''Siamotyrannus'' (meaning "Siamese tyrant") is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

from the early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...

Discovery and naming

In 1993, Somchai Traimwichanon found a partial skeleton of a large theropod at the Phu Wiang 9 site in

Khon Kaen Khon Kaen ( th, ขอนแก่น, ) is one of the four major cities of Isan, Thailand, also known as the "big four of Isan", the others being Udon Thani, Nakhon Ratchasima, and Ubon Ratchathani. It is the capital of Khon Kaen province and ...

. In 1996, Eric Buffetaut,

Varavudh Suteethorn Varavudh Suteethorn, or Warawut Suteethorn ( Thai:วราวุธ สุธีธร; born 10 October 1948) is a Thai geologist and palaeontologist. He is the current director of the Palaeontological Research and Education Centre, Mahasarakham ...

and Haiyan Tong named and described the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specim ...

''Siamotyrannus isanensis''. The generic name is derived from the old Thai kingdom of

Siam Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is bo ...

, and a Latinised Greek ''tyrannus'', meaning "tyrant", in reference to a presumed membership of the

Tyrannosauridae Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...

. The specific name is derived from Thai ''isan'', "northeastern part", referring to the provenance from northeast Thailand. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

, PW9-1, was found in the

Sao Khua Formation The Sao Khua Formation () is a middle member of the Khorat Group. It consists of an alteration of pale red to yellowish-gray, fine to medium-grained sandstone and grayish-reddish brown siltstone and clay. Rare pale red to light gray conglomerates ...

, dating from the

Berriasian In the geological timescale, the Berriasian is an age/stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ag ...

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...

. It includes the left half of the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

, five rear dorsal

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

, the

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part o ...

with five sacrals, and thirteen front tail vertebrae. In 1998, a tibia and some individual teeth were referred to the species.

Description

''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. In 2016 Molina-Pérez and Larramendi gave a higher estimation of 10 meters (33 ft) and 1.75 tonnes (1.93 short tons). A possible

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...

, unique derived trait, is the possession of two vertical ridges on the ilium. The second and third sacrals are strongly transversely flattened.

Classification

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 In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ha ...

that define the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

, 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 Allosauridae is a family of medium to large bipedal, carnivorous allosauroid theropod dinosaurs from the Late Jurassic. Allosauridae is a fairly old taxonomic group, having been first named by the American paleontologist Othniel Charles Marsh in ...

or a sinraptorid. In 2012 Matthew Carrano e.a. found a position in the Metriacanthosaurinae. Later studies considers its position uncertain within Avetheropoda, finding it to be either an allosauroid or a primitive coelurosaur.

References

Metriacanthosaurids Early Cretaceous dinosaurs of Asia Cretaceous Thailand Fossils of Thailand Fossil taxa described in 1996 Taxa named by Éric Buffetaut Taxa named by Varavudh Suteethorn {{theropod-stub