Concavispina

Concavispina is an extinct genus of thalattosaur reptile from the early Late Triassic (Carnian stage) Xiaowa Formation of Guangling, Guizhou, southern China. It contains a single species, Concavispina biseridens. It is known only from the holotype ZMNH M8804, a nearly complete 364 cm long skeleton. Concavispina can be differentiated from other thalattosaurs by possessing two rows of blunt teeth on the anterior part of the maxilla (upper jaw bone) and a V-shaped notch on the dorsal margin of each neural spine in the dorsal (back) vertebrae. Both its generic and specific names refer to these autapomorphies (unique characteristics), as Concavispina means "concave spine" and biseridens means "two rows of teeth". It is thought to be most closely related to Xinpusaurus, as both taxa share three derived characters: a maxilla that is curved upward at its anterior end, a proximal of the humerus or upper arm bone that is wider than the distal end, and the presence of less than five cervical or neck vertebrae.

Concavispina has a large skull, short neck, short limbs, and long tail. ZMNH M8804 is 364 centimetres (11.94 ft) long, making Concavispina biseridens the second longest thalattosaur after Miodentosaurus brevis. It has the largest skull of any thalattosaur. The skull is 50 centimetres (20 in) long and about half the length of the torso. Concavispina has less than five cervical vertebrae and its pectoral girdle is very close to its skull. 114 caudal or tail vertebrae are present, although the tip of the tail is not preserved. The tail makes up most of the length of the body. It is laterally compressed due to the long neural spines and chevrons that extend from each vertebra. The radius and ulna and fibula and tibula (the bones which make up the lower forelimb and lower hindlimb, respectively) are very short and very wide compared to the humerus and femur (which make up the upper forelimb and upper hindlimb). The carpal (wrist) and tarsal (ankle) bones are weakly jointed.