ORIGINAL SPOTTED PATTERNS ON MIDDLE DEVONIAN PHACOPID TRILOBITES FROM WESTERN AND CENTRAL NEW YORK

Preserved color markings in Paleozoic fossils are extremely rare and have been hypothesized to reflect muscular attachment scars, diagenetic artifacts unrelated to biological features 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 western and central New York. The small (~0.24 mm) circular markings appear either brown on a lighter cuticle, or white on a darker cuticle. Spots are non-randomly distributed on the entire dorsal exoskeleton and the regions of occurrence are bilaterally symmetrical and are not correlated with the locations of tubercles or with other surface features (e.g., frontal auxiliary impressions, or facial sutures). Thin section, SEM imaging, and EDX and WDS elemental analyses show spots to be spheres embedded within the primary layer of the cuticle below the prismatic layer and composed of microcrystalline low-Mg calcite. Surrounding cuticle (also low-Mg calcite) exhibits coarser crystallinity, lamellar structures, pore canals, and likely organic matrix as expected for unaltered trilobite skeletons. Potential diagenetic mineralogies or microstructures were not observed making diagenesis an unlikely explanation. Spot distribution, morphology and position in cuticle rule out a relationship with sites of musculature attachment/insertion. We suggest that the spots represent original biologic structures manifest as either crystallographic/optical loci resulting in structural color spots or potentially clear spots embedded in the cuticle that contrasted with a pigmented exoskeleton and may have served as windows to an underlying epidermis. The spots likely served as camouflage by providing a disruptive pattern to their dorsal surfaces serving to break-up the trilobite’s visual outline and also to reduce the starkness of shadows caused by surface relief.