The first marine invertebrates, fish, amphibians, reptiles, birds and mammals appeared 550 million years ago. Dinosaurs ruled the Earth in the Jurassic period, some 150 million years ago and among these, the first sea turtles appeared. The extinction of dinosaurs in the Cretaceous period, 65 million years ago, allowed other groups of animals to evolve.
The oldest sea turtle record in world, Santanachelys Gaffney (Hirayama1998), was recently found in the interior of the Ceará region, northern Brazil. This species belongs to the Cryptodira suborder and is the first and oldest record of the extinct Protostegidae family of sea turtles that inhabited South America, approximately 110 million years ago.
Another fossil record belonging to the same Cryptodira suborder was discovered in Angola. This species, named Angolachelys mbaxi belongs to the first group of turtles that evolved in the southern hemisphere, after the opening of the South Atlantic about 90 million years ago. Furthermore, it represents the oldest group of marine Cryptodira turtles in Africa.
The biggest sea turtle species known to have existed in the Cretaceous period, between 75 and 65 million years ago was Archelon ischyrus, whose name means "large turtle"; this species belonged to the Protostegidae family, and shared its habitat with the great aquatic reptiles Plesiosaurus and Ichthyosaurus. Archelon individuals measured between 10 and 13 feet long, weighed 1.5 to 2 tons, were carnivorous and inhabited the oceans where North America currently is.
Black Hills Institute of Geological Research, Inc.
Also during the Cretaceous period, four families of sea turtles (Toxochelyidae, Protostegidae, Cheloniidae and Dermochelyidae) were established but only the latter two have survived. All genus and species that exist today, evolved in the Eocene and Pleistocene periods, between 60 and 10 million years ago. They adapted to the life in the oceans, forests, swamps and deserts, representing a great success in the world of reptiles. Along with marine snakes and iguanas, sea turtles are the only surviving reptiles adapted to marine life.
Most turtle species that existed in the Cretaceous period are now extinct. The turtles that survived colonised all the oceans and gave rise to the various species. Nowadays, sea turtles inhabit the oceans and reproduce on the beaches of the tropical, subtropical and temperate climates.
Currently, only seven species, representing two families, still exist. They are the only living members of the evolutionary radiation that happened within the Cryptodira group, which resulted in a dramatic and quick increase in taxonomic diversity of these animals.
The Dermochelyidae family is represented by one single species, Dermochelys coriacea, whilst the remaining six species belong to the Cheloniidae family: Chelonia mydas, Caretta caretta, Eretmochelys imbricata, Lepidochelys olivacea, Lepidochelys kempii and Natator depressus.
The black turtle or the Eastern Pacific green turtle is recognised by some experts as a distinct species, Chelonia agassizii. However, the data published so far on morphological, biochemical and genetic characteristics does not support this classification. It suggests instead that the black turtle populations belong to the Chelonia mydas species.