THE HOLLOW NEWEL STATE IN GASTROPODS: WHEN SNAILS ARE OPEN-AXIS

Terminology for features associated with the coiling axis of gastropods is ambiguous and incomplete. Herein we describe the state of having no inner shell wall as a “hollow newel” construction, following the terminology applied to the analogous situation in spiral staircases lacking a central supporting column. For shells with the hollow newel axial state, a basal opening of the shell permits access to the body chamber of the shell many whorls beyond the aperture. We also clarify other states that can describe the relationship of a shell to its coiling axis, including having a solid columella, an umbilicus, a hollow columella, or a partially resorbed columella, and distinguish these from the hollow newel state.

The first observation of a hollow newel occurred during a systematic analysis of Plio-Pleistocene turritellids from Florida and the Atlantic Coastal Plain in the species “Turritella alumensis †”. We searched for additional hollow newel taxa by slicing shells axially, with micro-CT scans, and by designing a test using a sewing needle. The hollow newel state appears to be a phylogenetically informative character in at least one group of turritellids, where it characterizes a clade first identified based on molecular phylogenetics, which we name Caviturritella. The hollow newel state also occurs in other gastropod taxa, though it appears to be rare among those with high spires. We examine potential evolutionary scenarios for the origin of hollow newel morphology in turritellids, although the apparent single origin of the character and its apparent subsequent retention limits our ability to test these hypotheses without additional developmental data. This feature appears to facilitate achievement of large body size relative to co-occurring turritellid species and may therefore be a means of reallocating calcification effort to axial growth. The observation of this apparently novel, unnamed character in an abundant and commonly occurring family of gastropods illustrates the need for continued descriptive and observational malacology.