31 July 2008

A fossil snail fits in between, or, maybe not

Altaba, C.R. (2007). A new genus and species of Enidae (Gastopoda: Pulmonata) from the Quaternary of the Balearic Islands (Western Mediterranean). Zootaxa 1595: 43-52.

The land snail family Enidae is a taxonomic headache. The range of the family extends from Northern Africa to England in the west and all the way to Japan in the east. I have been delving into the systematics of the Turkish taxa—around where the family seems to be especially crowded—on and off for several years. Often the difficulty is with the generic placement of the new or previously known species. As is often the case in most other snail groups, until recently the generic designations were based mostly on shell characteristics, but when malacologists started paying more attention to internal anatomy, a minor chaos was the result. Imagine, what may happen when, sooner or later, DNA barcoding enters the game.

For now, the best, but perhaps not the wisest, way out seems to be to erect a new genus to accommodate those taxa that otherwise don’t fit in. I am not criticizing this approach, because I too have done it (here).

In this paper from September 2007, Cristian Altaba describes yet another new enid genus and species, but this time for a fossil shell, Balearena gymnesica, which is endemic to Upper Pleistocene sediments on Mallorca (Majorca) Island in the western Mediterranean Sea. The new species has a shell quite similar to the shells of the species in the genus Mastus of the Aegean islands and western Anatolia and in Napaeus of the Canary Islands and Azores. However, one significant trait distinguishes B. gymnesica from both Mastus and Napaeus: the protoconch, or the embryonic whorls, of Balearena have spiral lines on bottom halves of whorls, whereas the Mastus and Napaeus protoconches are always devoid of such microsculpture.

Balearena gymnesica, enture shell (1) and protoconch. [Figs. 1-2 from Altaba, 2008]

According to the paper, 2 species in Mauronapaeus, a Northwest African enid genus, have spiral lines decorating their embryonic whorls. Altaba concludes:
[Balearena fills] the gap in shell morphology between Mastus, Napaeus and Mauronapaeus. Remarkably, Balearena occupies a geographically central position between them.
Relying on the complex paleogeography of the Mediterranean area, Altaba proposes that Mastus, Mauronapaeus and Balearena are products of vicariance events. This means that they evolved from an earlier ancestral group that was split apart when the various land masses surrounding the Tethys Sea were being pushed and pulled every which way during the formation of the Mediterranean and the Aegean Seas in the Miocene period. The snails have retained their general shell shape, because it is presumed to be good for burrowing.

Live Mastus carneolus half buried in soil and mosses. Photographed in Istanbul, Turkey in May 2007.

I will offer a slightly different hypothesis. Mastus and Balearena may instead represent 2 lineages that were split long before the vicariance processes of the Miocene. Their similar shell shapes may be products of convergent evolution that has independently made them good burrowers. As Altaba also notes, the protoconch microsculpture probably doesn’t have anything to do with burrowing, or for that matter, have any function at all. The fine spiral lines of the protoconch are more likely to represent the still developing mantle anatomy of the snail embryo at the time of the secretion of its embryonic shell. This, if true, points to a deeper phylogenetic separation, rooted in developmental differences, between the Mauronapaeus-Balearena group and Mastus.

Someone should start looking at the genes of these snails.



Cristian Altaba e-mailed this comment:
"Thank you very much for your kind review. I agree that any biogeographical scenario for such an ill-known family as the Enidae is purely hypothetical. In fact, I would like to work more on the extant North African taxa -but unfortunately they were reported from the most conflictive regions in Algeria! At any rate, I also agree that looking at internal anatomy has given a lot of insight into this family's diversity, but there will probably be bigger changes with the addition of DNA characters. Actually, land snail phylogenies rely on a few poorly understood characters, so no wonder phylogenies based on DNA provide some radical changes, as happens in the Helicoidea. Regarding the Balearic fossil, the first step was to recognize it is not the widespread Mastus pupa, and thus not a remnant of a hypothetical pan-Mediterranean melting pot in the improbable late-Miocene desiccation of this basin. All we have is the subfossil shell... and even looking at the apical sculpture took a lot of time in the field and through the scope! The most promising part of the paper may be the application of geometric morphometrics, a tool that can be used to discriminate extant taxa."

Frank Anderson said...

Send me some tissue! I still have some reagents sitting around, and I can probably rope a student into doing the actual work.

I guess the hard part would be getting a good sampling of the relevant taxa, but if that could be done, I'm game.

Kevin Bonham said...

I'd be very interested in reading any studies that have examined the strength of genetic separation in examples like this (same family, similar shell form, radically different protoconch).

There are very many such examples in the Australian Charopidae and the normal practice (which I agree with) is to regard such strong protoconch differences as automatically significant at generic level in view of the unlikelihood that there is much evolutionary pressure affecting protoconch fsculpture. A consequence of this is that there appear to be lots of undescribed genera that only have one or two species in them.