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

Abstract

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 that are nonadaptive sequelae of adaptive features of organisms, including those that are structurally necessary, those that are developmentally correlated, and nondeterministic by-products that are correlated to features under selection.In turritellids, septa are constructed in microstructural continuity with secondary internal 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 appears 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 previous assertions that septa are less common in modern or high-spired shells.

A new method for quantifying heterochrony in evolutionary lineages

“…History (as represented by phylogeny) and ecology are both significant sources of variation among heterochronic weights but do not interact (Table 4), indicating that although they both exert influence on the distribution of heterochronic weights, they do so with conflicting signals. This conflict is due to the impact of the quasi-independent genealogical and ecological biological hierarchies (Congreve et al 2018), whereby historical contingency limits the morphological framework for subsequent adaptation to ecological pressures (Gould and Lewontin 1979; Eldredge and Salthe 1984; Anderson and Allmon 2018). Logically, and as hinted at by the data discussed here, this tension between competing hierarchies can also extend to developmental frameworks as the mediating factors by which morphologic phenotypes are expressed.…”

Convergence, parallelism, and function of extreme parietal callus in diverse groups of Cenozoic Gastropoda

“…The appearance of a feature in distantly related clades is typically taken as indicative of an adaptive response (notwithstanding exceptions; e.g., Gould and Lewontin 1979; Anderson and Allmon 2018 and references therein), though this observation does not necessarily provide evidence of the possible function(s) (Geary et al 2002; Tseng 2013). Directly establishing utility through hypothesis formation and experimentation (the “paradigm method” [Rudwick 2018]; e.g., Arnold 1983; Wainwright and Reilly 1994) may help determine whether the feature under investigation is indeed adaptive, exaptive, or nonaptive, and if it is adaptive, how selection may have acted to establish the feature (Wake 1991; Wake et al 2011).…”

Phylogeny and Systematics of Fossil and Recent Vermicularia (Caenogastropoda: Turritellidae)

“…Larger shells commonly have a high number of internal septa. Although the presence of septa is neither unusual for turritellids nor for other vermiform snails, some Vermicularia have a noticeably high frequency of septation (see Anderson & Allmon, 2018).…”