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:
, 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.
, Skeleton in ventral view. b
, Body in ventral view. c
, 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.