Meet Odontochelys semitestacea. This little creature spends its days  splashing in Late Triassic swamps with a host of other reptiles. Under the surface lies its best  defense against attack: a hard shell on its belly. Odontochelys is an early ancestor  of the turtle. Its half-shelled body illustrates an  important point about the modern turtle: it actually has two shells that develop  totally separately while the turtle is still an embryo. Both are extensions of the animal’s  skeleton, and together they are made  of almost 60 bones. Like other embryos, turtle embryos are made of  undifferentiated cells that become specific cell types, and then organs and tissues, through gene activity and communication between cells. At first, turtle embryos look very similar to those of other reptiles, birds, and mammals, except for a bulge of cells called  the carapacial ridge. The ridge expands around the body  between the neck and lower back, creating a disc shape. It guides the formation of the upper part of the turtle’s shell, called the carapace, likely by attracting  the cells that will become ribs. Instead of curving downwards  to make a regular rib cage, the ribs move outwards towards the  carapacial ridge. They then secrete a signaling protein that converts surrounding cells  into bone-forming cells. These fifty bones grow until they meet  and connect with sutures. A ring of bone solidifies  the carapace’s edges. The outer layer of skin cells produces  the scales, known as scutes, that cover the carapace. The development of the bottom half  of the shell, the plastron, is driven by neural crest cells, which can produce a variety of different  cell types including neurons, cartilage and bone. A thick shield of these cells  spreads across the belly, coming together in regions that produce nine plate-like bones. Eventually, these connect to the  carapace by sutures. A turtle’s shell has obvious advantages for guarding against predators, but the rigid casing also presents  some challenges. As the turtle grows, the sutures between the bones  of the carapace and plastron spread. Most mammals and reptiles rely on a flexible rib cage that expands to allow them to breathe, but turtles use abdominal muscles  attached to the shell instead: one to breathe in,  and one to breathe out. So how did the shell evolve? Though there are still gaps in the  fossil record, the first step seems to have been  a thickening of the ribs. The oldest known turtle ancestor, a creature called Eunotosaurus africanus, lived 260 million years ago and looked  almost nothing like a modern turtle, but it had a set of broad, flat ribs that anchored the muscles  of its powerful forearms. Eunotosaurus was likely  a burrowing creature, digging homes for itself in what’s  now southern Africa. Odontochelys semitestacea illustrates  another, later step in turtle evolution, with thick ribs like Eunotosaurus  plus a belly plate for protection. Our first fossil evidence of the full  shell characteristic of modern turtles is about 210 million years old, and belongs to a species called  Proganochelys quenstedti, whose ribs had fused. Proganochelys could move  between water and land. Unlike modern turtles, it couldn’t retract its head into its shell, but had defensive spines on its neck. Modern turtle shells are almost as diverse as the turtles themselves. Sea turtles have flatter, lighter shells  for streamlined gliding through the water. Land-dwelling tortoises, meanwhile, have domed shells that can slip free of predators’ jaws and help them turn right-side up if  they fall on their backs. Leatherback and softshell turtles have shells without the ring of bone  around the edge of the carapace or the tough scutes covering it, making it easier for them to  squeeze into tight spaces.