New research is telling paleontologists about the evolutionary steps that led to the development of today’s turtles and their uniquely hard shells.
A study led by Dr. Tyler Lyson from Yale University and the Smithsonian Institution, published in the journal Current Biology, compared the morphology of modern turtles to that of ancient turtle predecessors.
Eunotosaurus africanus is a 260-million-year old South African reptile that is one of the earliest ancestors of the turtle. Lyson examined more than 45 Eunotosaurus specimens, ultimately coming to the conclusion that the turtle shell came into existence by the broadening of vertebrae and ribs.
When a turtle shell develops, the creature’s ribs broaden, then its vertebrae broaden, and lastly it attains bony scales on the edge of its shell. Eunotosaurus has, just like the modern turtle, broadened ribs and vertebrae; and also like today’s turtles, it lacks muscles between its ribs. But Eunotosaurus didn’t have broadened neural spines and bony scales—this came in later descendants. Eventually all the components of the shell became connected and resulted in the hard shell that turtles display today.
The turtle shell is an evolutionary novelty—no other creature features a body part analogous to it. When the ribs fused together over time, as the turtle’s did to evolve into the modern shell, the turtle could no longer use its ribs to breathe, as other animals do. It had to develop a new way to breathe—an abdominal muscle that surrounds the lungs.
Read Lyson’s study in the journal Current Biology.
Two videos by the study lead author Tyler Lyson, entitled Evolution of the Turtle Shell, show the intermediate species that evolved new bone structures and eventually led to the formation of the modern-day turtle shell. Find the bone-structure video here and an illustrated video here.
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