A transitional turtle, Pappochelys

Turtles are nifty animals, with a remarkable adaptation: they've taken their ribs and shifted them outside their appendicular skeleton, flattened and expanded them, and turned them into a shell. It's a clever twist, and it doesn't require any magic -- just a shift in timing during development, with a little extra signaling. The molecular biology and development explain mechanistically how it happened, and we also have fossils of some of the in-between states.

Odontochelys, a 220 million year old fossil, for instance, is a good example of a turtle ancestor that's got some of the bits but not all. It has a well developed plastron, the belly armor of a turtle, but it doesn't have a shell -- it has broadened ribs that form a kind of flexible bony plate under the skin.

And now we have even older ancestor, from 240 million years ago, called Pappochelys. It lacks the plastron, too, instead having an array of ventral 'ribs', called gastralia. What caught the attention of the researchers was the true ribs. They also are flattened and broadened -- they look like curvy cricket bats.

a, Trunk rib in dorsal view (SMNS 92067); b, trunk rib in anterior view (SMNS 92068); c, d, trunk ribs in ventral view (c, SMNS 92063; d, SMNS 91360)

It looks distinctly lizardy in the reconstructions, but when you look at the bones you get the impression of a bony box, a kind of underlying lorica segmentata.

I also really like this diagram that illustrates the gradual acquisition of turtle-like features over the long history of this clade.

a, Restoration of the skeleton of Pappochelys in lateral view (as yet unknown elements in white; preserved bones in grey; trunk ribs and gastralia highlighted in black); b, successive appearance of key features of the turtle body plan; c, plastron of Odontochelys and reconstructed ventral bones of the shoulder girdle and gastralia set in Pappochelys (elements of the shoulder girdle and their homologues are indicated in a darker shade of grey).

Take a look at that skull: it's got two small openings in the back. It's a diapsid!

Schoch RR, Sues HD (2015) A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature doi: 10.1038/nature14472. [Epub ahead of print]