DOI: 10.5061/dryad.gqnk98stc
title: Data from: Wide gape in the Ordovician brachiopod Rafinesquina explains how unattached filter-feeding strophomenoids thrived on muddy substrates

description: 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
 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 constraints 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
 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 also hint that other strophomenoids with
 similar interarea angles lived with their shells widely agape.
description: We employed a range of techniques for the analysis of the hinge
 morphology and function of <em>Rafinesquina</em>. 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
 <em>Rafinesquina</em> 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 data
 set. Small silicified specimens are often articulated,
 and we examined a suite of such articulated specimens visually and scanned
 them using a SkyScan 1172 X-ray microCT. 3D models of these specimens were
 examined to gain insights into hinge articulation and those data are
 available in MorphoSource. Because such specimens had between-valve
 silicification that could not always be distinguished from shell material,
 we mainly used these analyses to guide our approaches for serial
 sectioning and interpreting unsilicified specimens. The
 most useful results were obtained through serial sectioning and serial
 grinding of articulated specimens, whereby we generated a series of
 sagittal sections through each brachiopod. We employed a slice spacing of
 ~3 mm in initial work, and based on those results decreased our
 slice spacing to ~0.5 mm. One 3D model based on serial sectioning work is
 presented herein, illustrated by different articulations of an oversize
 printed model. Slices from this work also permitted analysis of the
 function of each section in two dimensions and calculation of the
 centre of mass at different gapes. For mass calculations, when density is
 uniform, the centre of mass is equal to the centre of volume. Volumetric
 centres were computed before and after remeshing the model to a voxel size
 of 0.4 mm to smoothly interpolate between slices. Table S1 shows
 that remeshing did not substantially alter the computed volume. The centre
 of volume of the valves was calculated with gape angles of 0° (closed),
 20°, and 45°, to compute how gape alters the distribution of mass for the
 highest density component of the living brachiopods - the
 shell. Additional methodological information is in Appendix S1.
description: ## 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_CMC51999-3: High resolution, uncropped, unlabeled image of
 *Rafinesquina* specimen CMC IP51999-3 in matrix, from Stonelick Creek,
 Ohio, USA. Scale bars are 1 mm. Also included is a folder
 ("Peels") with transmitted light photomicrographs of the peels
 taken approximately every 3 mm, showing sequential cross-sectional views
 through the sample, and a folder with scans of the slabbed cross-sections
 ("Slices") through the same sample.   Fig09_CMC98737: Three
 folders are included, that contain scans of the slabbed cross-sections
 ("scans") through *Rafinesquina* specimen CMC IP98737 from
 Manchester, Indiana, USA, tracings of the valves derived from these
 sections ("Tracings"), and exploratory models ("3D
 files") used to assess the potential articulation of the valves.
 Fig10_CMC98741: Five folders are included of data pertaining
 to *Rafinesquina* specimen CMC IP98741 from Orangeburg Road, Kentucky,
 USA, including folders that contain transmitted light photomicrographs of
 the peels ("Peels") taken approximately every 0.5 mm, scans of
 the slabbed cross-sections ("Aligned Scans"), tracings of the
 pedicle valve ("AlignedPed") and brachial valve
 ("AlignedBrach") derived from these data, and exploratory models
 ("3D models") used to assess the potential articulation of the
 valves Fig11A_CMC98742: High resolution, uncropped, unlabeled images of
 ventral valve of *Rafinesquina* specimen CMC IP98742 from Orangeburg Road,
 Kentucky, USA, including three close-ups of the hinge area. Scale bars are
 1 mm. Fig11B_CMC98743: Additional SEM images of silicified ventral valve
 of *Rafinesquina* specimen CMC IP98743 from USGS locality 7812-CO,
 Cynthiana, Kentucky, USA. Fig11C_CMC98744: Additional SEM images of
 silicified dorsal valve of *Rafinesquina* specimen CMC IP98744 from USGS
 locality 7812-CO, Cynthiana, Kentucky, USA. Fig11DE_CMC98745_CMC98746:
 Five folders are included of data pertaining to *Rafinesquina* specimen
 CMC IP98745 and CMC IP98756 from USGS locality 7812-CO, Cynthiana,
 Kentucky, USA, including folders that contain high resolution images of
 the two valves, with muscle scars highlighted ("080416 muscle
 trace"), high resolution images of the hinge area with hinge elements
 highlighted ("080419 Hinge Parts"), and images showing
 exploratory realignment of the valves in different modes of articulation
 ("080419 In Articulated Positions").   Fig11I_CMC98747: High
 resolution images of articulated silicified *Rafinesquina* specimen CMC
 IP98747 from USGS locality 7812-CO, Cynthiana, Kentucky, USA, including
 close-ups of the interarea, dental plate and associated features.
 Fig13_CMC98748: Five folders are included of data pertaining
 to *Rafinesquina* specimen CMC IP98748 from St Catherine, Kentucky, USA,
 including folders that contain scans of the slabbed cross-sections
 ("SliceScansAligned") of the specimen, tracings of the pedicle
 valve ("SliceTrace Pedicle") and brachial valve
 ("SliceTraceBrachial") derived from these sections, exploratory
 models ("3D files") used to assess the potential articulation of
 the valves, and photographs of the printed models
 ("PhotosPrintedModel") in different degrees of valve opening.