The shell bone histology of fossil and extant marine turtles revisited

Abstract

Modern turtles exhibit a broad scope of ecological adaptations, including coastal marine and pelagic habitats, and, during their evolutionary history, turtles repeatedly exploited the aquatic environment as well. Although some pleurodiran clades also ventured into the marine realm, it is the cryptodires that did so most extensively. Among those, three major radiation phases are distinguished, with the first phase consisting of basal eucryptodiran taxa inhabiting littoral or near costal environments (Late Jurassic, Europe); the second phase including more open marine chelonioids (starting in the late Early Cretaceous, mainly North America and Eurasia); and the third phase (starting in the Palaeocene/Eocene, global distribution) including the highly-nested chelonioids, such as the modern cheloniid and dermochelyid turtles and closest relatives. A review of previously published as well as unpublished data of shell microstructures of these groups and those of some of the earliest aquatic turtles from the Middle Jurassic, Heckerochelys romani and Eileanchelys waldmani, show that bones are strongly influenced functionally as a result of life spent in an aquatic medium, whereas there are little to no characters of systematic value in the bones. We confirm the general tetrapod pattern that pelagic forms tend to show osteoporotic-like shell structures and neritic forms tend to have more bone ballast, especially by retaining a thickened external compacta.