As George Angehr said to The Straight Dope: “They are also one of the most distinctive life forms on the planet. My herpetology professor started his ‘Identification Key to the Reptiles’ with the couplet: ‘A. Turtles. Any damn fool knows a turtle. B. Other reptiles.'”
And it is true that the turtle may well have the most distinctive profile in the animal kingdom. It’s well deserved, since their entire anatomy has been radically altered in order to support their large, bony shells. But this rigid shell presents a real difficulty for turtle anatomy. The hips and shoulders have had to move inside the rib cage, which is itself fused with the spine and bony ossicles to form the carapace (upper shell). This, in turn, creates serious breathing problems. Most vertebrates force air into and out of their lungs through expanding and contracting the body cavity. You can see it yourself in the rise and fall of someone’s chest. Turtles have sacrificed this ability in the name of rigid protection, severely limiting their respiratory abilities.
However, this has lead to some interesting adaptations. For one thing, turtles are masters of going without. Turtle muscles and cells can store much more oxygen than normal and can tolerate much higher levels of carbon dioxide. And, in tough times, they can go without oxygen altogether. Most animals depend primarily on aerobic oxygen-based respiration to power their cells and turn sugars into energy. Turtles, however, have become masters of anaerobic respiration and can survive in totally oxygen-starved environments for hours, sometimes days. This may also be linked to their general lethargy and longevity, since aerobic respiration is generally both more efficient and more damaging to cells.
But in perhaps the strangest adaptation of all, some of the aquatic varieties have adapted away from using their lungs altogether. Certain turtles have developed specialized pouches on the inside of their cloaca (combination gastrointestinal/reproductive tract) that can extract oxygen out of the water, like gills. The blood vessels are so fine and so close to the surface that gases can pass between the bloodstream and environment. In some species this butt-breathing can account for as much as two-thirds of their respiration.
We have yet to totally understand turtle evolution as missing links between turtles and other reptiles are few in number and often incomplete. But Angehr was not lying. Turtles are one of the most iconic animals on earth and, perhaps, one of the most interesting evolutionary studies.