On the Origin and Evolution of the Ophidia

Alessandro Palci
With well over 3,400 described species, snakes undoubtedly represent one of the most successful groups of reptiles. Much has been written about their ecology, behavior, anatomy, relationships and evolution. However, despite the debate about the origin of this taxonomic group dating back to the second half of the XIX century, no consensus has been reached, yet. Scenarios that portray the first snakes as evolving from aquatic lacertilian ancestors are countered with others that see the first
more » ... s as the result of long-term adaptations to a burrowing, cryptic lifestyle. The supporters of the first type of scenario found their evidence mostly in osteological comparisons of non-burrowing snakes with extinct aquatic lizards (e.g., mosasaurs, dolichosaurs and adriosaurs), while the supporters of the second type of scenario base their conclusions mostly on anatomical comparisons between modern legless squamates and burrowing snakes. The debate is further complicated by the scarcity of well-preserved fossil remains that may help elucidate the origin of the group, and by the contradictory interpretations that different authors have provided after examination of the same fossil specimens. Therefore, the goal of this work was that of analyzing all the evidence that has been put forward so far in support of the two main origin scenarios, critically evaluate the contradictory evidence provided by different researchers, examine first-hand all the pivotal extant and fossil taxa that have been used in the formulation and support of each scenario, and provide a phylogenetic analysis of snakes within squamate reptiles that is based both on molecular and morphological data. The examination of over 400 iii specimens of squamates, including both extant and fossil species, lead to the following results: (1) the redescription of several important fossil taxa (Pachyrhachis, Eupodophis, Haasiophis, Najash, and Dinilysia), and the identification, in some of them, of anatomical features never reported before (e.g., chevron bones in Haasiophis, mental foramina and a sacral vertebra in Pachyrhachis); (2) the discovery of new material (a pelvic girdle) attributable to the fossil species Wonambi naracoortensis, a snake whose pelvic anatomy was previously unknown; (3) the retrieval of evidence that supports a reinterpretation of the circumorbital bones of snakes, with particular regard to the "postorbital" and the "supraorbital", here reinterpreted as primary homologues of the jugal and postfrontal, respectively; (4) a detailed assessment of what constitutes the "crista circumfenestralis" of snakes and how this anatomical feature varies within the Ophidia; (5) new hypotheses regarding the ingroup relationships of snakes, which imply a possible convergent evolution of the macrostomatan skull condition, and the possibility that scolecophidians may represent an aberrant lineage of alethinophidian snakes. iv
doi:10.7939/r3ng4h314 fatcat:hj7boaqbejccrpl5p4xkahgedu