Nucleotide sequences from the cytochrome b gene of mitochondrial DNA were employed to resolve phylogenetic controversies and to assess molecular evolutionary rates in marine turtles (Chelonioidea). Findings of special relevance to conservation biology include discovery of a distant relationship between Natator and other cheloniid species, the paraphyly of Chelonia mydas with respect to Chelonia agassizi, and genetic distinctiveness of Lepidochelys kempi from Lepidochelys olivacea. A longstanding debate in evolutionary ecology was resolved by phylogenetic mapping of dietary habits, which indicates that the spongivore Eretmochelys imbricata evolved from a carnivorous rather than a herbivorous ancestor. Sequence divergences at intergeneric and interfamilial levels, when assessed against fossil-based separation times, support previous suggestions (from microevolutionary comparisons) that mitochondrial DNA in marine turtles evolves much more slowly than under the ''conventional'' vertebrate clock. This slow pace of nucleotide replacement is consistent with recent hypotheses linking substitution rate to generation length and metabolic pace.
