Abstract

Strophomenoid brachiopods had thin, concavo-convex shells, were ubiquitous colonizers of Palaeozoic muddy seafloors, and are hypothesized to have filter-fed in a concave-upward orientation. This orientation would elevate their line of commissure out of potentially lethal lophophore-clogging mud. The paradox is that epi- biont distributions on strophomenoids support a convex-upward life position, as do studies of strophome- noid stability and trace fossils formed by strophomenoid sediment-clearing. A premise of the concave-upward orientation hypothesis is a narrow gape, which causes narrow, high velocity inhalant currents, leaving strophomenoids vul- nerable to sediment entrainment. Herein we investigate the gape angle of Rafinesquina using serial thin sections and peels, silicified specimens, computer modelling, SEM analysis, x-ray microCT, and 3D printing. Hinge line structure suggests that, conservatively, Rafinesquina could gape 40– 45°. Such a gape occurred when diductor muscle contraction could not cause any further rotation, hinge teeth and crenulations were disengaged, and interareas interlocked. In contrast, when closed, hinge teeth were locked in hinge sockets. This wide gape eliminates con- straints on feeding orientation. In either convex-up or concave-up orientation, Rafinesquina could feed with slow, diffuse inhalant currents incapable of disturbing sediment, and could snap valves shut to forcefully expel enough water to clear sediment from the mantle cavity, explaining previ- ously described moat-shaped trace fossils associated with shells. Our findings demonstrate that Rafinesquina gaped at an angle approximately equal to the angle between the two interareas when the valves are closed. Our analyses hint that other strophomenoids with similar interarea angles also lived with their shells widely agape.

Document Type

Article

Publication Date

4-2024

Notes/Citation Information

© 2024 The Authors

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Digital Object Identifier (DOI)

https://doi.org/10.1111/pala.12697

Funding Information 

We are grateful to B. Hunda for help with collections at the Cincinnati Museum Center, and S. Felton of the Cincinnati Dry Dredgers for access to the Steve Felton and Tom Weaver collections of Rafinesquina. M. Harrison assisted in field collections. This scholarship was supported by patrons of Earth Sciences at the DMNS and the David B. Jones Foundation. Jisuo Jin and David Harper commented on an earlier manu- script draft.

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