The influence of classification on the evolutionary interpretation of structure - A re-evaluation of the evolution of the palliai cavity of gastropod molluscs

Abstract

The gastropod mantle, or palliai, cavity and its associated structures have served as a phylobase for studies of gastropod relationships for well over 100 years. We review C. M. Yonge's model for the evolution of the gastropod palliai cavity published a little more than 50 years ago, as well as its subsequent mutation by other authors. We then use a recently published (Ponder Lindberg 1997) phylogenetic hypothesis of gastropod relationships to explore character transformations of attributes associated with the palliai cavity. Significant features of the evolution of the gastropod palliai cavity are the reduction or loss of structures (gill, osphradium, hypobranchial gland) and associated neural and reno-vascular systems on the right side of the cavity, and mechanisms for coping with an increase in overall body size in many clades. The loss of palliai cavity structures has occurred independently in several major clades, the patellogastropods, neritopsines, cocculinoideans, and apogastropods, and probably more than once in the vetigastropods. Evolution of the palliai cavity and associated structures is discussed for each of the clades in which largely different solutions are found to enable the achievement of larger body size. A seeming contradiction - reduction of gills with increasing respiratory demand due to increasing body size - is a feature of the group. We also examine possible linkages between the evolution of the palliai cavity and other morphological characters that were not suspect as a priori correlates of one another. The uncritical application of a current taxonomy to results obtained from applying the comparative method used to study form and function has been a significant hindrance to our understanding of evolution in the last several decades. C. M. Yonge's scenario published in 1947 was close to our phylogenetically based hypothesis. However, when it was later forced into agreement with the dominant classification of the last half-century (Thiele 1929-35), most of the points of agreement between the original scenario of Yonge and our phylogenetic hypothesis vanished, with four separate derivations reduced to a single event. This is an example of a Procrustean evolutionary scenario - fitting the data to a classification scheme, with taxonomy rather than phylogeny used as the bed.