Wednesday, November 27, 2013

A second look at the Tethyan limbed snakes

Artist's reconstruction of Pachyrhachis problematicus.
Three fossil marine snakes with hind-limbs Pachyrhachis problematicus, Eupodophis descouensi, and Haasiophis terrasanctus. are known from the upper Cretaceous (Cenomanian) of the Middle East. All of them were collected from the region that formed the ancient Tethys Sea. The first species was described in 1979 and a reduced limb lizard,but later recognized as a snake, the others were described in 2000. The phylogenetic status of these snakes has been controversial. In a new paper Palci et al. (2013) have re-evaluated the fossil anatomy of these three species and have drawn some interesting conclusions.  They found no evidence of a laterosphenoid in Haasiophis and Eupodophis; Pachyrhachis and Eupodophis retain a jugal; Haasiophis and Eupodophis, have  chevron bones in the tail; Haasiophis has a large number of unfused intercentra along the anterior portion of the body's vertebral column; and Pachyrhachis has numerous mental foramina on the dentary, as well as at least one sacral vertebra with unfused sacral ribs.

 The authors ran three phylogenetic analyses using alternative out groups (varanoids; iguanians; and skinks+ amphisbaenids, + dibamids) to polarize the character transformations. The in-group consisted of all well-preserved fossil snakes from the Cretaceous, the madtsoiids (mostly Gondwanan snakes with a fossil record extending from the Upper Cretaceous to late Pleistocene from South America, Africa, India, Australia and Southern Europe), and taxa that are representative of all major groups of living snakes. The analyses suggested Pachyrhachis, Eupodophis, and Haasiophis are either a series of stem taxa at the base of the radiation of snakes, or they are members of a clade of fossil snakes that are the sister group to all living alethinopidians (all living snakes minus the scolecophidians).

The authors also found  free intercentra located at the base of the anterior pre-cloacal vertebrae of Haasiophis terrasanctus. If these intercentra are homologous with the cervical intercentra of limbed squamates, this would suggest snakes experienced a considerable amount of axial elongation that involved not only the dorsal but also the cervical region, a point that is supported by the posterior extension of some cervical muscles in snakes. Of interest, a similar pattern of axial elongation as been observed  in dolichosaurs, adriosaurs, and pontosaurs—a group of lizards that have been considered close relatives of snakes since the 19th century. The authors comment that they do not consider these lizards ancestral to snakes, but they may have a close phylogenetic relationship with them.

Citation
Alessandro Palci , Michael W. Caldwell and Randall L. Nydam (2013) Reevaluation of the anatomy of the Cenomanian (Upper Cretaceous) hind-limbed marine fossil snakes Pachyrhachis, Haasiophis, and Eupodophis. Journal of Vertebrate Paleontology, 33:6, 1328-1342.


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