The Oldest Water Turtle:
Odontochelys semitestacea
The ancient water turtle called Odontochelys semitestacea is the oldest known ancestor of the tortoises and turtles. This creature lived in what is now China about 220 million years ago. This is considerably later than the former contender for the position of ancestral turtle, the tiny and now unrelated Eunotosaurus.
Three very well preserved and virtually complete skeletons of this animal have been found. This rules out the possibility that the animal was a freak or the result of two animals becoming fossilized together, one on top of the other.This animal was not grouped with turtles and tortoises just on overall looks. That mistake had already been made and corrected with Eunotosaurus. The state of the plastron developing before the upper part of the shell (called the carapace) is formed is also seen in the embryonic development of modern turtles, indicating that Odontochelys is at least part of the turtle lineage, if not the direct ancestor. Similarities With TurtlesIf you are at all familiar with what turtles look like underneath, the plastron (bottom part of the shell) is immediately recognizable as belonging to a turtle. Odontochelys also had bony plates covering its spine, effectively forming an armored strip down the center of its upper body. The ribs were broad and curved, forming the familiar dome like shape of today's turtle, but the ribs did not have a bony outer covering. Another clear indication that this animal is a turtle is the hook on the fifth tarsal of the back foot. This seems like an obscure thing to classify an animal on, but that tarsal is a feature of all members of the turtle family.
Differences From TurtlesThis creature had teeth which no modern turtles possess. The skull was also longer as was the tail.
An Aquatic LifestyleThe presence of the armored belly on this water turtle indicates it lived in water. The plastron offered it protection from predators attacking from below. If Odontochelys was a land based animal, this armor would serve no function at all. From this it's reasonable to assume that turtles evolved in aquatic environments, and that the shell or at least the bottom of it was well formed by the time these reptiles re-emerged onto land.
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