Fig. 1: Posidonia Shale ichthyosaur specimen within a concretion.

a Photograph of polished surface, sliced along the transverse plane. Image taken by authors. b Schematic annotation of a representing skeletal elements and lithological components. Note the deformation of distal ribs in the outer concretion and rim. The rim is composed of calcite cement and shale, indicating concretion growth around the skeleton began in the subsurface. The shale sampled overlaid the concretion and is not shown in the figure. Scale bar for a–b 10 cm. c 3D isometric view of the sampled concretion section mapped by CT, showing bones before and after destructive sampling. Note anterior vertebral centra and neural spines were present, but not visible on CT due to insufficient density difference, seemingly due to lack of barite. d Schematic annotation of c indicating sampled vertebrae, ribs, and concretion matrix. Vertebra positions are approximately drawn based on visual identification after the concretion fractured, exposing the vertebral column. Stratigraphic origin of the sample is shown in Supplementary Fig. 1, CT model of the sample prior to destructive sampling shown in Supplementary Fig. 3, sampling process shown in Supplementary Fig. 4, and subsamples shown in Supplementary Fig. 5.