Omphalosaurus

Omphalosaurus’ highly specialized dentition indicates that they were durophagous animals.[6] Their teeth were optimized for heavy wear, and CT scans indicate they had high rates of replacement to deal with a hard diet. However, they lacked the gripping dentition needed to grab prey, and the narrow jaw and anterior tooth placement do not match the short, massive skulls and jaws of other species with the strong bite force required to break shells. The combination of highly worn teeth and low bite force is more similar to herbivores and ornithopod dinosaurs. Like ornithopods, Omphalosaurus have a high rate of tooth replacement and smooth secondary occlusal surfaces, but the lack of fibrous marine plants during the Middle Triassic make it unlikely that it was herbivorous. Ammonites and pseudoplanctonic halobiid bivalves were, on the contrary, common in Omphalosaurus’ range and time period, and their shells were hard but thin. Sander and Faber hypothesized that Omphalosaurus could have had fleshy cheeks and used suction feeding to make up for the lack of grasping dentition, and could then proceed to grind through the shells, allowing them to feed on these animals.[2] Recent evidence suggests that they focused their hunting on ammonites over bivalves, the latter of which is preferred by placodonts.[10]

Like other early Ichthyosaurs, there is no evidence of avascular necrosis in Omphalosaurus, indicating that they were likely not subjected to decompression sickness. Rothschild et al. attributed this to the lack of large aquatic predators in the early to middle Triassic, which meant that Omphalosaurus would not have needed to quickly dive to escape. Early Ichthyosaurs would have only or almost only had slow movement up and down the water column or may have had physiological protection for quick water pressure changes.[11]

• Omphalosaurus nevadanus is the type specimen of the genus, and one of the two species found in the marine Prida Formation in the Humboldt Range of Nevada, United States. O. nevadanus originates from the Middle Triassic Anisian beds and was first described by Merriam in 1906.[6] The fossils consist of the inferior portion of the cranium with anterior cervical vertebrae and a portion of the mandible that has lost the articular and is broken before the symphesis. The angular, supraangular, dentary, and splenial are all visible on the mandible.[12] Unlike the other species, it has a rounded posterior margin of the dentary symphesis. O. nevadanus is much larger than O. nettarhynchus and has more numerous teeth.[6]
• Omphalosaurus nettarhynchus is the second species found in the Prida Formation, originating in the Middle Triassic Spathian beds. It was first described by Mazin and Bucher in 1987. The fossil consists only of a fragmentary lower jaw, but O. nettarhynchus is distinguishable from other species due to its smaller size, relatively few but large teeth, and laterally expanded jaw symphysis.[6]
• Omphalosaurus merriami originates from the marine Sticky Keep Formation in Svalbard, and was described by Maisch in 2010. It lived in the lower Triassic period. The fossil consists of jaw fragments that were initially thought to be part of Pessopteryx nisseri, which it was found with, until the jaw was found to be distinctly Omphalosaurian. The jaw fragments have three rows of teeth with smooth enamel and tooth roots containing plicidentine.[13]
• An Omphalosaurus sp. left humerus from the Middle Triassic was found in the Muschelkalk of Franconia, Germany, specifically in the Hohenlohe beds of the Garnberg Quarry. It is from the late Ladinian and was found by Hagdorn in 1984, then described by Sander and Faber in 1998. The humerus displayed the typical Ichthyosaurian cancellous bone structure, but its specific species remains unclear.[6]

• Omphalosaurus wolfi is a Middle Triassic, early Ladinian species found in the Northern Alpes on Dürrnberg Mountain, in Lercheck Limestone.[6] It was found by G. Wolf in 1991 and described by Tichy in 1995.[14] The specimen consists of several presacral vertebrae that are deeply amphiceolous and short compared to height and width, ribs that were swollen and hollow, and a disarticulated skull[6] with a partially articulated lower jaw.[2] O. wolfi’s dentition resembles that of O. nisseri, while its lower jaw elements are similar to O. nevadanus, the only other species found with significant skull material.[6] Ten other unidentified skull bones[2] and a bone that may be a humerus were also found.[6] O. wolfi was suggested to actually be O. cf. nevadanus by Sander and Faber in 2003, but Maisch argued that re-investigation of cranial material was needed and that O. wolfi should stand as a species until then.[13]
• Omphalosaurus peyeri was from the Middle Anisian period. It was buried in the porous arenitic limestone of the Schaumkalk beds at Rüdersdorf, which are thought to be from a shallow aquatic environment that surfaced episodically, meaning that O. peyeri was likely a coastal inhabitant. The fossil is an incomplete posterior portion of the left maxilla with 3 mature, mushroom-shaped teeth and several more replacement teeth below. Unlike the other Omphalosaurus species, O. peyeri had just one row of functional and relatively few replacement teeth.[15] Because of this, it was initially thought to be a placodont until it was described as Omphalosaurus by Maisch and Lehmann in 2002[16] due to the typically Omphalosaurus tooth shape and orange peel texture of the enamel crown. It is thought to be the most basal Omphalosaurus species,[15] however its classification is debated by Wintrich and Sander due to the placodont-like columnar enamel.[4][10]

• Pessopteryx nisseri was a Spitsbergen fossil composed of several species found and described by Wiman in 2010, including the jaw fragments now attributed to O. merriami.[13] It was considered O. nisseri by Wiman and Mazin, but it is now accepted to be its own species and Pessopteryx its own genus based on the limb and pectoral girdle fossils that are of Ichthyosaurian nature.[17]