Microwear Analysis and Functional Morphology of Claws of the Florida Stone Crab, Menippe mercenaria

Microwear analyses are often employed for studies of scratches, pits, and other scars on the surfaces of vertebrate teeth, which has proven an effective method for reconstructing the diet of extinct and extant taxa. Here, we apply a similar methodology to study the functional and taphonomic significance of wear patterns on the claws of the Florida stone crab, Menippe mercenaria. The taphonomic effects on microwear accumulation were investigated by tumbling a dactyl from a modern M. mercenaria in sediment for different time periods. We also compared microwear patterns between the pincer and crusher claws with both modern and fossil specimens at different magnifications (20×, 52×, and 100×). In order to preserve the specimens, we made molds of the inner and outer claw surfaces with a high-resolution polyvinylsiloxane compound and subsequent casts were made with an epoxy resin. Scanning electron microscopy was used to image the center region of each studied dactyl mold, which was combined with microwear analysis for quantification. The number of scratches, mean scratch length, and angular dispersion were recorded. In addition, fossil claws were examined for biomineralization (i.e., calcification) using energy dispersive x-ray spectroscopy. We found that tumbling time (up to 64 min) had little effect on microwear patterns and that microwear patterns only significantly differed between pincer and crusher claw types at 20× magnification. Paradoxically, the crusher claw had a higher concentration of calcium (i.e., is a harder material) but also had significantly more microwear scratches. We interpret this pattern as natural selection favoring increased calcification of the claw most prone to wear and damage during normal functioning, hence the greater extent of cuticle damage on the claw augmented with extensive calcification. We suggest that future studies carefully consider wear mark patterns on fossilized cuticle in combination with the local lithology before drawing conclusions about functional morphology.