Chapter 8 - Distribution and Diversity of Cretaceous Chelonioids

Abstract

This chapter discusses distribution and diversity of Cretaceous chelonioids. The Cretaceous Chelonioidea, which includes at least 22 valid genera, is divided into the three monophyletic families—Cheloniidae, Protostegidae, and Dermochelyidae. The cheloniids are well diversified and abundant in the Late Cretaceous of North America and Western Europe, whereas they are absent from the Japanese Cretaceous. The Protostegidae and Dermochelyidae are considered to be more closely related to each other than either is to the cheloniids. They possess a very elongated coracoid, a pelvic girdle with a huge lateral process of the pubis, and a very long plastron. The skull of protostegids may be modified to feed on hard-shelled animals, judging from the extensive contact between pterygoid and quadrate, and the strong processus trochlearis oticum as seen in the living batagurid Malayemys, which is a typical molluscivorous freshwater turtle. The protostegids were cosmopolitan as were the dominant chelonioids from the Aptian to the Turonian; however, they drastically declined and became extinct during the Maastrichtian. The Mesozoic dermochelyids were dominant only in the Late Cretaceous of Japan. It has been suggested that most Mesozoic sea turtles are endemic, and, cosmopolitan chelonioid species are quite rare.