Najash rionegrina, a snake with legs

It’s a busy time for transitional fossil news—first they find a fishapod, and now we’ve got a Cretaceous snake with legs and a pelvis. One’s in the process of gaining legs, the other is in the early stages of losing them.

Najash rionegrina was discovered in a terrestrial fossil deposit in Argentina, which is important in the ongoing debate about whether snakes evolved from marine or terrestrial ancestors. The specimen isn’t entirely complete (but enough material is present to unambiguously identify it as a snake), consisting of a partial skull and a section of trunk. It has a sacrum! It has a pelvic girdle! It has hindlimbs, with femora, fibulae, and tibiae! It’s a definitive snake with legs, and it’s the oldest snake yet found.

It’s described in a short paper that consists mostly of fine anatomical details. I won’t go into them, except to report the abstract and show you what these lovely bones look like.

It has commonly been thought that snakes underwent progressive loss of their limbs by gradual diminution of their use. However, recent developmental and palaeontological discoveries suggest a more complex scenario of limb reduction, still poorly documented in the fossil record. Here we report a fossil snake with a sacrum supporting a pelvic girdle and robust, functional legs outside the ribcage. The new fossil, from the Upper Cretaceous period of Patagonia, fills an important gap in the evolutionary progression towards limblessness because other known fossil snakes with developed hindlimbs, the marine Haasiophis, Pachyrhachis and Eupodophis, lack a sacral region. Phylogenetic analysis shows that the new fossil is the most primitive (basal) snake known and that all other limbed fossil snakes are closer to the more advanced macrostomatan snakes, a group including boas, pythons and colubroids. The new fossil retains several features associated with a subterranean or surface dwelling life that are also present in primitive extant snake lineages, supporting the hypothesis of a terrestrial rather than marine origin of snakes.

Here are the bones themselves.

(click for larger image)

a, Dorsal view. b, Ventral view. The left pelvic and limb elements show signs of healed traumatisms, with a large callus formation on the fractured femur. The disarticulated fibula is not visible in these views. Scale bar, 50 mm. Abbreviations: cav, first caudal vertebra; fem, femur; ili, ilium; isc, ischium; ly1?ly3, first, second, and third lymphapophyses; plz, sacral pleurapophysis; pub, pubis; r, rib; sav, sacral vertebrae; tib, tibia; tro, trocanter; psv, last presacral vertebra.

And a cladistic analysis places this animal as a clearly primitive snake.

The result is expressed in a strict consensus of two equally parsimonious trees (tree length of 270 steps, ensemble consistency index of 0.526, and retention index of 0.654). Bremer support and bootstrap percentages are given in the nodes (see Methods and Supplementary Information). Reconstructions of the pelvis and hindlimb elements of Najash, Pachyrhachis and a boine snake are illustrated for comparison.

It must be rough being a creationist right now—the data against their mythology just never stop coming.

Apesteguía S, Zaher H (2006) A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440:1037-1040.

(you can also find more at The Lancelet)