Professor, Dept of Biobiogical Sciences and Earth and Atmospheric Sciences
University of Alberta, Edmonton
The origin of snakes: morphology and the fossil record
The origin of snakes from within squamate reptiles is an excellent example of a major evolutionary transition; however, investigating the origins of a major clade, such as snakes, is problematic for the simple reason that the processes leading to macroevolutionary change result in significant morphological differences between the studied clade and its closest relative or sister clade. Modern and fossil snakes possess a number of squamate synapomorphies that strongly support their inclusion in, and thus origin from, the reptilian clade of lizards commonly referred to as Squamata. However, they are also distinguished from all other lizards by a large number of distinctive anatomical features, many of which are also significantly modified and specialized among the various clades of snakes thus defining potential ingroup relationships. This high degree of distinctiveness and specialization makes it difficult to interpret characters and hypothesize character states for use in phylogenetic analyses seeking to address the ingroup and sistergroup relationships of modern and fossil snakes. The application of molecular data sets to the phylogeny of modern snakes has in some cases corroborated morphology-based phylogenies, but it has also proposed new sistergroup and ingroup relationships not predicted by morphological data. In the absence of robust phylogenetic hypotheses (i.e., morphology, molecules or combined) the induction of origins scenarios becomes even more problematic. Despite the inherent difficulties of studying the evolutionary transitions leading to the origins of a major vertebrate clade, there has been a resurgence of interest on the problem of snake phylogeny and origins, recently driven almost completely by the discovery and description of new fossil snakes with hindlimbs, and the recharacterization of previously known fossil snake taxa. Anatomical features present in a variety of fossil snakes and lizards have re-invigorated the debate and are reviewed here alongside new data obtained from the analysis of the anatomy and molecules of extant snakes and lizards. Recent hypotheses of modern and fossil snake ingroup and sistergroup relationships, and the origins scenarios they implicitly or explicitly support, are reviewed and contrasted. A first-level problem concerns the sistergroup relationship of scolecophidian snakes. Are they a distinct clade of basal snakes, or are they derived snakes nested within a clade of higher snakes? All other sistergroup relationships and origins hypotheses for snakes are dependent on the answer to this problem.