ISANOSAURUS.

ISANOSAURUS

Acrylics and Inks on cardboard for Dinopress Vol 4.

What does a primitive Triassic sauropod look like? We've got used to the familiar prosauropod image of Plateosaurus and think of it as the perfect model for a sauropod ancestor. Nevertheless, the feet and stance of prosauropods show that perhaps they were already too specialised to be true ancestors. So we have to search for an even more ancient common ancestor for both sauropods and prosauropods. George Olshevsky has a different idea from the normally accepted scenario: No prosauropod could have served as the ancestor of any known sauropod without a considerable number of evolutionary reversals. Sauropods were more primitive than prosauropods. Were then sauropods ancestral to the prosauropods?
George and I have worked together in this new reconstruction of Isanosaurus, the Triassic sauropod from Thailand. Unfortunately its remains are very incomplete and this reconstruction remains controversial. Just what exactly is it about these remains that makes Isanosaurus a sauropod? Among other things, the femur and the lone dorsal neural spine show more similarities to sauropods (albeit less "advanced") than to prosauropods.
The body as a whole was probably built like that of the primitive African sauropod Vulcanodon (with primitive skin ornaments and armour). Isolated vertebrae remains point to a shorter neck and tail and the skull (still unknown) probably would have the nostrils towards the front.
Moreover, in Isanosaurus the femoral head is at an angle of about 45 degrees to the axis of the femur. It articulated upward as well as laterally with the pelvis. So it made the femur (inserted in a partially open acetabulum) bow slightly outwards. George interpreted this to mean that sauropods diverged from the common dinosaur ancestor even before the fully erect hind limb had evolved, developed a fully erect posture later and independently than other dinosaurs and retained the ancestral five functional toes!
Controversial to say the least. George is presenting Isanosaurus not only as a basal Sauropod but also sauropods as the animals closest to the basal essence of all Dinosauria.
For the full discussion and detailed analysis of this article see the excellent Japanese publication Dinopress Vol 4 http://www.dinopress.com/

 

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