History And Fossil Record
Turtles first appear in the fossil record of the Triassic period, from about 215 million years ago. This gives them an older fossil history than any other living kind of four-legged animal. Turtles were already present when the first dinosaurs appeared, and they shared the ancient seas with ichthyosaurs, watched pterosaurs soar overhead, and saw the first small, furry mammals. Undergoing relatively little change themselves, turtles witnessed the evolution of birds from feathered dinosaurs, and they were present as some of the early mammals evolved into elephants, whales, bats, and even human beings.
The earliest known fossil turtles (Proganochelys) are from late Triassic (Norian) sedimentary deposits in Germany. These ancestral turtles had a 3 ft (1m) shell length, and were terrestrial or marsh-dwelling animals. Like modern turtles, the shell of the fossil animals was composed of a rounded upper half (or carapace) and a flattened lower one (plastron). The carapace of the earliest fossil turtles incorporated 10 vertebrae, their associated ribs, and additional bone between the ribs. The skull was solidly constructed, without temporal openings, and the jaws were toothless and presumably beaked as in modern turtles. There were, however, small teeth in the palate, which are not found among living turtles. The eight cervical
(neck) vertebrae were primitive, in that they were not modified to allow the head to withdraw into the shell.
Turtles are fairly large animals, and have a great deal of bone. In addition, they often occur in aquatic or marshy habitats where their bones are likely to be buried and preserved. Consequently, their fossils are found relatively frequently in ancient sedimentary deposits from the Jurassic and younger eras. Many fossils are found in Cretaceous deposits of North America.
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