Seeing out of the shell

The turtle shell, or carapace, is a surprising novelty in the vertebrate world, and is emblematic of the entire Order, Testudines. No other species carries its home and its armor in such a long-suffering manner making its morphology very recognisable. Yet, the animals of this ancient and special clade should be recognised as much for their ocular adaptations, which have implications for the basal radiations of reptiles.

Extant turtles evolved from the oldest known turtle of the Jurassic over 210 million years ago, but this could not have been the first turtle. The carapaces of these (mostly) vegetarians developed from fused ribs to encase the body in a bony or leathery shell that surely evolved for protection, but could not have appeared without a period of gradual change.

The turtles are likely to have descended from a stem reptilian creature that had descended from the early anurans (frogs) and before that, the slow moving tetrapod pioneers emerging from an aquatic environment as terrestrial vertebrates.

Although the order of descent from the first stem reptile is controversial, it probably radiated into several different branches …