Simosaurus gaillardoti (von Meyer 1842) was recognized as the sister taxon to the eusauropterygians (Nothosaurus, Lariosaurus, Pistosaurus). Here, derived from a sister to Lariosaurus and Nothosaurus, Simosaurus phylogenetically preceded Pistosaurus and Plesiosaurus.
Distinct from Lariosaurus, the skull of Simosaurus had a wider rostrum and was not so flat. The cheek area was more robust with a relatively smaller orbit. An elongated premaxillary ascending process separated the nasals from each other and contacted the frontals. The postorbital did not contact the parietal. The internal naris were circular.
There were more cervicals and dorsals.
The forelimbs were longer than the hind limbs. The radius, ulna and hand were relatively longer.
The family tree of the Enaliosauria is here. The complete reptile family tree is here.