Now this is an interesting beast. It’s a 220 million year old fossil from China of an animal that is distinctly turtle-like. Here’s a look at its dorsal side:
a, Skeleton in dorsal view. b, Skull in dorsal view. c, Skull in ventral view. d, Body in dorsal view. Teeth on the upper jaw and palatal elements were scratched out during excavation. Abbreviations: ar, articular; as, astragalus; ca, calcaneum; d, dentary; dep, dorsal process of epiplastron; dsc, dorsal process of scapula; ep, epiplastron; fe, femur; fi, fibula; gpep, gular projection of epiplastron; hu, humerus; hyo, hyoplastron; hyp, hypoplastron; il, ilium; ipt, interpterygoid vacuity; j, jugal; ldv, last dorsal vertebra; m, maxilla; n, nasal; na, naris; op, opisthotic; p, parietal; phyis, posterolateral process of hypoischium; pm, premaxilla; po, postorbital; prf, prefrontal; q, quadrate; sq, squamosal; st, supratemporal; sv1, 1st sacral vertebra; ti, tibia; ul, ulna; vot, vomerine teeth; I, V, 1st and 5th metatarsals.
Notice in the skull: it's got teeth, not just a beak like modern turtles. The back is also odd, for a turtle. The ribs are flattened and broadened, but…no shell! It's a turtle without a shell!
Flip it over. There's another specimen, and we can look down on its ventral side, and there it is — a plastron, or the belly armor.
a, Skeleton in ventral view. b, Body in ventral view. c, d, Skull in ventral and slightly lateral views. Abbreviations as in Fig. 1, plus: ao, anal opening; bo, basioccipital; bs, basisphenoid; cav, caudal vertebrae; che, chevron; ent, entoplastron; hyis, hypoischium; is, ischium; meso I and II, mesoplastra 1 and 2; pr, prootic; prq, pterygoid ramus of quadrate; pt, pterygoid; pu, pubis; qj, quadratojugal; qrp, quadrate ramus of pterygoid; ra, radius; trpt, transverse process of pterygoid; xi, xiphiplastron; I, IV and V, 1st, 4th and 5th digits.
So, what we have here is a long-legged, toothed reptile with an elongate body, and it also has a plastron like a turtle, and hints in the bony structure of the spine of the carapace-to-be. It also fits perfectly with the embryology: modern turtles form the plastron first, and the carapace second. This is a _beautiful_ transitional form. I'd love to have some swimming in the streams near me. And here's a reconstruction of what they would have looked like, way back in the Triassic.
Li C, Wu X-C, Rieppel O, Wang L-T, Zhao L-J (2008) An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.
