Although no fossils of Dermochelys coriacea have been discovered, other leatherbacks are known from as early as the Eocene of Africa, Europe, and North America (Cosmochelys, Egyptemys, Eosphargis, Psephophorus); the Ogliocene of Peru (Natemys); and Dermochelys pseudostracion has been found in French Miocene deposits (Romer, 1956; Pritchard, 1980). A recent study by Wood et al. (1996) has shown the fossil history and ancestry of Dermochelyidae to be much more complicated than formerly thought. Until now there has been a tendency to refer most fossil leatherbacks to the extinct genus Psephophorus with the implication that some member of that genus eventually evolved into Dermochelys. However, cladistic analysis by Wood et al. (1996) shows that most taxa previously placed in Psephophorus are not assignable to that genus, and that Psephophorus is definitely not the immediate ancestor of the extant Dermochelys. Their analysis also revealed several developing lines of leatherbacks. The oldest known Cretaceous leatherbacks had shell morphologies resembling those of other marine turtles. Subsequent evolution led to several distinct lineages, of which only one has survived (Dermochelys). The genus Natemys represents a separate line unlike typical dermochelyids, and is characterized by a fully ossified plastral mosaic composed of large numbers of small ossicles. Several other dermochelyid lineages have modified carapaces with various types of ridge morphologies and other associated bony features. According to Wood et al. (1996), the main shell features in the evolution of Dermochelys are: 1) a progressive decrease in shell thickness; 2) a concurrent decrease in the size of the individual ossicles (and thereafter an overall increase in the number of ossicles forming the carapace); 3) a progressive increase in the prominence of the anteroposterior ridges, which are formed as flexures of the entire carapacial bony mosaic instead of being expressed only on the dorsal surface of the carapace; 4) the developing of undulating crests on the ridges; and 5) an increase in the number of ossicles between adjacent ridges.
Nowadays this family is represented by a single extant genus, Dermochelys Blainville, 1816, having only one living species, Dermochelys coriacea, the leatherback. This is the largest living turtle; a male stranded in 1988 at Harlech, Wales had a total length of 291 cm and weighed 916 kg. Dermochelys ranges widely in tropical and subtropical seas.
Both immunoprecipitation and electrophoretic tests of serum have shown Dermochelys to be the most distinct species of living sea turtle (Frair, 1979, 1982b). However, these tests also indicate a closeness that caused Frair (1982b) to suggest that the family Dermochelyidae be either placed in a superfamily containing all living sea turtles or relegated to subfamily rank within the family Cheloniidae. Zangerl (1980) also proposed that Dermochelys be placed in the same family as the cheloniids, and chromosome morphology supports this view (Bickham, 1979; Medrano et al., 1987). The minimum chronologic separation of modern hard-shelled sea-turtle lines is probably no less than 30 million years, and that leading to leatherbacks is probably at least 50 million years old (Carr, 1982). Studies on the comparative histology of the nasal epithelium of Dermochelys and species of the Cheloniidae by Saint-Girons (1991) supports the conclusion that the leatherback was adapted to an aquatic existence long before the cheloniids.