The Triassic marine reptile Omphalosaurus: osteology, jaw anatomy, and evidence for ichthyosaurian affinities

ABSTRACT

Omphalosaurus is a marine reptile known from the latest Early Triassic to the Middle Triassic. It has a wide distribution in the northern hemisphere, occurring in the eastern Pacific realm, in Spitsbergen, and in the western Tethys, but remains poorly known. A recently discovered partial skeleton from the northern Alps is the only specimen showing cranial and postcranial bones together. It is described here in detail and compared to the other known material, especially the holotype skull from Nevada.

Although numerous disarticulated skull bones are present in the Alpine specimen, their identification and interpretation is difficult. The lower jaw and the dentition are sufficiently well preserved to reconstruct morphology, function, and tooth replacement. Remarkably, the snout of Omphalosaurus was rather elongate with the crushing dentition placed anteriorly. The dentition is concentrated along the skull midline and forms a convex upper and concave lower pavement of button-like teeth, set in the premaxillae and the dentaries, respectively. The combination of the forward placement of the dentition and the high tooth replacement rates (as deduced from CT scans) suggests highly abrasive food requring relatively low masticatory forces.

Although ichthyosaurian affinities of Omphalosaurus have been controversial, the postcranial skeleton of the Alpine specimen and an improved understanding of cranial and dental anatomy provides further evidence for such affinities. Omphalosaurus possesses four synapomorphies placing it within the ichthyosaurs (preorbitai region distinctly longer than postorbital region, pterygoid flange strongly reduced, interpterygoid vacuity absent or strongly reduced, cervical ribs without distinctive anterior process) and five characters suggestive of ichthyosaurian affinities (splenial contributing extensively to jaw symphysis, shortened vertebral centra, dorsal ribs single-headed and articulating largely with centrum, lack of compact bone, microunit enamel).