Decay and mineralization of mantis shrimps (Stomatopoda; Crustacea); a key to their fossil record

Abstract

Experiments were carried out on decay and early diagenetic mineralization in the stomatopod Neogonodactylus as a basis for the interpretation of fossil specimens of mantis shrimps. Neogonodactylus has a robust cuticle that is heavily mineralized in places in contrast to the decapods Crangon and Palaemon, which have been the subject of similar taphonomic experiments. Decay over 25 weeks resulted in a continuum of morphological stages: (1) swollen, due to osmotic uptake; (2) ruptured, as the exoskeleton split; and (3) partially decomposed, including weakening of the cuticle, decay of the soft-tissues, and disarticulation and fragmentation of the exoskeleton. Two categories of mineralization occurred; the precipitation of crystal bundles of calcium carbonate, and the replacement of soft-tissue and cuticle in calcium phosphate. Calcium carbonate formed as: (1) crystal bundles of various shapes; (2) crusts on the outside of the cuticle; and (3) an amorphous crystalline mass within the cuticle. The amount of soft-tissue mineralized in calcium phosphate increased throughout the experiment. In the later stages the hepatopancreas was often completely mineralized. Muscle tissue was replaced to a lesser extent and mostly in small fragments. Nerve ganglia were occasionally mineralized. The degree of mineralization was much greater than in decapod shrimps, presumably reflecting the higher calcium content and relatively phosphorus-rich cuticle of the stomatopod. The stages of morphological decay observed in these experiments can be identified in fossil stomatopods. SEM observations of fossil material have revealed phosphatized soft tissues similar to those found in the decaying carcasses. The experiments indicate that stomatopods have a relatively high fossilization potential. Their scarce and fragmentary fossil record must be a function of factors other than decay and degradation.