Siamotyrannus

Discovery and naming

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

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.

The holotype, PW9-1, was found in the Sao Khua Formation, dating from the Berriasian-Barremian. It includes the left half of the pelvis, five rear dorsal vertebrae, the sacrum 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, 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 that define the clade, 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 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. In his 2024 review of theropod relationships, Cau recovered it as a tetanuran outside of the Orionides, as a close relative of Siamraptor. In the same year, the describers of Alpkarakush included Siamotyrannus within Metriacanthosauridae based on their phylogenetic analysis.

References

References

1. (1996). "The earliest known tyrannosaur from the Lower Cretaceous of Thailand". Nature.
2. Buffetaut, E. and Suteethorn, V., 1998, "Early Cretaceous dinosaurs from Thailand and their bearing on the early evolution and biogeographical history of some groups of Cretaceous dinosaurs", In: Lucas, Kirkland and Estep, (eds.). ''Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History Bulletin 14''. p. 205-210
3. Paul, G.S., 2010, ''The Princeton Field Guide to Dinosaurs'', Princeton University Press p. 91
4. Molina-Pérez and Larramendi. (2016). "Récords y curiosidades de los dinosaurios Terópodos y otros dinosauromorfos". Larousse.
5. Rauhut, Oliver W. M. Special Papers in Palaeontology: ''The Interrelationships and Evolution of Basal Theropod Dinosaurs'' (No. 69). The Palaeontological Association: 2003
6. 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
7. (2012). "The phylogeny of Tetanurae (Dinosauria: Theropoda)". Journal of Systematic Palaeontology.
8. (May 2019). "Two new basal coelurosaurian theropod dinosaurs from the Lower Cretaceous Sao Khua Formation of Thailand". Acta Palaeontologica Polonica.
9. (2024). "A Unified Framework for Predatory Dinosaur Macroevolution". Bollettino della Società Paleontologica Italiana.
10. (2024-08-01). "A new theropod dinosaur from the Callovian Balabansai Formation of Kyrgyzstan". [[Zoological Journal of the Linnean Society]].