Donaldina: Palaeozoic Turrets

Within the last few decades, we've developed a reasonably good idea of what are the primary subdivisions of gastropods alive today. One such generally accepted lineage is the Heterobranchia (or, depending on the author, the Heterostropha), a group that includes (among others) the air-breathing pulmonates as well as the marine sea slugs and bubble shells. In the fossil record, the roots of this lineage extend well back into the Palaeozoic with early members recognisable by their distinctive mode of shell development. The larval shell, the protoconch, of these forms spirals in the opposite direction from the mature teleoconch, so the animal will start its life sinistral (spiralling left) and end it dextral (spiralling right; if you're having difficulty imagining how this works, the protoconch often ends up sitting upside down relative to the teleoconch). Among the earliest heterobranchs in the fossil record is the genus Donaldina.

Specimen of Donaldina (1.2 mm in height), with close-up of protoconch, from Bandel et al. (2002).

Fossils of Donaldina have been found around the world and the genus persisted for a long time. The earliest potential Donaldina have been described from the Early Devonian but their inclusion in the genus is uncertain (Bandel et al. 2002). The protoconch on these early forms is poorly preserved and it is uncertain whether they truly showed a heterobranch development. The genus was definitely present by the early Carboniferous and persisted into the lower Permian. This is an impressive length of time: the Carboniferous alone last for around sixty million years.

Donaldina was a genus of small, high-spired gastropods, less than a centimetre in height. Many early members of the Caenogastropoda, the likely sister group of the Heterobranchia, also had shells of this kind and it may have represented the ancestral form for the two lineages. The sinistral protoconch of Donaldina was almost planispiral (spiralling in a flat plane) and completed between one and two whorls. The multi-whorled, dextral teleoconch was characterised by an ornament of spiral cords, usually only on the lower half of the whorl.

So what were Donaldina doing with their time when alive? Modern high-spired gastropods occupy a range of lifestyles, including free-living grazers, burrowers, or sedentary forms that live as filter feeders or parasites of other animals (Signor 1982). The morphology of Donaldina suggests that it is unlikely to be a burrower. The whorls are individually rounded whereas those of habitual burrowers tend to be flattened so the shell moves more smoothly through the sediment. The ornamentation on the underside of the whorl would presumably also have presented resistance to burrowing. The shape of the aperture in Donaldina is more suggestive of a free roamer, as a sinus in the upper part of the outer margin would have allowed the animal to pull back into its shell while the plane of the aperture was held as flat as possible against the substrate to protect against predators. Overall, the lifestyle of Donaldina may not have been dissimilar to that of the modern mudsnails of Cerithium and similar genera, crawling about in search of algae and other tasty morsels.

REFERENCES

Bandel, K., A. Nützel & T. E. Yancey. 2002. Larval shells and shell microstructures of exceptionally well-preserved Late Carboniferous gastropods from the Buckhorn Asphalt Deposit (Oklahoma, USA). Senckenbergiana Lethaea 82 (2): 639–689.

Signor, P. W., III. 1982. Resolution of life habits using multiple morphologic criteria: shell form and life-mode in turritelliform gastropods. Paleobiology 8 (4): 378–388.

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