Published 2 December 2009
Alaskiella medfraensis is a member of the Porcellioidea, a superfamily of gastropods containing the Palaeozoic Porcelliidae and the Mesozoic Cirroidea. Porcellioids are distinguishable from other gastropods by virtue of being the only heterostrophic vetigastropods. I’ll explain what that means, and I apologise in advance if it’s a little hard to follow. I know it confuses the hell out me.
I’ve previously explained the difference between dextral and sinistral gastropods and how to distinguish the two. The method that I described therein is the correct one for orthostrophic shells. Orthostrophic growth is the standard gastropod growth pattern with the shell growing in a downwards spiral. However, some types of gastropod are hyperstrophic which essentially means that from a developmental perspective they grow upwards rather than downwards. Because the shell’s “correct” orientation is therefore rotated 180° from that of an orthostrophic shell, if you hold a hyperstrophic shell with the aperture downwards a dextral shell is going to appear sinistral and vice versa. In life, of course, the aperture will still be held downwards in a hyperstrophic gastropod, but they can still be distinguished because the positions of the organs (gastropods are bilaterally asymmetrical) will be reversed—the stuff that you’d expect to see on the left side will instead be on the right. In the case of fossil gastropods, where you can’t look at the organs, distinguishing hyperstrophic shells becomes a lot more difficult and I have to confess that I really don’t see how they do it (apparently the shape and orientation of the aperture may offer some indications).
Heterostrophic shells like Alaskiella start off life growing upwards like a hyperstrophic shell but then change the direction of growth to downwards. As a result of this, they also change the direction of spiralling—so Alaskiella starts off spiralling upwards dextrally before it spirals downwards sinistrally. Among living gastropods, heterostrophic coiling is characteristic of the Heterobranchia, the clade that includes opisthobranchs (sea slugs and related animals) and pulmonates (lung-breathing snails). However, porcellioids developed heterostrophy independently of heterobranchs; instead, they belong to the Vetigastropoda, the clade including trochids (top shells) and turbinids (cat’s-eye or turban shells). The main characters showing porcellioids to be vetigastropods are the presence of a vetigastropod-type protoconch (the embryonic shell) and nacre (the shiny inner layer of, for instance, a paua shell Haliotis iris; Frýda et al. 2008). Neither of these features is really visible in the figure above (nacre has only been identified as preserved in one cirrid genus so far) but you can readily see the large selenizone, the groove running around the outer edge of the whorls, which tends to be a characteristic of vetigastropods*. Porcellioids can also be distinguished from heterobranchs in that while heterobranchs change the direction of coiling at the transition point between the protoconch and the teleoconch (the post-embryonic or post-larval shell), porcellioids change directly during the early part of the teleoconch (Frýda & Blodgett 2004).
*To be scrupulously correct, not all vetigastropods have a selenizone and not every gastropod with a selenizone (which may be a groove or may be a row of openings) is a vetigastropod. However, in general, non-vetigastropods with selenizones have only small ones.
Alaskiella medfraensis differs from other porcellioids in that other porcellioids have the axis of coiling of the protoconch parallel to that of the teleoconch, but Alaskiella has the axes offset at an angle. The change in the axis of coiling is what gives Alaskiella its characteristic looped look where the shell changes direction, which always puts me in mind of the looped peak that tends to form at the top of a meringue when you spoon it out.
Frýda, J., & R. B. Blodgett. 1998. Two new cirroidean genera (Vetigastropoda, Archaeogastropoda) from the Emsian (late Early Devonian) of Alaska with notes on the early phylogeny of Cirroidea. Journal of Paleontology 72 (2): 265–273.
Frýda, J., & R. B. Blodgett. 2004. New Emsian (Late Early Devonian) gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell Terrane), and their paleobiogeographic affinities and evolutionary significance. Journal of Paleontology 78 (1): 111–132.
Frýda, J., R. B. Blodgett, A. C. Lenz & Š. Manda. 2008. New porcellioidean gastropods from Early Devonian of Royal Creek area, Yukon Territory, Canada, with notes on their early phylogeny. Journal of Paleontology 82 (3): 595–603.