Open Paleo

Siamotyrannus

Buffetaut et al., 1996 sie-AM-o-ti-RAN-us

From the old Thai kingdom of Siam combined with Latinized Greek tyrannus, "tyrant" — means "Siamese tyrant"

137.1–125.8 million years ago Valanginian–Hauterivian, Early Cretaceous
Early Cretaceous stages

Siamotyrannus is a genus of possibly allosauroid theropod dinosaur from the Early Cretaceous Sao Khua Formation of Thailand.

Under dispute

Siamotyrannus isanensis was described as an early tyrannosaurid (Buffetaut and colleagues, 1996), but analyses since recover it among the allosauroids (Rauhut, 2003), as a metriacanthosaurine (Carrano, Benson and Sampson, 2012; Rauhut and colleagues, 2024). Samathi and colleagues (2019) regard it as an avetheropod, possibly a basal coelurosaur rather than a basal allosauroid.

Diagnosis

  1. Long and relatively low ilium with incipient subhorizontal medioventral shelf on anterior blade
  2. Pubis with long straight shaft terminating in massive distal boot, more developed anteriorly than posteriorly
  3. Obturator foramen of pubis open ventrally but largely encircled by proximal bony hook
  4. Ischium slender and curved with well-defined scar on proximodorsal edge for flexor tibialis internus insertion
  5. Anterior caudal vertebrae with tall slender neural spines
  6. Mid-caudal vertebrae with small dorsal process on neural arch anterior to main neural spine
  7. Anterior chevrons long straight and slender

Species

Siamotyrannus isanensis Buffetaut et al., 1996

Type species

From Thai 'isan' meaning 'northeastern part', referring to the provenance of the holotype from northeastern Thailand.

Holotype
PW9-1 , Department of Mineral Resources
Material
Partial skeleton: left half of pelvis, sacrum, 13 anteriormost caudal vertebrae with chevron bones, five dorsal vertebrae.
Horizon
Sao Khua Formation, Valanginian–Hauterivian, Early Cretaceous , 137.1–125.8 million years ago
Locality
Phu Wiang 9, Khon Kaen, Thailand

References

  1. Buffetaut, E.; Suteethorn, V.; Tong, H. (1996). The earliest known tyrannosaur from the Lower Cretaceous of Thailand. Nature 381(6584): 689-691. Springer Science and Business Media LLC. doi:10.1038/381689a0

    Named Siamotyrannus as an early tyrannosaurid.

    buffetaut1996a
  2. Molina-Pérez, R.; Larramendi, A. (2019). Dinosaur Facts and Figures: The Theropods and Other Dinosauriformes. Princeton University Press.

    Source of the hip height estimate.

    molina-pérez2019a
  3. 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. Informa UK Limited. doi:10.1080/14772019.2011.630927

    Placed Siamotyrannus within Metriacanthosauridae as a metriacanthosaurine.

    carrano2012a
  4. Rauhut, O. W. M.; Bakirov, A. A.; Wings, O.; Fernandes, A. E.; Hübner, T. R. (2024). A new theropod dinosaur from the Callovian Balabansai Formation of Kyrgyzstan. Zoological Journal of the Linnean Society 201(4). Oxford University Press (OUP). doi:10.1093/zoolinnean/zlae090

    Reanalysis placed Siamotyrannus within Metriacanthosaurinae.

    rauhut2024a
  5. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Theropods. Princeton University Press.

    Places Siamotyrannus in Metriacanthosaurinae.

    jones2026c
  6. Rauhut, O. W. M. (2003). The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology 69: 1-213. The Palaeontological Association.

    Found Siamotyrannus an allosauroid in the complete-matrix trees, and deleted it from the final analysis as poorly known.

    rauhut2003b
  7. Samathi, A.; Chanthasit, P.; Sander, P. (2019). Two new basal coelurosaurian theropod dinosaurs from the Early Cretaceous Sao Khua Formation of Thailand. Acta Palaeontologica Polonica 64. Polska Akademia Nauk Instytut Paleobiologii (Institute of Paleobiology, Polish Academy of Sciences). doi:10.4202/app.00540.2018

    Regard Siamotyrannus as an avetheropod, possibly a basal coelurosaur or a basal allosauroid.

    samathi2019a