrannus isanensis | |
Siamotyrannus (meaning "Siamese tyrant") is agenus of possiblyallosauroidtheropoddinosaur from theEarly CretaceousSao Khua Formation ofThailand.
In 1993, Somchai Traimwichanon found a partial skeleton of a large theropod at the Phu Wiang 9 site inKhon Kaen.
In 1996,Eric Buffetaut,Varavudh Suteethorn andHaiyan Tong named and described thetype speciesSiamotyrannus isanensis. The generic name is derived from the old Thai kingdom ofSiam, and a Latinised Greektyrannus, meaning "tyrant", in reference to a presumed membership of theTyrannosauridae. Thespecific name is derived fromThaiisan, "northeastern part", referring to the provenance from northeast Thailand.[1]
Theholotype, PW9-1, was found in theSao Khua Formation, dating from theBerriasian-Barremian. It includes the left half of thepelvis, five rear dorsalvertebrae, thesacrum with five sacrals, and thirteen front tail vertebrae.[1] In 1998, a tibia and some individual teeth were referred to the species.[2]
Siamotyrannus is a large theropod. Buffetaut estimated its length at seven meters. In 2010,Gregory S. Paul estimated the length at 6 meters (20 ft), the weight at 500 kg.[3] In 2016 Molina-Pérez and Larramendi gave a higher estimation of 10 meters (33 ft) and 1.75 tonnes (1.93 short tons).[4] A possibleautapomorphy, unique derived trait, is the possession of two vertical ridges on the ilium. The second and third sacrals are strongly transversely flattened.
As evidenced by its name, it was originally thought to be atyrannosauroid and even atyrannosaurid,[1] though due to lacking some of the primary tyrannosauroidsynapomorphies that define theclade, its position there is not certain.[5] Some analyses have categorizedSiamotyrannus as a primitivecarnosaur rather than a basal tyrannosauroid, and it has several features that may determine it to be anallosaurid or asinraptorid.[6] In 2012Matthew Carrano et al. found a position in theMetriacanthosaurinae.[7] Later studies consider its position uncertain withinAvetheropoda, finding it to be either an allosauroid or a primitivecoelurosaur.[8] In his 2024 review of theropod relationships, Cau recovered it as atetanuran outside of theOrionides, as a close relative ofSiamraptor.[9] In the same year, the describers ofAlpkarakush includedSiamotyrannus withinMetriacanthosauridae based on their phylogenetic analysis.[10]
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