Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye

Abstract

The success of vertebrates is linked to the evolution of a camera-style eye and sophisticated visual system. In the absence of useful data from fossils, scenarios for evolutionary assembly of the vertebrate eye have been based necessarily on evidence from development, molecular genetics and comparative anatomy in living vertebrates. Unfortunately, steps in the transition from a light-sensitive 'eye spot' in invertebrate chordates to an image-forming camera-style eye in jawed vertebrates are constrained only by hagfish and lampreys (cyclostomes), which are interpreted to reflect either an intermediate or degenerate condition. Here, we report-based on evidence of size, shape, preservation mode and localized occurrence-the presence of melanosomes (pigment-bearing organelles) in fossil cyclostome eyes. Time of flight secondary ion mass spectrometry analyses reveal secondary ions with a relative intensity characteristic of melanin as revealed through principal components analyses. Our data support the hypotheses that extant hagfish eyes are degenerate, not rudimentary, that cyclostomes are monophyletic, and that the ancestral vertebrate had a functional visual system. We also demonstrate integument pigmentation in fossil lampreys, opening up the exciting possibility of investigating colour patterning in Palaeozoic vertebrates. The examples we report add to the record of melanosome preservation in Carboniferous fossils and attest to surprising durability of melanosomes and biomolecular melanin.

Keywords: Mazon Creek; cyclostomes; melanosomes; retinal pigment epithelium.

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 [–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.)

Figure 2.

Bars of colour patterning along the dorsal surface on Mayomyzon pieckoensis. The dorsoventrally orientated ‘stripes’ (black arrows) comprise carbon of amorphous and elliptical microbody textures. (a) Specimen ROMV56787, (b) specimen PF5687a, (c) specimen ROMV56828a, (d) specimen PF5539, (e) specimen ROMV56788b and (f) specimen ROMV56800b. (a–f) Scale bars, 5 mm. (g–i) SEM images of melanosomes from the stripes in specimen ROMV56800b demonstrating relatively uniform size and shape of microbodies. (g) BSE image of melanosomes. Scale bar, 1 µm. (h) Secondary Electron image of melansomes. Scale bar, 1 µm. (i) BSE image showing that melanosomes are composed of many small melanin granules. Scale bar, 500 nm. e, eye; b, branchial structure; oc, otic capsule, as, axial structure. See also the electronic supplementary material, figure S1. (Online version in colour.)

Figure 3.

TOF-SIMS analysis of melanosomes in the eye of Mayomyzon LEIUG123268. (a) Complete specimen of Mayomyzon LEIUG123268. (b) Optical overview of head and region analysed under the TOF-SIMS (optical view and total yield; scale bars, 100 µm). (c) Comparison of TOF-SIMS spectrum from the eye in comparison with a fossil melanin reference (Jurassic cephalopod ink). (d) Representative examples of spatial distribution of secondary ion fragments previously assigned to fossil melanin [15]: , C₆H⁻, C₆N⁻ and C₁₂H⁻, together with matrix-related fragments: FeO⁻, ⁵⁷FeO⁻, and Si₄O₂H⁻. Scale bar, 100 µm. See also the electronic supplementary material, figures S4 and S6 for additional maps and spectra. (Online version in colour.)