Just When Did Turtles Get Their Shells?

Peter Suciu for redOrbit.com — Your Universe Online

While there are several species that have shells, the turtle is the only animal to form such a shell on the outside of its body through a fusion of modified ribs, vertebrae and shoulder girdle bone. This makes the turtle´s shell a unique modification, one that has fascinated and confounded biologists for more than two centuries.

How and when it originated has remained mostly conjecture.

However, a team of researchers led by a Smithsonian scientist recently discovered that the beginnings of the turtle shell began some 40 million years earlier than previously thought. The findings of this research, titled “Evolutionary Origin of the Turtle Shell,” were published in the May 30 issue of Current Biology.

To date, the oldest known fossil turtle dates back about 210 million years, but this example features a fully formed shell. This provided little insight into the early shell evolution.

“The turtle shell is a complex structure whose initial transformations started over 260 million years ago in the Permian period,” explained Tyler Lyson of Yale University and a postdoctoral researcher at the Smithsonian. “Like other complex structures, the shell evolved over millions of years and was gradually modified into its present-day shape.”

However, in 2008 the 220 million-year-old fossil remains of an early turtle species, Odontochelys semitestacea, were discovered in China. This fossil suggested that the turtle had a fully developed plastron, which is the belly portion of the shell, yet only had a partial carapace made up of distinctively broadened ribs and vertebrae on its back.

This knowledge led the researchers to recently discovered specimens of the Eunotosaurus africanus, a South African species 40 million years older than O. semitestacea. This other specimen also had distinctively broadened ribs, and this indicated to the researchers that it shared many features only found in turtles.

This included no intercostal muscles that run in between the ribs, paired belly ribs and a specialized mode of rib development. To the researchers, this indicated that Eunotosaurus represents one of the first species to form the evolutionary branch of turtles. This further takes the turtle and its shell back another 40 million or years.

Lyson noted that Eunotosaurus fills an approximately 30-55 million year gap in the turtle fossil record, and that there are several anatomical and development features that indicate that this could be an early representative of the turtle´s linage. However, he added this morphology is intermediate between the specialized shell found in modern turtles to those primitive shells found in other vertebrates. In this way the Eunotosaurus could help bridge that gap between turtles and other reptiles.

If a shell works so well for turtles, the question then becomes why other animals don´t have shells?

“The reason, I think, that more animals don´t form a shell via the broadening and eventually suturing together of the ribs is that the ribs of mammals and lizards are used to help ventilate the lungs,” Lyson added. “If you incorporate your ribs into a protective shell, then you have to find a new way to breathe!”

Lyson and his colleagues will next look to investigate various other aspects of turtles´ respiratory systems, which allows the creatures to manage with their ribs locked up into a protective outer shell.

“It is clear that this novel lung ventilation mechanism evolved in tandem with the origin of the turtle shell,” Lyson noted.