The new genetic research was led by Eugenia Naro-Maciel, a marine biodiversity scientist at the American Museum of Natural History.

Naro-Maciel and colleagues used five nuclear DNA markers and two mitochondrial markers to test the evolutionary relationships of all species of marine turtles -- leatherback, flatback, green, hawksbill, loggerhead, Kemp's Ridley, and Olive Ridley.

The researchers said their findings form a well-supported phylogenetic tree, or cladogram, that tells the story of sea turtle evolution.

"The evolution of a specialized diet appears to have occurred three times, independently," said Naro-Maciel. "Many sea turtles are carnivorous generalists. However, hawksbills tend to have a diet of glass -- they eat toxic sponges -- while the leatherback consumes jellyfish and the green grazes mainly on algae or sea grass."

All sea turtles are listed as endangered on the International Union for Conservation of Nature's "Red List of Threatened Species," some of them as critically endangered.

The research that included scientists from the University of Canberra is detailed in the journal Molecular Phylogenetics and Evolution.