Phylogenetic Relationships of the Earliest Anisostrophically Coiled Gastropods

Abstract

In order to explore the phylogenetic relationships among early gastropods, cladistic analyses were conducted of nearly 300 “archaeogastropod” species known from the latest Cambrian through the Silurian. The study includes an extended outgroup analysis of Cambrian molluscs. The resulting estimates of gastropod phylogeny differ not only from traditional ideas about early gastropod relationships, but also from most alternative notions. Outgroup analyses suggest that gastropods had ancestors among the Tergomya (= Monoplacophora of many workers) of the Middle or Late Cambrian. Putative gastropods from older strata (e.g., the Pelagiellida and early Onychochilidae) apparently are not closely related to gastropods. The hypothesized ancestor of gastropods possessed dextral-coiling, septation, a deep sinus, and a peripheral band. An anal slit is commonly described as a synapomorphy of gastropods that many clades subsequently lost; however, this study suggests that the slit is a rare, highly derived, and polyphyletic character among early Paleozoic species, and that the ancestors of most “advanced” clades (e.g., the Apogastropoda) never had slits. This study suggests that two major subclades evolved by the earliest Ordovician. The diagnoses and definitions of these two subclades best correspond to the traditional diagnoses and definitions of the Euomphalina and Murchisoniina. The Pleurotomarioidea is not a paraphyletic ancestral taxon as typically suggested, but instead it is a polyphyletic assemblage derived multiple times from “euomphalinae” and “murchisoniinae” species. The Bellerophontina is at least diphyletic, as the taxon includes both the ancestors of “archaeogastropods” and a clade of planispiral species that is secondarily derived from “archaeogastropods.” Macluritoids sensu stricto represent a restricted subclade of the “euomphalinae”; other supposed macluritoids evolved among different euomphalinae subclades or are not gastropods. Early Paleozoic species previously classified as caenogastropods (i.e., the Loxonematoidea and Subulitoidea) represent separate murchisoniinae subclades, with some putative members of the Subulitoidea derived within the Loxonematoidea. Early Paleozoic species assigned to the Trochoidea also represent several subclades, with most of those clades having evolved from the “euomphalinae.” An extensive taxonomic revision is presented, which removes all early Paleozoic taxa from the Pleurotomariina and broadly expands the definitions of the Euomphalina and Murchisoniina.