Temporal range: Early Cretaceous
An artist's restoration of Siamotyrannus isanensis
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Clade: Carnosauria
Superfamily: Allosauroidea
Family: Metriacanthosauridae
Subfamily: Metriacanthosaurinae
Genus: Siamotyrannus
Buffetaut, Suteethorn & Tong, 1996
Binomial name
Siamotyrannus isanensis
Buffetaut, Suteethorn & Tong, 1996

Siamotyrannus, name meaning "Siamese tyrant", is an extinct genus of theropod dinosaur which lived during the early Cretaceous period in what is now Thailand.

Siamotyrannus was a large theropod, with estimates suggests an average length of 7 meters (23 feet) and a weight of around half a ton.[1]

Siamotyrannus is a large theropod. Buffetaut estimated its length at seven meters. In 2010, Gregory S. Paul estimated the length at six metres, the weight at half a ton. A possible autapomorphy, 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 a tyrannosauroid and even a tyrannosaurid, though due to lacking some of the primary tyrannosauroid synapomorphies that define the clade, its position here is not certain. Some analyses have categorized Siamotyrannus as a primitive carnosaur rather than a basal tyrannosauroid, and it has several features that may determine it to be an allosaurid or a sinraptorid. In 2012 Matthew Carrano e.a. found a position in the Metriacanthosauridae.


In 1996, Eric Buffetaut, Varavudh Suteethorn and Haiyan Tong named and described the type species Siamotyrannus isanensis. The generic name is derived from the old Thai kingdom of Siam, and a Latinised Greek tyrannus, meaning "tyrant", in reference to a presumed membership of the Tyrannosauridae. The specific name is derived from Thai isan, "northeastern part", referring to the provenance from northeast Thailand.


As highlighted by its genus name, Siamotyrannus was originally identified as a tyrannosaur.[2] However, as it lacks some of the fundamental features of a tyrannosaur, its position in the group is not clear.[3] Some scientists refer to Siamotyrannus as a primitive carnosaur, rather than a basal tyrannosauroid, as it possesses several features that occur in some allosaurids and sinraptorids.[4] In 2012, Matthew Carrano e.a. found a position in the Metriacanthosauridae[5]




  1. Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 91
  2. Buffetaut, E., Suteethorn, V. and Tong, H. 1996. The earliest known tyrannosaur from the Lower Cretaceous of Thailand. Nature 381: 689-691
  3. Rauhut, Oliver W. M. Special Papers in Palaeontology: The Interrelationships and Evolution of Basal Theropod Dinosaurs (No. 69). The Palaeontological Association: 2003
  4. Holtz, Thomas R. et al. (2004). "Basal Tetanurae." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. p. 101 ISBN 0-520-24209-2
  5. Carrano, M. T.; Benson, R.B.J.; Sampson, S.D., 2012, "The phylogeny of Tetanurae (Dinosauria: Theropoda)". Journal of Systematic Palaeontology 10(2): 211–300
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