A fossil Toxochelys from about 75 million years ago

The evolutionary origin of turtles has been called one of the great unanswered questions of evolutionary biology. The scientific community does not know from what group of primitive reptiles’ turtles descended.

The oldest known turtle fossils, discovered in Germany, Greenland, and Thailand (countries once part of the supercontinent Pangea), belong to the extinct genus Proganochelys and are believed to be about 210 million years old. Proganochelys looked very much like modern turtles but possessed something that none of today’s turtles have - tiny teeth on the roof of the mouth. It also had spikes on its neck and tail. Proganochelys may have been terrestrial or an amphibious swamp-dweller. That all turtles lay their eggs on land is a strong clue that the first turtle was a terrestrial animal. Evidence shows, however, that the first turtles must have evolved even earlier than 210 million years ago.

A little over 200 million years ago, at the beginning of the Jurassic period, a mass extinction occurred, the second in the earth’s history. Both dinosaurs and turtles survived and thrived. Turtles flourished throughout the Age of Reptiles, or Mesozoic era (245 to 65 million years ago), producing many more species and families than there are today, including such bizarre forms as the horned turtles, or meiolaniids. These had goat-like or steer-like horns, giving the skulls the appearance of devil masks. Most of these families were aquatic, living in both freshwater and marine habitats. The adaptation to living in the sea occurred not once, but several times in distinct groups of turtles, so the transition from life in a coastal marsh to swimming in the ocean must not have been too difficult.

Proganochelys - the earliest known turtle

Proganochelys, the earliest known turtle, is believed to be an evolutionary dead-end, and not an ancestor ofmodern turtles. Both Proganochelys and today’s turtles must be descended from earlier ancestors whose fossilized remains have not yet been found.

Scientists speculate that about the time turtles had achieved their greatest diversity (approximately 65 million years ago), a giant asteroid may have collided with the earth, causing the next mass extinction. Dinosaurs were wiped out, except for the group that developed into birds, but some turtles survived and continued to evolve.

Sea Turtle Beginnings

The oldest marine turtle fossils found date from the Jurassic period (208 to 145 million years ago). By that time, the main lineage of turtles had split into two branches: the side-neck turtles (pleurodires), which protect the head by folding the neck and head over to one side, and the hidden-neck or arch-neck turtles (cryptodires), which pull the neck into a vertical S-curve and retract the head straight back between the shoulders. The side-necked turtles produced many seagoing species during the Cretaceous period (145 to 65 million years ago), but these died out. Modern pleurodires live in freshwater. Jurassic sea turtles belonged to the hidden-neck group, the group to which more turtles belong today. Many families once made up the hidden-neck group, but most died out by the early part of the Cretaceous period.

Four important families of hidden-neck sea turtles survived into the mid-Cretaceous period. Two of these families, the Dermochelyidae and the Cheloniidae, have modern descendants. The leather-back 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 giant sea turtle Protostega

The giant sea turtle Protostega, from the late Cretaceous period (80 to 70 millionyears ago), could grow as long as 4.2m (14 feet). Most modern turtles have a solid bone layer in the carapace, but Protostega, like Toxochelys, had large gaps between the bones, which reduced the turtle’s weight, possibly giving it an advantage in open sea habitat.

The extinct Protostegidae may be related to the Dermochelyidae. Like the toxochelyids, many of the protestegids had frame-type shells with gaps between the bones, and probably lived in open ocean. Although later protostegids were large to gigantic sea turtles with vast heads, the earliest known protostegid, Santanachelys, from 110 million years ago, was only 20 cm (8 inches) long. Like modern fresh-water turtles, Santanachelys had highly flexible flippers with movable digits, but, like marine-dwelling turtles, it also had large salt-excreting glands, showing 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 - the largest sea turtle that ever lived

Archelon, the largest sea turtle that ever lived (about 75 million years ago) and may have weighed between two to five tons.

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 4.5m (15 feet) long from beak to tail, with a span of 5.25m (16.5 feet) between the extended tips of its massive flippers. Estimates of the creature’s weight range from 2 to 5 metric tons (4,500 to 11,000 pounds). Archelon’s enormous head alone could be 1m (3.3 feet) long. The turtle may have used its formidable curved beak to crush ammonites, shelled mollusks related to the chambered nautilus.

Many types of ammonites occupied the seas during most of the Mesozoic era, but they disappeared 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 has survived the mass extinction that eliminated the dinosaurs and the last of the giant fishlike 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.