Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 31, Number 11;   November, 2004

Turtle History

Four important families of hidden-neck sea turtles did survive into the mid-Cretaceous period. Two of these families, the Dermochelyidae and the Cheloniidae, have modem descendants. The leatherback sea turtle is the only surviving member of the Dermochelyidae. All other modern sea turtles belong to the Cheloniidae.

Extinct Species
The extinct Toxochelyidae appear to be related to the Cheloniidae. They were small- to medium-sized, round-shelled sea turtles. Some had upper shells of solid bone like modern sea turtles, while other members of the family had much lighter upper shells with large openings, most likely an adaptation for open ocean existence. In life these openings would have been covered with skin or horny plates. Toxochelys, the best known member of this family, had eye sockets that faced up, suggesting that the turtle may have been a bottom-dweller. The toxochelyids died out by the late Eocene (56 to 37 million years ago).

The extinct Protostegidae may be related to the Dermochelyidae. Like the toxochelyids, many of the protostegids had frame-type shells with gaps between the bones, and probably lived in open ocean. Although most later protostegids were large to gigantic sea turtles with huge heads, the earliest known protostegid, Santanachelys, from 110 million ears ago, was only 8 inches (20 cm) long. Like modem fresh-water turtles, Santanachelys had highly flexible flippers with movable digits, but, like marine-dwelling turtles, it also had large salt-excreting glands, indicating that it lived in the ocean. Later protostegids had semi-rigid flippers like modern sea turtles. This family includes the giants Protostega and Archelon, the latter being the largest sea turtle that ever lived.

Archelon inhabited the Western Interior Seaway, or Niobrara Sea, which covered the middle of the North American continent, separating the Rocky Mountains from the eastern half of the continent, and connecting the Arctic Ocean to the Gulf of Mexico. One Archelon fossil is 15 feet (4.5 m) long from beak to tail, with a span of 16.5 feet (5.25 m) between the extended tips of its massive flippers. Estimates of the creature's weight range from 4,500 to 11,000 pounds (2-5 metric tons). Archelon's huge head alone could be 3.3 feet (1 m) long. The turtle may have used its formidable curved beak to crush ammonites, shelled molluscs related to the chambered nautilus.

Many types of ammonites occupied the seas during most of the Mesozoic era, but they began to disappear toward the end of the Cretaceous period. This may explain why the protostegid turtles disappeared at about the same time. Only one species of protostegid is known to have survived the mass extinction that eliminated the dinosaurs and the last of the giant fish-like reptiles at the end of the Cretaceous period. Eventually it too disappeared, leaving the leatherback and cheloniid lines (and a variety of terrestrial and freshwater turtles) to carry on to modern times.

Source
Sea Turtles of the World, © Doug Perrine, 2003.