But now things must return to normality, and it being Monday it's time for a Taxon of the Week post. The last such post two weeks ago was on the fossil gastropod family Scoliostomatidae, and this week's highlight post continues with that gooey molluscan goodness with another group of gastropods, the Stylommatophora. While you may not be familiar with the name, Stylommatophora are actually the most instantly recognisable of all gastropod groups, for this is the group that includes the significant majority of land snails. Aydin Örstan has recently presented a series of posts discussing the difficulties of classing many gastropods (or other moisture-associated organisms for that matter) as "terrestrial" or "aquatic", so I should probably qualify that last statement by stressing that "land snails" here refers to fully terrestrialised taxa that do not have an aquatic component to their life cycle. The name "Stylommatophora" refers to what must be the second thing that any child learns about snails (after learning that they carry their house on their back), that their eyes are on the end of stalks. More than one group of stylommatophorans has reduced or lost the shell - these, of course, are the slugs and semi-slugs (yes, "semi-slug" is a valid term, though unfortunately, to the best of my knowledge "slugi" is not). Such shell loss has occured multiple times. The most influential classification of stylommatophorans was the 1900 classification by Pilsbry that divided them into three groups based on the anatomy of the excretory system, the Orthurethra, Sigmurethra and Heterurethra. Pilsbry regarded the orthurethran straight ureter as ancestral to the sigmurethran sigmoid ureter, but more recent molecular phylogenies have supported the reverse - orthurethrans are a monophyletic group within the paraphyletic "sigmurethrans", with the earliest division within the stylommatophorans being between the "achatinoid" and "non-achatinoid" clades (Wade et al., 2001, 2006). The Elasmognatha (≈Heterurethra) are a small group of two families, the shelled Succineidae and shell-less Athoracophoridae, whose status as a monophyletic group is well-supported but whose position relative to other stylommatophorans is not.
This newer division into achatinoids and non-achatinoids does not appear to be well-supported morphologically, though non-achatinoids tend to have better developed copulatory organs than achatinoids. For instance, those families with members that inject each other with calcareous "love darts" are all non-achatinoids. However, it is debatable whether and to what to degree this represents phylogenetic versus functional considerations. All stylommatophorans are functional hermaphrodites, but mating behaviour differs between taxa that mate face-to-face and inseminate each other simultaneously or those in which one individual mounts the other and insemination may be simultaneous, sequential or unilateral. As well as only occurring in achatinoids, love-darting only occurs in face-to-face copulators - and all face-to-face copulators are non-achatinoids. The function of the love darts is poorly known, but studies in Helix aspersa have shown that they induce faster uptake of received sperm by the darted individual, and one of the leading suggestions is that they discourage reproductive "cheats" that attempt to donate sperm while not taking up their partner's. Such behaviour is really only a consideration in species that inseminate simultaneously, and while many achatinoids do have simultaneous insemination their shell-mounting behaviour may be less conducive to forcing reciprocity in sperm uptake (Davison et al., 2005).
Their fully terrestrial habits (not to mention the absence of a shell in many species) mean that stylommatophorans have a much poorer fossil record than other gastropods, and their time of origin is a little doubtful. While Solem & Yochelson assigned some Carboniferous and Permian fossil snails to extant stylommatophoran families, this assignation is not well-supported and the next record of land snails is not until over 100 million years later in the Cretaceous (Wade et al., 2006). As discussed in the scoliostomatid post, the elucidation of relationships between Palaeozoic and later gastropods and distinguishing true relationships from convergences is generally a process fraught with difficulty.
Davison, A., C. M. Wade, P. B. Mordan & S. Chiba. 2005. Sex and darts in slugs and snails (Mollusca: Gastropoda: Stylommatophora). Journal of Zoology 267 (4): 329-338.
Wade, C. M., P. B. Mordan & B. Clarke. 2001. A phylogeny of the land snails (Gastropoda: Pulmonata). Proceedings of the Royal Society of London Series B - Biological Sciences 268 (1465): 413-422.
Wade, C. M., P. B. Mordan & F. Naggs. 2006. Evolutionary relationships among the pulmonate land snails and slugs (Pulmonata, Stylommatophora). Biological Journal of the Linnean Society 87 (4): 593-610.