Original spotted patterns on Middle Devonian phacopid trilobites from western and central New York

Abstract

Preserved color markings in Paleozoic fossils are rare and have been hypothesized to refl ect muscular attachment scars, diagenetic artifacts, or the altered remains of biochromes (organic pigments) or sclerochromes (structural colors) embedded in fossilized skeletal remains. More than 25 exceptionally well preserved phacopid trilobites with spotted patterns are described from the Middle Devonian of western and central New York State (USA). The small (~0.23 mm) circular markings appear either brown on a lighter cuticle, or white on a darker cuticle. Thin section, scanning electron microscope imaging, energy-dispersive X-ray spectroscopy, and wavelength-dispersive spectroscopy elemental analyses show spots to be microcrystalline low-Mg calcite spheres embedded within the primary layer of the cuticle below the prismatic layer. Surrounding exoskeleton (low-Mg calcite) exhibits coarser crystallinity, typical lamellar structures, pore canals, and possible organic matrix, as expected for relatively unaltered trilobite skeletons. Potential diagenetic mineralogies or microstructures were not observed, making diagenesis an unlikely explanation. Spot distribution, morphology, and position in exoskeleton rule out a relationship with sites of musculature attachment and/ or insertion. We suggest that spots represent original biologic structures manifested as either crystallographic or optical loci resulting in sclerochrome spots or possibly clear spots embedded in the cuticle that contrasted with a pigmented exoskeleton and may have served as windows to an underlying epidermis.

Colour Patterns in Devonian Trilobites

“…1A), while the ground colour of the trilobite is a bright to medium brown. Our observtions accord with, and confirm the phenomena described by McRoberts et al [17] and the authors mentioned before.…”

“…Most known specimens of Eldredgeops crassituberculata (Stumm, 1953) [24], are homogeneously dark brown, but the two individuals documented here have a spotted exoskeleton, like those described by previous authors [15][16][17]. These come from the Middle Devonian Silica Shale, Silvana, Ohio.…”

“…This is indeed a problem, as exemplified by the different preservation of the eye-lenses (which are green) and the exoskeleton (which is red) in some silicified Moroccan phacopids [21]. McRoberts et al [17] provide compelling evidence both from thin sections and constant patterning in 25 specimens of Eldredgeops rana, that the structures seen are at least of original biological origin. These take the form of regularly arranged circular spots within the cuticle as can be seen in McRoberts documentation as in the specimen described here.…”

“…A comparison with the congener E. rana investigated by McRoberts [17] might be interesting at this point. While the cuticle of one group of E. rana was much brighter than those of the E. crassituberculata (E.c.)…”

“…103], and most recently in 2013 by McRoberts et al [17] in phacopid trilobites, Eldredgeops rana (Green 1832) [18] and in Greenops boothi (Green 1837) [19] from the Devonian of New York, Esker [15] described dark spots all over the exoskeletons wherever these were intact and unweathered. These were reported as comparable to those described by Teichert [14] in the pygidia of Ditomopyge meridionalis from the Permian of Australia.…”