How Did the Turtle Get Its Shell?

In On the Origin of Species, Darwin asserted: “Monstrosities cannot be separated by any clear line of distinction from mere variations” (1). But encased in its shell, the turtle appears to be just such a monstrosity. No other animal, living or extinct, has its body enclosed within a bony shell that is similarly constructed in its entirety. Over the last few years, developmental biologists have started to tackle the question of how the turtle shell evolved. On page 193 of this issue, Nagashima et al. (2) provide a detailed account of muscular and skeletal changes during the embryogenesis of the modern turtle, and in drawing parallels between these early developmental changes and what is seen in ancestral turtles, provide insights into how turtle shell evolution might have occurred.