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

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

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

from the early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...

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 (, ) is the capital of Khon Kaen province and the List of municipalities in Thailand#Largest cities by urban population, fourth-largest city in Thailand. It is one of the four major cities of Isan, Northeast Thailand, also known as the ...

. In 1996, Eric Buffetaut, Varavudh Suteethorn and

Haiyan Tong Haiyan may refer to: Places *Haiyan, Guangdong (), a town in Guangdong *Haiyan County, Qinghai (), a county of Haibei Tibetan Autonomous Prefecture, Qinghai *Haiyan County, Zhejiang (), a county of Jiaxing, Zhejiang People with the given name Haiy ...

named and described the

type species In International_Code_of_Zoological_Nomenclature, 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 spe ...

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

Siam Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...

, 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 fifteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...

. 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 ''isan'', "northeastern part", referring to the provenance from northeast Thailand. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

, PW9-1, was found in the Sao Khua Formation, 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 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...

. It includes the left half of the

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

, five rear dorsal

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

, the

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a 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 of the pelvic cavity, ...

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 Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology. He is best known for his work and research on theropoda, theropod dinosaurs and his detailed illustrations, both l ...

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 Synapomorphy, 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 (cladistics), outgroup taxa, not ...

, 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 supercontinen ...

and even a

tyrannosaurid Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to fifteen 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 ...

that define the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

, its position there 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 an extinct 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 ...

or a sinraptorid. In 2012 Matthew Carrano et al. found a position in the Metriacanthosaurinae. Later studies consider its position uncertain within Avetheropoda, 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, ty ...

. In his 2024 review of theropod relationships, Cau recovered it as a

tetanuran Tetanurae (/ˌtɛtəˈnjuːriː/ or "stiff tails") is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, and coelurosaurs (which includes tyrannosauroids, ornithomimosaurs, compsognathids and maniraptoran ...

outside of the Orionides, as a close relative of '' Siamraptor''. In the same year, the describers of '' Alpkarakush'' included ''Siamotyrannus'' within

Metriacanthosauridae Metriacanthosauridae (Greek for "moderately-spined lizards") is an extinct family of allosauroid theropod dinosaurs that lived in Europe and Asia from the Middle Jurassic to the Early Cretaceous. The family is split into two subgroups: Metriacan ...

based on their phylogenetic analysis.

References

{{Taxonbar, from=Q579201 Metriacanthosauridae Dinosaur genera Early Cretaceous dinosaurs Taxa named by Éric Buffetaut Taxa named by Varavudh Suteethorn Fossil taxa described in 1996 Dinosaurs of Thailand