Snails Letting It All Hang Out

This week's highlight taxon is the Scoliostomatidae, a distinctive small family of six genera of Devonian gastropods. For those unfamiliar with geological stratigraphy, the Devonian was Very Long Ago. It was during the Devonian that plants started making a real go of it in the terrestrial environment (they had arrived there earlier, but had so far been very half-hearted about the whole thing), while it wasn't until towards the end of the Devonian that some fishy thing gave thought to the possibility of investing in some piggly-wiggly toes. Of course, as marine organisms, Scoliostomatidae lived in an environment that had been long stocked with everything it needed and could have quite happily ignored the development of terrestrialised upstarts. The family is only known from what is called the "Old World Realm", the section of Devonian geography that incorporated what is now Australia, Asia, Europe, western North America, and the Morocco-India fringe of Gondwana (eastern North America was then part of the separate Appalachian realm). These were tropical seas at the time, and the Scoliostomatidae would have been warm-water taxa.

The Scoliostomatidae weren't recognised as a family until 2002, even though Scoliostoma itself had first been described as far back as 1838 (Frýda et al., 2002). Members of the family share a unique morphology - while most of the shell grows as a standard conical shape, the very last whorl of the shell undergoes a drastic change in direction, growing outwards and backwards (and in the case of four genera forming the subfamily Scoliostomatinae, upwards as well) from the rest of the shell. The image at the top of this post (from Frýda et al., 2002) shows Pseudomitchellia macqueeni, an inch-long member of the Mitchelliinae, the other two genera of Scoliostomatidae distinguished from the Scoliostomatinae in that the last coil did not have an upwards curve. In the left-hand photo in particular you can see how the aperture is facing in the wrong direction. This has the minor side-effect of making scoliostomatids buck the trend for the usual means of distinguishing dextral and sinistral gastropod shells - even though the aperture appears at first glance to be on the left-hand side, examining the rest of the shell demonstrates that it is actually on the right.

Though their distinctive morphology clearly unites the Scoliostomatidae as a group, the relationships of the family to other gastropods are completely unknown. This is sadly not unusual among Palaeozoic gastropods, especially such early forms as this. There was a time when gastropods were classified according to large-scale features of the shell, but study of Recent taxa has shown that such features are prone to significant homoplasy and are usually indicative of ecology rather than phylogeny. Instead, gastropod classification is more reliably based on such things as internal anatomy and the morphology of the protoconch, the larval shell which is visible as a small morphologically distinct region at the very tip of the adult shell. Internal anatomy is of course not preserved in fossils beyond possibly such basic points as muscle attachment scars. The protoconch is much more useful in studying fossil shells, but its small size and delicate nature means that it often fails to be preserved, and the chance of preservation becomes significantly reduced the older the fossil is (just to make things even more difficult, some gastropods shed the protoconch when they reach maturity). Many (if not most) Palaeozoic-only families are simply too old for more than a minuscule chance of protoconch preservation.

What was the ecological significance of the uncoiled scoliostomatid shell? In Recent gastropod families showing a loss of standard coiling, such as the worm-like Vermetidae and Siliquariidae, the uncoiled shell is related to a sessile filter-feeding lifestyle. Similarly uncoiled gastropods were even more widespread in the Palaeozoic than they were afterwards, or at least included representatives of a more diverse array of families. Indeed, a greater diversity of filter-feeders compared to today was a characteristic of Palaeozoic marine faunas in general, their disappearance generally attributed to an increase in predation pressure with the appearance of faster and more active predators. For Scoliostomatidae, the displaced aperture means that the bulk of the animal would have no longer been in line with the centre of gravity for the shell, meaning that they would have also had reduced motility and would have probably been fairly sedentary. However, in most filter-feeding uncoiled gastropods the uncoiled whorls became irregular in their growth. Scoliostomatidae retained a well-defined growth form with the aperture close to the remainder of the shell. The lifestyle of scoliostomatids remains unknown. I am going to speculate here that scoliostomatids may have been semi-sessile soft-sediment-feeders (if only because I like the sound of the phrase). If the aperture is directed downwards, then the remainder of the shell lies more or less flat on the substrate. The edge of the aperture would have also been more or less flush with the substrate, offering quite effective protection.

REFERENCES

Frýda, J., R. B. Blodgett & A. C. Lenz. 2002. New Early Devonian gastropods from the families Crassimarginatidae (new family) and Scoliostomatidae (new family), Royal Creek area, Yukon Territory, Canada. Journal of Paleontology 76 (2): 246-255.