ALL POWER TO SHIELDS: THE GROWTH AND ECOLOGY OF TURRITELLA ABRUPTA (MIOCENE), THE LARGEST, HEAVIEST-SHELLED TURRITELLID GASTROPOD

The Miocene turritellid Turritella abrupta can exceed 20 cm in length, and is the widest, thickest-shelled turritellid species, fossil or Recent. This appears to have provided protection against drilling and peeling predation, as T. abrupta has higher rates of failed drilling predation attempts and breakage-repair than co-occurring turritellid species.

Isotopic sclerochronology of two specimens from Panama and Venezuela indicates that this large size was not achieved solely through increased longevity, as over 10 cm in length was achieved in the first year of growth, with a subsequent decline in growth rate, typical of turritellids. This growth rate is achieved despite significant deposition of initial (as opposed to secondary) shell material at a thickness of 3 mm, as determined by examining specimens of varying sizes possessing aperture scars (indicating complete shells). In comparison, Recent tropical species such as Turritella leucostoma and T. terebra initially deposit only ~1-1.5 mm thick shells. Maximum lifespan observed for T. abrupta is 4.5 years, but the existence of larger specimens suggests a maximum lifespan greater than 5 years. The first year of growth represents an estimated 30-35 g of shell material, derived from specimens with aperture scars or no secondary thickening and mathematical approximations. In contrast, other tropical Turritella species deposit ~5-6 g of shell material in the first year of growth.

Unlike many other turritellid species, T. abrupta is never found at high abundance, typically representing only 1-5% of turritellid individuals in localities where they occur. This suggests that increased shell deposition was achieved in part by decreased energetic investment in reproductive output. Despite this low abundance, T. abrupta are widespread, ranging from the Gulf of California to Peru, and at least as far east as Venezuela. This range appears to contract from the lower (~15-16 Ma) to middle Miocene (~11-12 Ma), perhaps due to climatic cooling. The combination of apparently low reproductive output with long lifespan and high survivability suggests that T. abrupta represents an unusual exploration of K-selectivity in the clade.