Fossil fills gap in turtle shell history

Eunotosaurus fossil and living turtle
Turtles are the only animals to form a shell that incorporates their ribs and vertebrae. These bones broaden, and eventually the ribs suture together with other ribs and the vertebrae to form the major portion of the turtle shell.
Dr Tyler Lyson
Fossil finds of an extinct South African reptile, Eunotosaurus africanus, have helped to shed light on the evolutionary process by which the turtle got its shell…

A paper in the latest edition of the Cell Press journal Current Biology reports that the evolutionary gap in our knowledge of how members of the order Chelonii, or Testudines, got their shells. Broadly referred to as ‘turtles’, this group of reptiles includes tortoises, sea turtles, terrapins and their relatives.

The initial transformations which led to the formation of the turtle shell began more than 260 million years ago in the Middle Permian. Like other complex structures, such as feathers, the shell – which is made up of approximately 50 bones – evolved over millions of years and was gradually modified into its present day shape.

"Turtles are the only animals to form a shell that incorporates their ribs and vertebrae," said lead author Dr Tyler Lyson, from Yale University and the Smithsonian Institution. "These bones broaden, and eventually the ribs suture together with other ribs and the vertebrae to form the major portion of the turtle shell."

Other animals, including armadillos and various lizards, form a shell via the acquisition of more and more bony scales called osteoderms; these are two very different ways of forming a shell.

Eunotosaurus neatly fills the gap
 

Until quite recently, the oldest turtles that had been found were around 210 million years old and possessed fully developed shells, making it difficult to determine the sequence of events in the evolution of the shell. This only changed in 2008, when a fossil of the Chinese Odontochelys semitestacea dating back to approximately 220 million years ago was found. It had a fully developed plastron – the belly side of the shell – but only a partial carapace covering its back.

"This discovery partially resolved the origin of the turtle shell in indicating that the shell did not form via the acquisition of osteoderms," explained Dr Lyson. "However, given it had a partial carapace and fully developed plastron, questions remained regarding the sequence of events in the origin of the turtle shell."

Furthermore, techniques to calculate rates of molecular change estimated that turtles diverged from other amniotes (egg-laying vertebrates which include mammals, turtles, lizards, crocodilians, and birds) approximately 250-275 million years ago, long before Odontochelys existed.

"Eunotosaurus, which has been estimated to be approximately 260 million years old, neatly fills this 30-55 million year gap in the early fossil record of turtles," noted Dr Lyson. "It has some important features of the turtle shell, but lacks other features which develop later in ontogeny. As such, Eunotosaurus is a good transitional fossil which bridges the morphological gap between turtles and other reptiles.

"The consilience between the fossil and developmental data indicates that first the ribs broadened, then the neural spines of the vertebrae broadened, and finally osteoderms on the perimeter of the shell formed and everything sutured together to form the modern day turtle shell."

Dr Lyson and his colleagues used new fossil specimens of Eunotosaurus in combination with histological techniques to determine that the reptile represents an early stem turtle.



"The new specimens revealed some interesting morphological features found only in turtles and Eunotosaurus including nine broadened ribs, nine elongate trunk vertebrae, ribs that are T-shaped in cross section, and paired gastralia that lack lateral and medial elements," he told ScienceOmega.com. "All of these morphological features strongly support the idea that Eunotosaurus represents an early stem turtle.

"Since we found tendinous insertion of muscle on one side of the rib and not the other, we inferred that Eunotosaurus lacked intercostal muscles. The only amniotes that lack intercostal muscles are turtles."

Dr Lyson admitted that he was surprised to find the ribs of Eunotosaurus only had tendinous insertion of muscles on one side and apparently lacked intercostal muscles. All other amniotes use their ribs to help ventilate their ribcage.

"Since turtles have locked up their ribs into an immobile protective shell, they use a muscular sling to ventilate their lungs instead," said Dr Lyson. "It is clear that this novel lung ventilation mechanism evolved in tandem with the origin of the turtle shell. Along with collaborators at the University of Utah and the Smithsonian Institution, I am now investigating various aspects of the turtles’ respiratory system."

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