Data from: When domes are spandrels: on septation in turritellids (Cerithioidea) and other gastropods

Although generally considered rare in gastropods, septation has long been
noted in turritellids, but functional hypotheses do not survive strong
scrutiny. Here we outline a methodology for testing spandrel hypotheses,
and apply it to the problem of turritellid septa. We follow Gould in using
“spandrel” as a term for all features which are non-adaptive sequelae of
adaptive features of organisms, including those which are structurally
necessary, those that are developmentally correlated, and
non-deterministic byproducts which are correlated to features under
selection. In turritellids, septa are constructed in microstructural
continuity with secondary thickening of the shell, are highly variable
features infraspecifically, and are strongly associated with degree of
shell thickening. We therefore conclude that rather than being themselves
adaptive, turritellid septa are spandrels of shell thickening. Turritellid
septa are composed of crossed lamellar aragonite, which appear to be
constructed by mantle epithelium over the visceral mass. Septation was
also found in 22 of 24 gastropod families examined from a broad
phylogenetic distribution. Septa thus appear to be a widespread feature of
caenogastropods, in strong contrast to the assertions that septa are less
common in modern or high-spired shells.