Abstract
Screening pigments are essential for vision in animals. Vertebrates use melanins bound in melanosomes as screening pigments, whereas cephalopods are assumed to use ommochromes. Preserved eye melanosomes in the controversial fossil Tullimonstrum (Mazon Creek, IL, USA) are partitioned by size and/or shape into distinct layers. These layers resemble tissue-specific melanosome populations considered unique to the vertebrate eye. Here, we show that extant cephalopod eyes also show tissue-specific size- and/or shape-specific partitioning of melanosomes; these differ from vertebrate melanosomes in the relative abundance of trace metals and in the binding environment of copper. Chemical signatures of melanosomes in the eyes of Tullimonstrum more closely resemble those of modern cephalopods than those of vertebrates, suggesting that an invertebrate affinity for Tullimonstrum is plausible. Melanosome chemistry may thus provide insights into the phylogenetic affinities of enigmatic fossils where melanosome size and/or shape are equivocal.
Highlights
Screening pigments are an essential component of visual systems in animals as they absorb errant light, allowing directional photoreception and the protection of photoreceptors from ultraviolet radiation [1]
X-ray absorption near edge structure (XANES) spectra were collected at beamline 2-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) from points of interest identified in the XRF maps
We explored variation in the oxidation state of Cu, a metal commonly associated with melanin [11], in selected fossil and modern taxa using XANES
Summary
Screening pigments are an essential component of visual systems in animals as they absorb errant light, allowing directional photoreception and the protection of photoreceptors from ultraviolet radiation [1]. The eyes of the enigmatic taxon Tullimonstrum gregarium (Carboniferous, Mazon Creek, IL, USA) exhibit successive layers of melanosomes of different geometries. Dissected samples of tissues from cephalopod eyes and choroid, iris, RPE and sclera from vertebrate eyes were fixed using 2.5% glutaraldehyde for 1 h at room temperature. The following were mapped at beamline 2-3: uncoverslipped and unstained thin sections of eyes from one individual of Dicentrachus and Octopus and from two individuals of Loligo and Petromyzon; the eyespots of one specimen of Tullimonstrum (CKGM F 6426), Knightia sp. XANES spectra were collected at beamline 2-3 at the SSRL from points of interest identified in the XRF maps This was achieved by driving the incident beam energy through the Cu K edge in a stepwise fashion and recording the emitted intensity of the Kα line as a function of incident energy [16]. Spectra were processed using the software package ATHENA [17]
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