Omphalosaurus

Omphalosaurus (from the Greek root "Button Lizard", for their button-like teeth) is an extinct genus of marine reptile from the Early Triassic to Middle Triassic, thought to be in the order of Ichthyosauria. Most of what is known about "Omphalosaurus" is based on multiple jaw fragments, ribs, and vertebrae. Specimens of "Omphalosaurus" have been described from the western United States, Germany, Austria and the island of Spitsbergen off the northern coast of Norway.
Description.
"Omphalosaurus" are moderately large and plump marine reptiles and are best known for their highly specialized dentition compared to other Ichthyosaurs. The teeth are button-like, with a dome shape when viewed laterally and almost circular crowns that have an irregular enamel surface akin to the texture of an orange peel. Individual teeth do not exceed 12mm in diameter and are arranged in tooth plates exclusively on the premaxilla, which sit at 90º from each other, and dentary. Based on "O. nevadanus"’ well preserved and smooth palatine, it’s unlikely that "Omphalosaurus" had palatine teeth akin to Placodonts. Nonetheless, "Omphalosaurus" teeth could potentially number in the hundreds, and are concentrated along the skull midline. Each species has varying degrees of tooth organization, but "O. nevadanus" has the most neatly organized teeth, which most closely resemble distinct rows despite some unevenness. Attempts have been made to count the number of rows of teeth for the other species, but they are mostly irregularly patterned on the occlusal surface.
The upper tooth plates form a convex surface, while the lower plate is concave. They were previously thought to have short, broad jaws and powerful bite forces, but recent reconstruction indicates that the dentary symphysis is elongated and connects at an approximately 15º angle, giving the jaw a long “V” shape. If reconstructed, the lower jaw of "O. nevadanus" could potentially exceed 50 cm in length.
Jaw fragments have revealed that "Omphalosaurus" had a dental batteries that were optimized for constant wear, with high tooth replacement rates. "Omphalosaurus" is unusual in that their immature replacement teeth and mature teeth had different enamel microstructure. Like other Ichthyosaurs, "Omphalosaurus" have a microunit enamel in their mature teeth, while replacement teeth have columnar enamel. It is currently unknown how this transformation occurs.
Aside from dentition, "Omphalosaurus" is relatively poorly known, save for a small number of ribs and presacral vertebrae attributed to "O. wolfi." The ribs are swollen and hollow, which is a common characteristic in amniotes returning to water, and the vertebrae are deeply amphicoelous. "Omphalosaurus" have lost the neural arch atop the centra of the vertebrae. Their bones have woven-fibered bone tissue, indicating rapid rate of bone growth.
Paleobiology.
Diet.
"Omphalosaurus’" highly specialized dentition indicates that they were durophagous animals. 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. Recent evidence suggests that they focused their hunting on ammonites over bivalves, the latter of which is preferred by placodonts.
Decompression sickness.
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.
Discovery and classification.
The first fossil Omphalosaurus was found in 1902 by V. C. Osmont in Nevada, United States, and it was first described in 1906 by John C. Merriam. Merriam did not identify the fossil "O. nevadanus" as Ichthyosaurian, suggesting instead placodont or rhynchosaurus affinities. The first to identify "Omphalosaurus" as Ichthyosaur was Kuhn in 1934 and Mazin justified the grouping in 1983. In 1997 and 2000, Motani argued against the assignment, citing the lack of basal synapomorphies of Ichthyopterygia and suggesting sauropterygian affinities. However, Maisch described a new species in 2010 and restated its affinity with Ichthyosauria.
"Omphalosaurus" are currently considered small-to-medium-sized Ichthyosaurs. Like other Ichthyosaurs, they have deeply amphicoelous vertebrae with no distinct transverse processes, and their centra are shorter than they are wide. The ribs of Omphalosaurus share the dorsoventrally articulation of Ichthyosaurian family Shastasauridae and "O. wolfi" was shown to have the same cancellous bone structure as Ichthyosaurs, though this is common in several other aquatic species. One of the most distinct traits placing "Omphalosaurus" within Ichthyosauria is that they share the same microunit enamel in mature teeth that Ichthyosaurs are known to have, but that is rare in other reptiles.
The most prominent feature that has created controversy in the assignment of "Omphalosaurus" is the placement of the teeth. Unlike other Ichthyosaurs, for which teeth form distinct rows, "Omphalosaurus" teeth form an irregular pavement. Additionally, no other Ichthyosaurs have maxillary grinding surfaces at right angles of each other. The tooth crowns of "Omphalosaurus" are lower and more irregular than other durophagus Ichthyosaurs, and the enamel typically has an orange-peel textured surface rather than Ichthyosaur's typical longitudinal wrinkles. Omphalosaurus also have hollow ribs and humerus with prominent deltopectoral crest, neither of which are found in other Ichthyosaurs.