On the phylogenetic relationships of the Cretaceous snakes with legs, with special reference to Pachyrhachis problematicus (Squamata, Serpentes)

ABSTRACT

Pachyrhachis problematicus is an early Upper Cretaceous snake with legs from the Middle East. The taxon is involved in an increasingly controversial debate about the origin and higher-level interrelationships of snakes. Its status is problematic because it combines characters of advanced (macrostomatan) snakes with plesiomorphic squamate traits. Two competing hypotheses of relationships have consequently been proposed: (1) Pachyrhachis is the sister group of all other snakes, and links Serpentes with Mosasauroidea; and (2) Pachyrhachis is related to the advanced macrostomatan snakes and has no bearing on the issue of snake origins. In a recent paper published in this journal, Caldwell critically reviewed the status of Pachyrhachis as the sister-taxon of macrostomatan snakes, and concluded that Pachyrhachis is the sister-group of all other snakes instead. In the present paper, we review several aspects of character delimitations as proposed by Caldwell, and corroborate the macrostomatan affinities of Pachyrhachis.