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.
