Isanosaurus

''Isanosaurus'' means "North-eastern thailand lizard" was a sauropod dinosaur from Thailand. It was originally dated to approximately 210  million years ago during the Late Triassic (late Norian to Rhaetian stages), which would make it one of the oldest known sauropods. Its age was later considered uncertain, and may be as young as Late Jurassic. The only species is ''Isanosaurus attavipachi''. Though important for the understanding of sauropod origin and early evolution, ''Isanosaurus'' is poorly known. Exact relationships to other early sauropods remain unresolved.

Description

The only specimen includes a neck vertebra, a back vertebra and part of a second, six tail vertebra, two chevrons, fragmentary ribs, the right sternal plate, the right shoulder blade, and the left thigh bone (femur). This individual may have measured when alive; the thigh bone measures 76 centimetres in length. However, the vertebral neural arches have been found separated from the vertebral centra, indicating that these elements were not fused with each other; thus, this individual probably was not fully grown.

Early sauropodomorphs were primitively bipedal (two-legged). ''Isanosaurus'', being one of the first sauropods known, already shows a quadrupedal locomotion (with all four legs on the ground). The legs were column-like, as indicated by the robust and straight thigh bone. In prosauropods, but also in the very basal sauropod ''Antetonitrus'', the thigh bone was slightly sigmoidal (S-curved). Also, like in other sauropods, bony processes of the femur were reduced in ''Isanosaurus''; most notably, the lesser trochanter was lacking.

Additional important features can be found in the vertebrae. The neck vertebrae were distinctly Vertebral column#Classification, opisthocoelous (convex at the front and hollow at the back), forming ball-and-socket joints with neighbouring vertebrae. The tail vertebrae, on the other hand, were Vertebral column#Classification, amphicoelous (concave at both ends). The dorsal neural spines were very high, like those of some later sauropods, unlike the low prosauropod neural spines. The lateral sides of the vertebrae were concave, but not deeply excavated (a structure known as Vertebral column#In dinosaurs, pleurocoels) as in later sauropods.

Discovery

The specimen was found in dark red sandstone of the Nam Phong Formation near the village of Ban Non Thaworn, in Chaiyaphum Province. When discovered in 1998, the skeleton had unfortunately been mostly eroded away. With regard to vertebrate fossils, the Nam Phong Formation is poorly explored: besides ''Isanosaurus'', only two articulated ischium, ischia were found. Whether these ischia belong to ''Isanosaurus'' is unclear, because no pelvic bones are preserved in the holotype specimen.

''Isanosaurus'' was described by French palaeontologist Éric Buffetaut and colleagues in 2000. The name is derived from Isan (north-eastern Thailand); the species name honours P. Attavipach, a supporter of palaeontological research in Thailand and former Director General of the Thai Department of Mineral Resources.

Phylogeny

The following cladogram shows the position of ''Isanosaurus'' within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:

References

Peyre de Fabrègues, C. & Allain, R. 2020. ''Kholumolumo ellenbergerorum'', gen. et sp. nov., a new early sauropodomorph from the lower Elliot Formation (Upper Triassic) of Maphutseng, Lesotho. ''Journal of Vertebrate Paleontology'' e1732996. DOI: 10.1080/02724634.2019.1732996
Yates, A.M. & Kitching, J.W. 2003. The earliest known sauropod dinosaur and the first steps towards sauropod locomotion. ''Proceedings of the Royal Society of London B: Biological Sciences'' 270: 1753-1758.

External links

Restoration and discussion
of ''Isanosaurus'', by noted paleoartist Luis Rey.

{{Taxonbar, from=Q1555632

Sauropods
Norian life
Rhaetian life
Late Triassic dinosaurs of Asia
Fossils of Thailand
Fossil taxa described in 2000
Taxa named by Éric Buffetaut