a–d, Photographs (overview of specimen and enlargement of eyes, respectively) of specimen MHM-18a and MHM-18b (part and counterpart) immersed in Milli-Q water. Arrowheads in b and d indicate wrinkling along the perimeter of the optical surfaces. e, f, Photographs of specimen MHM-18-6a (part) immersed in Milli-Q water. g, h, Photographs of specimen FUM-N-15451 immersed in Milli-Q water (a–h, n = 23 fossils). i, FEG-SEM image and X-ray computed microtomographic rendering (inset; illustration is representative of six independently sampled datasets; the colour is artificial) of the external visual surface in specimen FUM-N-13923, showing convex ommatidial lenses. j, FEG-SEM image of the internal visual surface in FUM-N-13923. Ommatidial lenses are represented by concavities. k, Enlargement of a transversely sectioned cornea in specimen MHM-18-6a (for SEM EDS element maps; see Extended Data Fig. 2). l, Magnification of the area demarcated in k showing the granular composition of the facet wall (i–l, n = 4 fossils). Scale bars, 2 μm (l), 30 μm (inset of i, k), 100 μm (i, j), 200 μm (f), 300 μm (b, d, h), 1 mm (e, g), 2 mm (a, c).