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.
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