Figure 1.

Fossil cyclostomes from the Mazon Creek Lagerstätte. (a) Myxinikela siroka (PF15373). Scale bar, 5 mm. (b) Mayomyzon pieckoensis (ROMV56800b) showing clearly defined pigmented stripes along the dorsal surface. Scale bar, 5 mm. (c–e) Back-scattered electron (BSE) SEM images of melanosomes present in the eyes of (c) Myxinikela siroka (PF15373), (d) Mayomyzon pieckoensis (LEIUG 123268) and (e) Mayomyzon pieckoensis (ROM56806); note two distinct melanosome morphologies, which is typical of the RPE of fish [12–14]. All scale bars, 5 µm. Not all material preserved as carbon within the fossils shows these textures; for example, in Mayomyzon the oral disc and pharynx comprise sheet-like carbon with associated pyrite. Ellipsoid/oblate textures are not evident in carbon patches beyond the margins of the body in any of the taxa studied. (f) Radial TEM image of the retina of an extant fish (Rhinogobius). Dark pigment granules (melanosomes) are elliptical in the base of the image and spherical at the top of the image. Decay-induced collapse of the RPE would result in a fossilized structure with both elliptical and oblate melanosome morphologies. Scale bar, 5 µm. Image courtesy of Gengo Tanaka. e, eye; ot, oral tentacles; snc, forked subnasal cartilage; b, branchial structure; d, digestive organ; oc, otic capsule, od, oral disc; as, axial structure. (Online version in colour.)