Dr Katie Collins
Going Round the Twist: Analysing shell coiling in helicospiral gastropods
The molluscan class Gastropoda is, after the insects, the most diverse class in the animal kingdom, and one of the most disparate groups of animals overall in terms of body form. They have an excellent fossil record stretching back 500 million years, and are found in all habitats on Earth. They would seem to be an excellent model system for studies of morphological evolution in deep time, but are perhaps not used to their full potential. Part of the reason they can be so morphologically variable is their highly modifiable coiled shell, but this shell is also the reason that studies of gastropod evolution lag in many ways – the simple act of measuring traits on shells is made more difficult by the coil itself, and the more disparate a set of morphologies, the harder they are to examine in a single analysis.
The logarithmic helicospiral has been the dominant model of regularly coiled mollusc form since it was proposed by Moseley (1838) and popularised by Thompson (1942) and Raup (1966). It assumes that shells are isometric and grow exponentially and that the external form of the shell follows the internal shape, which implies that the parameters of the spiral can be reconstructed from the external whorl profile. Given the long existence and wide acceptance of this model, why is it that morphological studies of real (non-simulated) gastropod shells are so few and far between?
My coauthors and I, to answer this question, took measurements from a dataset of 176 fossil and modern gastropod shells, and our results show that the assumptions of the logarithmic helicospiral model fail widely across the diversity and disparity of gastropods. We construct an empirical morphospace of coiling using the parameters of whorl expansion rate, translation rate, and rate of increasing distance from coiling axis, plus rate of aperture shape change, from their best-fit models. We demonstrate that the best morphological model is not the same for each parameter.
To show the utility of the method in an evolutionary paleobiology context, we present a brief case study of change in shell form through geological time in the highly disparate austral family Struthiolariidae. Shell form parameters in the Struthiolariidae highlight a hitherto-neglected hypothesis of the relationship between Antarctic Perissodonta and the enigmatic Australian genus Tylospira, that fits the biogeographic and stratigraphic distribution of both genera.