Wide gape in the Ordovician brachiopod Rafinesquina explains how unattached filter-feeding strophomenoids thrived on muddy substrates

Strophomenoid brachiopods had thin, concavo-convex shells, were ubiquitous colonisers of Paleozoic muddy seafloors, and are hypothesised 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 epibiont distributions on strophomenoids support a convex-upward life position, as do studies of strophomenoid 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 vulnerable 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 3-D printing. Hinge line structure suggests that, conservatively, Rafinesquina could gape 40–45°. Such a gape occurred when diductor muscle contraction..., We employed a range of techniques for the analysis of the hinge morphology and function of Rafinesquina. We began with examination under a reflected light microscope, also documenting specimens by macrophotography, microphotography, and SEM imaging of the interiors of disarticulated valves with special attention to the articulating features of the hingeline and the distribution of muscle scars in the interior of the shells. We studied silicified Rafinesquina valves in the same way. Specimens from the large USGS collections of isolated valves were useful for comparing the features on opposing valves of similar size, as well as testing for fit. However, isolated valves do not always fit well even if similar in size because they did not come from the same individual. These specimens are also delicate. High resolution illustrations of all figured specimens in different orientations, as well as some of the exploratory scans and cross-sections used in this study, are available in this Dryad d..., , ## Images of figured specimens, exploratory scans, and cross-sections

These data are a suite of high resolution photographs of specimens (Figs. 2, 5, 11A, 11I), scans and models of serially sectioned specimens (Figs. 6, 9, 10, 13), SEM images (Figs. 11B, 11C), and hinge tracings (Fig. 11DE).

## Description of the data and file structure

Files are organized in nested folders and tied to the figures, with both raw and edited versions of images included. Folders include:

Fig02_CMC98738: High resolution, uncropped image of *Rafinesquina* specimen CMC IP98738, on the upper surface of a shell-sand bed from Brookville, Indiana, USA. CMC=Cincinnati Museum Center. Scale bars are 1 mm

Fig05AB_CMC98739: High resolution, uncropped, unlabeled images of *Rafinesquina* specimen CMC IP98739, including three close-ups of the hinge area. Scale bars are 1 mm.

Fig05CD_CMC98740: High resolution, uncropped, unlabeled image of *Rafinesquina* specimen CMC IP98740. Scale bars are 1 mm.

Fig06_CMC51999-...