The Marine Crocodilian Hyposaurus in North America

Publisher Summary This chapter discusses the marine crocodilian Hyposaurus in North America. The fossil genus Hyposaurus, a longirostrine mesosuchian crocodilian, is a characteristic and relatively common fossil from marine sediments of Maastrichtian through Danian ages. Its geographic range in the United States now extends from New Jersey to Alabama, in glauconite and chalk facies. A specimen that comprises the most perfectly preserved skull and jaws of Hyposaurus rogersii is analyzed. A diminutive seventh mandibular tooth is present only in the right dentary and it is completely absent from the left. This reduction of the seventh mandibular tooth is a synapomorphy of the Dyrosauridae. The edge of each dentary is slightly festooned. The mandibular symphysis varies from quadrangular to oval in transverse section, and is slightly wider than high. The type specimen of Hyposaurus fraterculus Cope is now known to be a juvenile Thoracosaurus, leaving H. rogersii Owen as the only North American species. It is notable as a marine guide fossil of short stratigraphic range, which does, however, span the Cretaceous-Tertiary boundary. It is found that one well-preserved specimen has yielded remarkably large gastroliths, prompting comparison to those of elasmosaurs, with potential contributions to stability in marine environments.