A Turtle's Shell
Turtles are well known for their shells- keratin houses that makes them look both comical and particularly ancient. Of all the backboned animals in the world, turtles are the only species to have shells. The shell is intricately connected with the turtles central nervous system (including their spinal cord and nerve endings), and is imperative for their survival. So how did they singularly evolve with both a backbone and their shells?
The reason why turtles look vaguely like ancient dinosaurs is simple: They are. The first and oldest documented species of turtle, Pappochelys (the first fully-shelled turtle) can be traced back to 240 million years ago, far earlier than when dinosaurs first roamed the earth. As Tyrannosaurus rex moved through North America, the first species of turtle had already been extinct for more than 140 million years.
A very ancient species now dubbed Eunotosaurus has more recently been considered as a remote precursor to the modern turtle. Eunotosaurus had no shell, but did have partially overlapping broad ribs, which curved up and vaguely resembled that of a turtle's underlying shell structure. This gave Euntosaurus exceptional arm and shoulder muscles, making it a much powerful digger in sand and mud.
Eunotosaurus fossil, ©Tyler Lyson
But scientists still grappled with understanding why turtles evolved to have shells. Until recently, fossils found after Euntosaurus already had fully-formed shells. While incredible, the findings gave little clarity into the shell's origin. It seemed unlikely that a complete shell would evolve all at once - so what happened in between?
Then, in 2015, a discovery was made in China: Odontechelys. This ancient fossil is dated back to over 220 million years ago, and is indisputably a precursor relative to the turtle we know today. Odontechelys lacked any distinct outer shell, but on closer inspection had a fully formed plastron (the flat underside part of the turtle shell): Odontechelys was a turtle with half a shell.
Work by Nobu Tamura, Digital
It's thought that perhaps Odontechelys indicates that turtles evolved primarily in the water. This would make sense, as many marine predators attack from below: A bottom shell would help protect from this. Odontechelys was the missing link connecting the evolutionary path between Euntosaurus and the turtles we know today.
What We Know
Today, we understand that turtle's shells seem to have evolved for two purposes: First, the overlapping, broadened ribcage allowed for a strengthened trunk for digging. Later, the overlapping plates broadened and formed bony plates which added additional protection from predators.
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