A Review of the Origin of Snakes

Abstract

The problem of the origin of snakes has been a central issue in herpetology ever since the innovative work of Cope (1869). Although the riddle appears to be rather easily solved if looked at from a somewhat superficial prespective, it must be admitted that an acceptable solution is no closer than at the beginning of this century, and while faith in the promise for progress of a purely observational approach vanishes, conceptual issues are more and more thrown into focus. Snakes are an example for the origin of a higher taxon obscured by a high degree of character incongruence, indicating commonness of convergence. Another source of conceptual confusion results from the interpretation of the results of comparative anatomical analysis in the light of an often incongruent discussion of the adaptive strategies involved in the origin of a higher taxon. These conceptual problems are illustrated by the fact that several unrelated lineages of lizards approach the ophidian ''structural plan'' (Bauplan) to a variable degree by a reduction of the paried appendages and by modifications of skull structure, correlated with a shift to limbless locomotion and sheltering or fossorial habits. Whereas such structural similarities may indeed document a coincidence of broad adaptive patterns in limbless lizards and early snakes, the analysis of the pattern of interrelationships based on homology and parsimony indicates that similarities shared by fossoriallizards and snakes are convergent. However, there is other, and independent, evidence supporting the contention that snakes had a burrowing or fossorial ancestry! The origin of snakes, like that of any other higher taxon, is a multifaceted problem, and has resisted a unifying evolutionary explanation. The strategy proposed in this paper therefore aims at a disentanglement of the various levels of argumentation, which will be considered separately. I have argued previously that ''pattern-thinking'' and ''processthinking'' are two incompatible but complementary ways of looking at nature, i.e., of gathering and interpreting or explaining observational data. The present review of snake origins is designed to exemplify the adoption of these two perspectives of nature successively rather than simultaneously, permitting the analysis of the origin of a higher taxon from different but complementary viewpoints. The solution arrived at is not monistic: there is no unifying view of the origin of snakes based on evolutionary reasoning-a view that has escaped scientific investigation. Instead it is both hoped and admitted that the treatment of snake origins from complementary viewpoints is more complete while aesthetically less satisfying. For example, the strategy adopted in this review permits the acknowledgement of convergent similarities between fossorial lizards and snakes without precluding a burrowing origin of snakes from an as yet unknown ancestor. ''Pattern-thinking'' is dominated by the search for homologies. Organisms are decomposed into their constituent elements, which in tum are compared in terms of their topographical relations. This structuralist ''way of seeing'' (Hughes and Lambert, 1984) abstracts from the form (causa formalis) and function (causa finalis) of the organs compared. Following the principle of logical subdivision, the homologies fall into the subordinated pattern of a strictly dichotomous hierarchy, which reflects the relative degrees of similarity. Viewed from an evolutionary perspective, the maximal congruence of the homologies involved in the analysis is interpreted as evidence of phylogenetic relationship. No actual ancestor is specified, and whereas relative degrees of phylogenetic relationships are indicated, the structuralist approach provides no arguments as to the adaptive causation of transformation. Instead, the subordinated hierarchy of homologies cuts across any continuity of transition from lizards or lizardlike ancestors to snakes. ''Process-thinking,'' on the other hand, focuses precisely on the continuity of transformation: it seeks to explain structural changes by an adaptive process, and thus corresponds to a functionalist ''way of seeing'' (Hughes and Lambert, 1984). A series of intermediate stages is expected to document the shift in form and function (i.e., the change of the causa formalis and finalis) in response to ecological and/or behavioral para meters. The transformation of homologies is hypothesized as a consequence of functional demands. While this way of reasoning is dependent on the concept of descent with modification and thus on some hypothesis of ancestry, it can be applied to isolated organs or organ systems in order to illustrate the adaptive shift involved in the origin of a higher taxon. There is more than one way oflooking at nature, and since theoretical premises must precede observation, each of these ''ways of seeing'' can only convey as much information as is embraced by its respective visual angle. If viewpoints differ, such as in pattern versus process analysis, it comes as no surprise that the data and their information content differ also. The admission of different viewpoints as incompatible implies the renunciation of a unifying view of nature as proclaimed by classical evolutionism; but the approach is rewarded by a more complete comprehension of the complexity of natural phenomena, since it permits their exploration from different and complementary perspectives.