A Guide for Terrestrial Gastropod Identification - University of ...

Phylogeny and Classification of North American Terrestrial
Gastropods
Kevin J. Roe, Iowa State University
After the Arthropoda, the Mollusca is considered to be the second most diverse of all
metazoan phyla, and the gastropods represent the most diverse group of mollusks consisting of
~80% of all mollusks (~80,000 species). It is perhaps no great surprise then that the
classification of such a large and diverse group of organisms has continued to change over the
years. In general, these changes have reflected our improved understanding of the evolutionary
relationships of gastropods. The past several decades have witnessed some dramatic shifts in our
understanding of the relationships of the major gastropod lineages, and with them the placement
and relationships of terrestrial snails. An understanding of the changes that have taken place in
the classification of terrestrial snails will prove to be invaluable when attempting to reconcile the
names and associated information contained in recent publications with that found in older, but
still very important ones.
In order to fully appreciate the value of a taxonomic classification, it is paramount to
realize that classifications schemes are, first and foremost, information retrieval systems. The
information they convey is of a set of relationships (all the species in a genus should be more
closely related to each other than to species in other genera, and likewise for genera within
families) that is arranged hierarchically. By looking at a classification one should be able to
understand something about the where the organism of interest fits within this hierarchy. In
more modern classifications, there has been some movement away from the use of traditional
rankings (class, order, suborder etc.). This fact should not cause any great concern, because what
is most important is the relative position of taxa within the hierarchy, not the categorical rank
associated with it.
Traditionally, gastropods were classified based on the presence and position of their
respiratory apparatus into three groups: Prosobranchia (gills in-front of the heart),
Opisthobranchia (gills behind the heart), and Pulmonata (lack gills, but use a highly vascularized
mantle for respiration). While convenient, this classification has been recognized to not reflect
the evolutionary relationships of gastropods (a misleading information retrieval system) and has
been abandoned in favor of classifications based on the objective analysis of both morphological
(e.g. Salvini-Plawen and Steiner, 1996; Ponder and Lindberg, 1997; Dayrat and Tillier, 2002)
and DNA (e.g. Tillier and Ponder, 1992; Dayrat et al. 2001) data. Current classifications place
pulmonates and opisthobranchs together in the clade Euthyneura. This group together with
pyramidelids and related snails form the Heterobranchia. Those interested in exploring higherlevel
gastropod systematics should consult the publications listed in this summary in particular,
Bieler (1992) and Ponder and Lindberg (1997). The use of ever more sophisticated methods of
analysis and high-tech tools has resulted in a refinement of our knowledge of the relationships of
gastropods, but, despite these improvements our understanding is far from complete and still
very much in a state of flux. The only way to improve the current state of knowledge of these
relationships is to continue to collect and analyze data, and just as importantly, to continue to
train researchers to be able to locate and identify gastropods.
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