Shellementary, My Dear Watson!

The turtle is certainly a bizarre creature. Along with longevity that rivals our own, they can grow up to 6.6 feet and weigh upwards of a ton. They are ectotherms (well, mostly,) and have fascinating reproductive rituals, which include migrating thousands of miles each year to nest at the same beach at which they were born. Turtles have been around for 220 million years, ranking them among some of the oldest species on earth. Turtles have no shortage of interesting evolutionary caveats, but perhaps the most fascinating aspect of the Testudine (the fancy, cladistic name for both Turtles and Tortoises) is their shell. We tend not to think much about the Turtle shell (or Carapace, as it is formally known), but it really is an evolutionary marvel. Not convinced? Keep reading.

Look Ma! No ribs!

Eunotosaurus

If you were take a look inside of a modern turtle, what you see may surprise you. In place of a rib cage (which serves to protect our inner organs from external abuse), turtles have developed a shell. Where on earth did this come from? Thanks to a recent study at Yale, we think that the turtles eldest ancestor was Eunotosaurus, a protoreptile (which is a subclass of Reptilla, the class to which turtles belong). This crazy creature looked like a lizard that swallowed a frisbee. Eunotosaurus possessed a broadened rib cage, which is thought to be the earliest precursor of the modern turtle shell. The story of the turtle shell continues with the now extinct Odontochelys, which had the lower shell (or plastron) of modern turtles, but it still lacked the distinctive carapace. In its place, it had groupings of bones in its skin called osteoderms. The presence of dermal bone in Odontochelys suggests that the turtle carapace is not exclusively rib material, but instead a concoction of dermal and skeletal bone. As the genus continued to evolve, greater investments of dermal bone resulted in the creatures we have today.

The other crazy aspect of a turtle's shell is their scapula. Turtles walk on all four legs - their back legs are attached to their pelvis, and their front legs are attached to their scapula. Most creatures have their scapula on the outside of their rib cage; however, this isn't true in the case of Turtles. A turtle's scapula are actually found *inside* their shells. How did this happen? There are many theories about what exactly caused this inversion, but the current theory is that it is due to a mutated expression of the Sonic Hedgehog (SHH) protein, which plays a role in notochord development. This change in expression caused the scapula to form inside the rib cage, which allowed turtles to maintain a solid carapace while still retaining the ability to walk. The incredible ingenuity of this system is marvelous to look at in the context of evolution. There are still things we don't know about the turtle shell, but what we do know shows that evolution truly is a wonderfully complicated, yet elegant system that produces results that reach further than our wildest imagination.

If you'd like to know more about how the turtle shell was formed, here's a cool video illustrating the process!

Posted by David Almanzar (1)