Extended Data Fig. 10: FEG-SEM and polarized light micrographs of trilobite, fossil crane-fly and extant decapod ommatidial lenses.

a, FEG-SEM micrograph of a holochroal eye (specimen LO 12437t) of the trilobite Telephina bicuspis (Middle Ordovician period, Norway). b, Enlargement of fractured ommatidial lenses (a, b, n = 3 fossils). c, Single-spot SEM EDS spectrum acquired from the lens marked by a yellow asterisk in b, showing carbon, oxygen and calcium intensities that are consistent with calcite (compare with Fig. 2b). Platinum and palladium peaks derive from the sample pre-treatment (coating). Data are representative of six independent measurements. d, Plane-polarized light microscopy section through ommatidial lenses in a holochroal eye (specimen LO 12439t) of the trilobite Nileus armadillo (Middle Ordovician period, Sweden). e, Equivalent cross-polarized light micrograph. f, Plane-polarized light microscopy section through ommatidial lenses in an eye (specimen LO 12438t) of T. bicuspis. g, Equivalent cross-polarized light micrograph. h, Plane-polarized light microscopy section through a second eye (specimen LO 12437t) of T. bicuspis. i, Equivalent cross-polarized light micrograph, showing calcite crystal growth beyond the preserved lenses (arrowheads; Supplementary Information). j, Plane-polarized light microscopy section through an ommatidial lens in a schizochroal eye (specimen LO 12440t) of the trilobite Phacops latifrons (Middle Devonian period, Germany). k, Equivalent cross-polarized light micrograph. l, Plane-polarized light microscopy section through ommatidial lenses in an eye of a fossil crane-fly (specimen FUM-N-13923). m, Equivalent cross-polarized light micrograph. Data in d–m are representative of three independent analyses. n, Cross-polarized light micrograph of calcite crystals in a compound eye of the extant sandy swimming crab, L. depurator (n = 3 specimens). Scale bars, 30 μm (l, n), 100 μm (d, f, h, j), 200 μm (b), 500 μm (a).