MULTIPLE FORMS OF HETEROCHRONY RESULT IN RAPID GROWTH AND VERMIFORM MORPHOLOGY IN VERMICULARIA (TURRITELLIDAE)

Vermicularia are turritellid gastropods with an uncoiled vermiform shape that represents a significant departure from the typical, tightly coiled, turritelline morphology, likely as a means of facilitating rapid upward growth (Gould, 1969). The vermiform habit represents such a substantial change that it has previously been recognized at the genus level, although other turritellines are then paraphyletic with respect to Vermicularia (Lieberman et al., 1993).Whatever its taxonomic treatment, the derived position of this habit allows us to study the ontogenetic mechanisms associated with such a major morphological change. A trend towards uncoiling in the latest whorls of other turritelline species suggests that Vermicularia is peramorphic with respect to other turritellines. This change in morphology may have been accomplished by a) uncoiling earlier than other turritellines (pre-displacement of uncoiling), b) extending the lifespan, allowing additional time for growth (hypermorphosis), c) increasing the rate of growth (acceleration of growth rate, or post-displacement of the decline in growth rate typically observed in other turritellines after the first year of growth), or d) some combination of these mechanisms.

Oxygen isotopic sclerochronologies were generated for V. spirata (Recent) and V. recta (Pliocene). This technique has been used to study the growth of more than a dozen extant and fossil turritelline species, and indicates that most turritelline species live for less than 3 years, with some modern extra-tropical species living up to 5. The individual of V. recta lived at least 5 years, with no significant decline in the rate of growth until the fifth year. The individual of V. spirata which was sampled, although moderately large, appears to have only lived 1 year, indicating extremely rapid growth, but not providing sufficient data to determine lifespan or to document changes in growth rate. These results indicate variation in the heterochronic pattern of attaining rapid upward growth in these two species of Vermicularia.