Living Primitive Fishes and Fishes From Deep Time

Abstract

The notion of “primitive living taxon,” or “living fossil,” largely stems from the evolutionary concepts that have pervaded systematics for nearly a century, notably the view that paraphyletic taxa are real and ancestral. Certain living taxa are regarded as primitive because some of their characters remain in a plesiomorphic state relative to their homologues in other, closely related living taxa, and this assessment rests on both out‐group comparison and fossil data. The biology of “primitive living taxa” is thus supposed to mirror that of the related fossil taxa they resemble. Physiologists, therefore, bet that the physiological functions of a reputedly “primitive living fish” are the same as those of its fossil anatomical proxies, but paleontologists often infer those of the latter on the basis of “primitive” living models. In some cases, such circular reasoning can be avoided by considering paleoenvironmental data that are inferred preferably from geochemical parameters. An overview of living and fossil vertebrate phylogeny, however stable it may seem, shows that there are several ways of defining and naming taxa, and that shared physiological characters of a crown group may not be extrapolated to its stem group, the divergence of which may be much earlier. Physiological characters are probably no more and no less homoplastic than morphological characters. Like the latter, they can be decomposed into series of states that can be included in fractioned and combined parsimony analyses, and can contribute to patterning the trees, instead of being interpreted a priori as adaptive and mapped as attributes on trees based on other kinds of characters.