Helveticosaurus zollingeri (Peyer 1955) Middle Triassic, ~225 mya, ~2 m in length, was a thalattosauriform derived from a sister to Askeptosaurus and was a sister to Vancleavea and Eusaurosphargis. Originally considered a basal placodont, Helveticosaurus has been difficult to categorize until now.
Distinct from Askeptosaurus, the skull of Helveticosaurus was shorter overall with a much shorter rostrum, a higher orbit on a deeper jugal and shorter postorbital area. The premaxillary fangs were deeper than the premaxilla. The maxillary teeth were not suitable for biting. They were hyperelongated, perhaps to act like baleen, filtering food in while letting water out. The lacrimal, prefrontal and postfrontal were all reduced.
The cervicals were shorter. There were at least 45 presacral vertebrae or 5 more than in Askeptosaurus. The neural spines were tall throughout.
The scapula was long, resembling the scapula in Placodus and Paraplacodus. The coracoid was more circular. The forelimb was slightly larger than the hind limb, although the manus and pes were subequal.
The pelvis was much larger ventrally. The ilium had a greatly reduced posterior process and a slightly expanded anterior process producing an inverted triangle shape. The femur, tibia and fibula were straighter, without greatly expanded ends. The tarsus was poorly ossified. Metatarsals 1-4 were nearly identical. Five phalanges appeared on pedal digit 4. The unguals were all smaller.
This bulkier, snub-nosed thalattosaur may have fed on rock scrapings, filtering the tiny particles with its cheek teeth.
A family tree of the Enaliosauria is here. A more complete tree of the Reptilia is here. The wide variety displayed by thalattosauriforms hints at a wider variety and several missing links yet to be discovered.