WHEN DOMES ARE SPANDRELS: ON SEPTATION IN TURRITELLIDS AND OTHER GASTROPODS
A variety of functional hypotheses have been proposed for septa in turritellids, including prevention of parasitic infestation after apical loss, protection against apex-damaging predation, and deposition as a response to excess calcium. None of these hypotheses survive strong scrutiny. Turritellid septa are thin and curved, inserted in varying number and position within a single species, and are also inserted far from the apex. They form in continuity with secondarily deposited crossed-lamellar shell material. We therefore propose that these dome-like structures are not themselves adaptive, but rather are associated with adaptive shell thickening; septa are therefore constructional artefacts (spandrels), rather than products of natural selection (adaptations). This is supported by the association of septa with more extensive shell thickening both among multiple species and also among multiple individuals of a single species.
The process of formation of turritellid septa is unclear because crossed-lamellar microstructures have been presumed to only be constructed in direct, or very close, contact with mantle tissue. However, turritellid mantle tissue is located 4-5whorls from the apical termination of body tissue, a difficulty not unique to turritellids. Either mantle tissue is capable of regulating the regular arrangement of lamellae extremely remotely, or another tissue is capable of performing this function.