EYES OR EYESPOTS: INVESTIGATION INTO THE VISUAL ORGANS OF GILPICHTHYS GREENEI, A STEM HAGFISH FROM THE LATE CARBONIFEROUS MAZON CREEK FOSSIL ASSEMBLAGE

The eyes of modern hagfish are small, rudimentary, and lack a number of features typical of vertebrate eyes, including retinal pigment, a lens, and a three-layered retina. Despite some debate about the origin of the simple hagfish eye, it is now generally accepted that the eyes of hagfish evolved from a more complex eye through vestigialization. The fossil record of hagfish eyes can help constrain the order and dynamics of this process; for example, Myxinikela siroka, a hagfish from the Late Carboniferous (307-309 Ma), Moscovian, Francis Creek Shale (FCS), had pigmented eyes, suggesting modern hagfish eyes lost their pigment over time. Here we use scanning electron microscopy (SEM) to investigate the eyes of Gilpichthys greenei, a putative stem hagfish from the FCS, with small (0.2-0.5 mm in diameter) visual organs that are variably described as eyes or eyespots, to help further elucidate early hagfish eyes. Under the SEM, the eyes of G. greenei are distinct from the surrounding body impression due to the precipitation of large crystals within the eyes, primarily pyrite crystals and framboids, and siderite crystals. This suggests that G. greenei had a vitreous body forming a localized microenvironment conducive to mineral precipitation, consistent with a vertebrate eye rather than an invertebrate eyespot. A lens, typically preserved in FCS vertebrates as a dark, raised, circular structure sometimes coated with kaolinite, is absent in all specimens of G. greenei studied here, in contrast with the initial description. Finally, the eyes of G. greenei often contained dense accumulations of small, oval to spherical microbodies, which superficially resemble fossil melanosomes. However, these are much more variable in size (ranging from ~0.2 to 1.1 µm in diameter) than is typical for melanosomes. More investigation is needed to determine if these are degenerate melanosomes or decay products. These results suggest that the stem hagfish G. greenei had eyes very similar to those of both Myxinikela and modern hagfish – small, with a posterior chamber but lacking a lens – and possibly exhibiting degenerate melanosome microstructure transitional between Myxinikela and modern hagfishes. This further supports the idea that many of the features of modern hagfish eyes were acquired early in hagfish evolution.