This spiny turtle, like other turtles, has an upper shell that forms as ribs widen and fuse into a bony plate.
Never mind Aesop and his fables. Japanese scientists are telling a new story of how the turtle got its shell. A shield from the elements and from predators, as well as a mineral reserve in low-oxygen environments, the turtle’s shell is unique in vertebrate anatomy. Still, a turtle’s embryo starts out looking like any spined animal’s—say, a chicken’s or a mouse’s. But about a third of the way through in-ovo development, says Shigeru Kuratani of the Riken Center for Developmental Biology, “an anatomical rule is violated” that remaps the animal’s physique. The ribs grow over the shoulder blades instead of under them as humans’ do, forcing the body wall to fold in on itself. What would have been an internal rib cage fuses into a bony plate under the skin and becomes a part of the turtle’s outer armor. In 2008 the fossil record delivered elegant support for this theory—and for another, more disputed one: that shells evolved from the bottom up. With a belly plate but an incomplete upper shell, 220-million-year-old Odontochelys semitestacea, found in China, seems an in-between form—one that looks a lot like an early stage in modern turtle development. More bony finds may someday tell the rest of the turtle’s story. —Jennifer S. Holland

Photo: Joel Sartore. Art: Hiram Henriquez
Source: Hiroshi Nagashima, Riken Center for Developmental Biology