Details on Isanosaurus attavipachi
| [ Citation | Etymology | Locality | Specimens | Diagnosis | Description | Relationships ] |
Citation
Isanosaurus Buffetaut, Suteethorn, Cuny, Tong, Le Loeuff, Khansubha and Jongautcharlyakui 2000
I. attavipachi Buffetaut, Suteethorn, Cuny, Tong, Le Loeuff, Khansubha and Jongautcharlyakui 2000
Etymology
“Attavipach’s northeastern Thailand lizard”, P. Attavipach being the former Director General of the Thai Department of Mineral Resources, who supports paleontological research, and Isan being the local name for northeastern Thailand.
Locality
Nam Phong Formation, Thailand, Late Norian-Rhaetian, Late Triassic
Specimens
Holotype
- Department of Mineral Resources, Thailand CH4 (6 m)
- posterior cervical centrum (154 mm)
- posterior dorsal centrum (85 mm)
- posterior dorsal neural arch (314 mm tall)
- fragmentary ribs
- six caudal centra
- two chevrons
- scapula (573 mm)
- sternal plate (197 mm)
- femur (760 mm)
Diagnosis
very prominent acuminate S-shaped fourth trochantor
Description
The posterior cervical centrum is opisthocoelous, with a strong ventral ridge and deeply concave sides. The posterior dorsal vertebra also lacks pleurocoels, but has a tall neural arch. The neural spine is transversely compressed with incipient posterolateral laminae (suprapostzygopophyseal lamina?) and a lamina extending from the transverse process to the base of the spine (spinodiapophyseal lamina?). The caudal centra are amphicoelous.
The scapula has a small rounded proximodorsal expansion (50% of scapular length). The sternal plate is subtriangular, with a low ridge on the outer surface. Unfortunately, its orientation in the figure is unknown.
The femur is straight and the shaft is compressed anteroposteriorly. The femoral head is prominent and dorsomedially inclined. The greater trochantor is massive and the lesser trochantor is lacking. Far from being deflected medially, as in titanosaurs, the proximal femur is deflected laterally. The fourth trochantor is placed proximally on the posteromedial surface of the shaft and has a distinct S-shaped morphology. It is intermediate between the hook-like shape of prosauropods and the low rounded form of other sauropods. The distal femur has a well-developed ectipicondyle and lacks a longitudinal crest proximal to the lateral condyle.
Relationships
Buffetaut et al. referred Isanosaurus to the Sauropoda, but simply stated it was a basal member probably more derived than Gongxianosaurus, as basal sauropod relationships are poorly known.
Isanosaurus is obviously a sauropod, based on the presence of: tall dorsal neural arches; suprapostzygopophyseal laminae; straight femur; dorsomedially projected femoral head; femoral shaft compressed anteroposteriorly; reduced fourth trochantor.
The exact relationship among basal sauropods (Barapasaurus, Gongxianosaurus, Kotasaurus, Kunmingosaurus, Ohmdenosaurus, Shunosaurus, Vulcanodon, Zizhongosaurus) is currently controversial as several genera have yet to be included in phylogenetic analyses, many are poorly known and Shunosaurus might belong to the Euhelopodidae. Isanosaurus can be compared to most of these to help determine where it might be placed, although a study of basal sauropod relationships is not within the scope of this post.
Gongxianosaurus lacks opisthocoelous cervical centra, which characterize Kotasaurus, Shunosaurus, Isanosaurus and Barapasaurus. Both Barapasaurus and Zizhongosaurus have transversely expanded dorsal neural spines, unlike Shunosaurus and Isanosaurus. Isanosaurus and Barapasaurus have spinodiapophyseal laminae on the posterior dorsal neural arches, unlike Shunosaurus and Zizhongosaurus. Vulcanodon has a lesser trochantor, unlike Gongxianosaurus, Shunosaurus, Isanosaurus and Barapasaurus. Isanosaurus cannot be compared with Kunmingosaurus or Ohmdenosaurus. Based on the characters listed above, Isanosaurus would seem to be more derived than Vulcanodon, Gongxianosaurus and Shunosaurus, but less derived than Barapasaurus. Its position relative to other species is equivocal. Of course, this is only involving the four characters which vary in basal sauropods and which are known in Isanosaurus. I’m sure an analysis including characters that are currently unknown in Isanosaurus would improve our understanding immensly. For now, I recommend classifying Isanosaurus as Sauropoda incertae sedis until a more detailed study is carried out.
There are also several aspects of Isanosaurus‘ morphology that deserve comment. The posteromedially placed fourth trochantor was thought by Upchurch (1998) to be a synapomorphy of a Cetiosaurus-Neosauropoda clade, convergently developed in Vulcanodon. The presence of this character in Isanosaurus and Kotasaurus shows it needs more study. Perhaps the three form a clade based on it, although Vulcanodon does seem more basal than Isanosaurus. The fourth trochantor is more prominent than Vulcanodon, Barapasaurus or Shunosaurus, which may support a more basal position for Isanosaurus. Finally, the ridged sternal plate is reminiscent of euhelopodids, although it is on the ventral surface as opposed to the dorsal surface. The condition is unknown in other basal sauropods however, which may have something to say regarding the monophyly of the Euhelopodidae.