A turtle’s shell makes for a great portable home, offering a huge amount of protection to the animal living inside. But this carapace is nothing more than an evolutionary byproduct of another adaptation. The turtle shell evolved to help it burrow into the earth.
"Just like the bird feather did not initially evolve for flight," paleontologist Tyler Lyson told the Denver Museum of Nature and Science, "the earliest beginnings of the turtle shell was not for protection but rather for digging underground to escape the harsh South African environment where these early proto-turtles lived."
The origin of the turtle shell has been uncovered by Lyson by studying fossils. The shell is actually a broadening of the animal’s ribs, but a protective shell is a rather abstract goal for natural selection to work towards. The problem is that the penalties for bigger ribs outweigh the advantages. Heavy ribs slow breathing, and slow the animal itself, making walking difficult. Until the shell has fully developed, those disadvantages would actually work against the evolving of a shell. A more practical, segmented shell like an armadillo’s would be more likely.
Was there perhaps an intermediary advantage that would coax the formation of bigger, stronger ribs?
"Ribs are generally pretty boring bones," says Lyson. "The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell."
In the turtle, the lower ribs grew to make the plastron, the chest-covering plate. Then the upper ribs expanded, joined the spine, and eventually grew over the shoulder blades.
When fossils of partially-shelled "proto-turtles"—Eunotosaurus—were discovered in South Africa, the 260-million-year-old imprints provided the missing link. Lyson saw that Eunotosaurus had big claws, and big ribs for anchoring strong arms. These arms were made from much thicker bone than the hind legs. He also deduced that the creatures had bony eye sockets, which meant small eyes that were insensitive to light. This all fit into a picture of the early turtle as an underground creature that dug, more mole-like than the turtle we know today.
So that’s how the turtle got its shell. Not as protection, but to make it better at digging and tunneling. That the shell now has a completely different use is the result of exaptation, the process of features acquiring functions they weren’t designed for. In the case of the turtle, it explains how something so cumbersome and unwieldy came to to be so useful.
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