The Lower Devonian (Lower Emsian, -400 Myr) roof slates of the Hunsrück in southeastern Germany have delivered a highly diverse and exceptionally preserved marine fauna that provides a unique snapshot into the anatomy and ecology of a wide range of Palaeozoic animals. Several of the described taxa, however, remain enigmatic in their affinity, at least until new pyritized features hidden under the surface of the slate are revealed using X-ray radiography or micro-computed tomography (µCT). Here we redescribe such an enigmatic fossil, the putative anostracan crustacean Gilsonicaris rhenanus Van Straelen, 1943. Using µCT scanning, we unveil unprecedented details of its anatomy, including a ventral oral opening and four pairs of recalcitrant jaw elements. These jaws are morphologically consistent with the scolecodonts of eunicidan polychaetes, which along with the gross anatomy of the body and head unambiguously identifies G. rhenanus as a polychaete rather than an arthropod. While this discovery firmly discards the Early Devonian record of crown anostracans in the fossil record, it adds a new record of eunicidan soft tissues, which are surprisingly rare considering the abundant microfossil record of scolecodonts.

X-ray radiography of the slate and local micro-computed tomography (µCT) of Gilsonicaris rhenanus Van Straelen, 1943 (accession number IRSNB a11647) were performed at the AST-RX imaging platform of the Muséum national d’Histoire naturelle (Paris, France), using a GE Sensing and an Inspection Technologies phoenix | x-ray v | tome | ×L240-180 CT scanner.

µCT scanning was limited to the articulated anterior part of the specimen, which also allowed for achieving higher resolution. 2600 projections were collected over 214° to account for the extremely flat nature of the specimen, using 3 averaged images per projection, 1 second of exposure time, and 1 skipped image before each projection. Voltage and current were set to 90 kV and 300 mA, respectively. The volume was reconstructed using the phoenix datos| xfi 2.0 reconstruction software, yielding an isotropic voxel size of 10.15 µm. The tomographic dataset was then exported into an 8-bit TIFF image stack of 48 coronal slices (the dataset –29 MB– is available on Morphosource; https://www.morphosource.org/concern/media/000523565).

Segmentation and 3D rendering were performed using the software MIMICS Innovation Suite 19.0 (Materialise) at the IPANEMA laboratory. Automatic thresholding and manual selection were used for the segmentation of the cuticle and mouthparts, respectively. 3D models are available on Sketchfab (https://skfb.ly/ow8w7 and https://skfb.ly/ow8xH).

We also assessed qualitatively µCT-derived density differences between the different materials present in the fossil using ImageJ, by extracting greyvalue histograms from areas of interest using the Freehand selection tool. The data are shown in Electronic supplementary material (ESM) figure S1 of the paper, but we provide in this Data Repository the histogram data and R script used to produce this figure.

Follow the R script "Gueriau-etal-Gilsonicaris_R-script.r" to load the histograms and generate the plot presented in figure S1i of the paper. This script opens data from the Excel file "Gueriau-etal-Gilsonicaris_histograms.xlsx". It includes at the beginning all needed libraries/packages needed to load and plot the histograms, which need to be installed prior to running the script. Comments throughout the script should make its use straightforward.