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, Poland, Austria and the island of Spitsbergen off the northern coast of Norway.
Description
Omphalosaurus is a moderately large and plump marine reptile, measuring 5 m (16 ft) long and weighing more than 783 kg (1,726 lb). It is best known for its 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 is 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. Furthermore, it seems likely that early Ichthyosaurs typically moved slowly up and down the water column, or may have had physiological protection for quick water pressure changes.
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