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

Diversity and higher-level systematics of land snails
Solem (1978) estimated that there are some 20,500 land pulmonates world-wide, nearly
twice the earlier estimate provided by Boss (1971). The focus of this workshop is on the
terrestrial snails of the continental United States. Estimates of the number of the number of land
snail species are available for several states and regions but accurate estimates for North America
or even the U.S are hard to come by. A back of the envelope enumeration of the species listed in
Turgeon et al. (1998) results in slightly less than 1100 native species in the U. S. and Canada,
with about 70 more non-native species. The increasing problem of invasive, non-native species
emphasizes the importance of recognizing all taxa at least at a basic level, and some effort should
be made to become familiar with these groups. The terrestrial snails were traditionally placed in
two large groups, the Prosobranchia, which has been discarded, and the Pulmonata, which has
been retained. The majority of the native land snails in the United States are placed within the
latter group. In most recent classifications, the Pulmonata is further divided into three major
lineages, the Basomatophora, Systellommatophora, and Stylommatophora based on the number
and kind of cephalic tentacles and the location of eyes on the head. In the Basomatophora, one
pair of contractile tentacles (they shrink, accordion-like) is present on the head and the eyes are
located at their base. In the Systellommatophora, the head has two pairs of contractile tentacles,
with eyes on tips of the upper pair. Lastly, the Stylommatophora also have two pairs of
tentacles, with eyes on tips of the upper pair, but in this case the tentacles are retractile (they can
be inverted, like the fingers of a glove).
The Stylommatophora was traditionally divided into four subgroups, the Orthurethra,
Mesurethra, Heterurethra, and Sigmurethra, based on the position and structure of the kidney and
ureter (Pilsbry, 1900; Baker, 1955). Of these groups only the Orthurethra has remained
consistently supported (e.g. Wade et al, 2001; 2006) and based on the most comprehensive
modern analyses the stylommatophorans are divided into two groups, a smaller achatinoid clade
and a much larger non-achatinoid clade. Traditionally, the majority of land snail species were
placed in the Sigmurethra, and this group was further divided into two groups, the Holopoda and
the Aulacopoda based on the morphology and placement of pedal grooves (Pilsbry, 1896; 1900).
In recent phylogenetic reconstructions, the achatinoid clade includes only holopod species,
whereas the non-achatinoid clade includes holopods and aulacopods.
To a casual observer the revisions to the classifications of terrestrial snails based on
recent phylogenetic analyses have apparently only resulted in the reshuffling of some taxa and
the loss of several named groups to wit: most land snails are pulmonates, most pulmonates are
stylommatophorans, and the majority of stylommatophorans are grouped together in a large nonachatinoid
clade. What is not readily apparent is that these new hypotheses inform us about the
way organisms evolve and how some characters that at first appear to be similar due to common
ancestry are actually derived independently. To fully appreciate the full potential that land snails
have to teach us about evolutionary processes requires a more detailed understanding of the
different families of land snails.
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