Growth Models of Recent Gastropods Shells: Implication for Ammonoids

Patterns of variation of shell shape and its associated growth features (like growth halts) in ammonoids and gastropods suggest that common rules of accretionary growth underlie the morphogenesis of the shell and its evolution in both clades. A longitudinal study of a population of recent gastropods (Muricidae) bred in laboratory for more than one year highlights a covariation between growth rhythm (frequency and amplitude of pulses of growth), growth halts spacing, aperture allometry and intensity of ornamentation. In particular, variation in growth rhythm is regarded as critical in generating the observed covariation between growth halts spacing and ornamentation: the most widely spaced the growth halts, the larger the apertural spines. A simple growth vector model is able to account for the integration and covariation of these shell characters. Moreover, in some highly variable ammonoids species, it has been shown that simple growth rules could underlie the evolutionary recurrent covariation of aperture shape, degree of coiling and intensity of the ornamentation (Buckman's law of covariation). Similarly, these three characters covary with the spacing between growth halts during the ontogeny of some ammonoid species. Some recurrent patterns of variation in ammonoids species could result from similar rules of growth tied to basic constraints of accretionary growth.