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

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of theropod

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 th ...

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 bo ...

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 The given name Eric, Erich, Erikk, Erik, Erick, or Eirik is derived from the Old Norse name ''Eiríkr'' (or ''Eríkr'' in Old East Norse due to monophthongization). The first element, ''ei-'' may be derived from the older Proto-Norse ''* ain ...

, Varavudh Suteethorn 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 specimen ...

''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 Mainland Southeast Asia, Indochinese Peninsula, spanning , with a population of almost 70 mi ...

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

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

is derived from

Thai Thai or THAI may refer to: * Of or from Thailand, a country in Southeast Asia ** Thai people, the dominant ethnic group of Thailand ** Thai language, a Tai-Kadai language spoken mainly in and around Thailand *** Thai script *** Thai (Unicode block ...

''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 several ...

, 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 ago) ...

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 characteristic i ...

, 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 ...

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 t ...

, 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 Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent b ...

and even a

tyrannosaurid 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 ...

, 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 have ...

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 term, ...

, its position here is not certain. Some analyses have categorized ''Siamotyrannus'' as a primitive

carnosaur Carnosauria is an extinct large group of predatory dinosaurs that lived during the Jurassic and Cretaceous periods. Starting from the 1990s, scientists have discovered some very large carnosaurs in the carcharodontosaurid family, such as ''Gig ...

rather than a basal tyrannosauroid, and it has several features that may determine it to be an allosaurid or a

sinraptorid Metriacanthosauridae is an extinct family of allosauroid theropod dinosaurs that lived from the Middle Jurassic to the Early Cretaceous. When broken down into its Greek roots, it means "moderately-spined lizards". The family is split into two sub ...

. In 2012

Matthew Carrano Matthew may refer to: * Matthew (given name) * Matthew (surname) * ''Matthew'' (ship), the replica of the ship sailed by John Cabot in 1497 * ''Matthew'' (album), a 2000 album by rapper Kool Keith * Matthew (elm cultivar), a cultivar of the Chi ...

e.a. found a position in the Metriacanthosaurinae. Later studies considers its position uncertain within

Avetheropoda Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs. Definition Avetheropoda was named by Gregory S. Paul in 1988, and was first defined as a clade by Currie and Padian i ...

, finding it to be either an allosauroid or a primitive

coelurosaur Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyrann ...

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