Gastropod phylogenetics: Progress, problems, and implications

Journal of Paleontology, Nov 2001 by Wagner, Peter J

ABSTRACT-Twentieth century fossil gastropod systematics relied extensively on neontological paradigms. However, recent appreciation of the extant gastropod diversity suggests that those early paradigms provided very unsound models. This likely is a greater problem for Paleozoic taxa than for Meso-Cenozoic gastropods because Meso-Cenozoic taxa frequently have easily recognized extant relatives whereas Paleozoic taxa frequently do not. Also, many of the taxa that apparently diverged in the Paleozoic now are limpets and retain little information about the morphologies of their coiled ancestors.

Snails could be a model taxon for investigating macroevolutionary patterns because of the Glade's dense fossil record. However, paleontologists usually study only adult shells (teleoconchs), and many malacologists maintain that teleoconch characters reflect phylogeny poorly if at all. This is important because many macroevolutionary hypotheses make their most specific predictions given phylogeny. Studies evaluating species- or genus-level relationships typically use more shell characters and states than do studies evaluating suprageneric relationships, as expected if shells evolve rapidly. Monte Carlo tests reject a null hypothesis that rates of homoplasy are equal among shell and soft-anatomy characters for two neogastropod Glades, but suggest that these rates differ by less than an order of magnitude. Finally, teleoconch characters fail to unite bellerophontiform species with gastropod muscle scars but successfully unites clusters bellerophontiform species with tergomyan muscle scars. These results corroborate the conventional wisdom that teleoconch character distributions reflect abundant homoplasy, but the results also suggest that these distributions reflect phylogeny, too.

If we can control the effects of homoplasy, then gastropods are an excellent "model" group for testing macroevolutionary hypotheses such as changing rates of evolution. Two obvious candidates are rates of morphologic evolution among basal neogastropods, and rates of molecular evolution within Glades radiating after the K/T mass extinction.

INTRODUCTION

In THE last 75 years, our views of gastropod evolution have become increasingly more complicated. Both neontological and paleontological data have contributed to this trend. As with any diverse taxon, numerous outstanding issues remain about gastropod evolution. Theoretical issues include which taxa even belong to the Gastropoda and macroevolutionary patterns such as the pervasiveness of trends and shifting rates of morphologic change and cladogenesis. Whether adult shell characters retain phylogenetic information remains an outstanding systematic issue. Improved understanding of gastropod phylogenetics goes far beyond satisfying the trivial curiosities of malacologists. Many generally interesting hypotheses (types of trends, rate shifts for character evolution or cladogenesis) require phylogenetic patterns to make explicit predictions about observable data. If adult shell characters provide reasonable inferences about fossil gastropod relationships, then the dense fossil record of gastropods coupled with neontological data will make gastropods a highly useful taxon for macroevolutionary studies (Bieler, 1992).

HISTORICAL OVERVIEW

Simple early consensus.-Nineteenth century studies of fossil gastropods formed many ideas about gastropod relationships based primarily on fossil data (e.g., Lindstrom, 1884; Koken, 1897; Ulrich and Scofield, 1897). However, a consensus view about basic gastropod phylogeny arose early in the 20th century (e.g., Naef, 1913; Thiele, 1929) (see Bieler [1992] and Ponder and Lindberg [1996] for reviews). The consensus was derived largely from neontological data, although it was considered generally consistent with the fossil record. The basic scheme was very simple (Fig. 1.1). Archaeogastropods similar to modem pleurotomarioids arose from bilaterally symmetrical molluscs, with torsion (i.e., two counter-clockwise rotations of the body early in embryology, resulting in the anus being above the head instead of behind the body) being the primary feature distinguishing gastropods from their ancestors. Numerous hypotheses exist positing torsion as either a larval (e.g., Garstang, 1929; Crofts, 1937; Ghiselin, 1966; Batten et al., 1967; Pennington and Chia, 1985) or adult (e.g., Runnegar, 1981; Stanley, 1982; Edlinger, 1988) adaptation, but none are entirely satisfactory.

The first gastropods were thought to be bilaterally symmetrical, or nearly so (like modern pleurotomarioids and scissurelloids). "Advanced" gastropods are highly asymmetrical, with de-coupling of left and right paired homologies and frequent reduction or loss of right organs. Within archaeogastropods, several groups, including the Patelloidea (true limpets), Neritoidea and Trochoidea independently acquired strong asymmetry. Another such group, the Caenogastropoda, yielded two major radiations: the Neogastropoda (from the Mesogastropoda) and heterostrophic gastropods. The former group includes much of present gastropod diversity. The latter group includes radically modified animals, the Opisthobranchia and Pulmonata, which undergo detorsion and essentially re-evolve bilateral symmetry. Many opisthobranchs show marked reduction or loss of the shell (with many losing it altogether), while pulmonates are specialized for terrestrial lifestyles.