VARIATION AND TAPHONOMIC IMPLICATIONS OF COMPOSITION IN MODERN AND FOSSIL MALACOSTRACAN CUTICLES

The crustacean fossil record is dominated by mineralized forms. These fossils, in turn, are mainly chelae or parts of chelae. Studies of the composition of modern crustacean cuticles have likewise been dominated by decapods. Here, we summarize major element chemical analyses (Ca, Mg, Na, P) using SEM-EDS of modern crustacean exoskeletons from Bermuda, Georgia (U.S.A.), and Lake Tanganyika, Burundi. Although most specimens were brachyuran and anomuran crabs, material also included examples of stomatopods, isopods, caridean shrimp, and Achelata. Transects were made of sections of the carapace, palm of the propodus, and tips of either the dactyl or propodus. Multiple transects were made of the Bermudan land crab Gecarcinus lateralis. Additional studies were made of fossil crabs from Texas and South Dakota and of crabs having undergone ten months of burial in marine sediments.

Overall, the tips of the chela were the thickest and most heavily mineralized regions and had the lowest phosphorus content. In nearly all cases, they also had the lowest ratios of Mg to Ca. One notable exception was Gecarcinus, where the denticles had elevated Mg:Ca values. Between taxa, Anomura and Brachyura had markedly different composition from Stomatopoda and possibly Isopoda, suggestive of independent evolutionary origins of biomineralization within the Crustacea. There were also significant differences in composition between crabs from Georgia and Bermuda, perhaps dues to differences in water temperature. The fossil crustaceans had much more phosphate in their cuticles, certainly as a result of diagenetic processes.

Composition should be a first order control on the fossil record of crustaceans. Future studies need to expand the taxonomic and geographic ranges of material examined, determine whether there are environmental controls on composition, and explore the relevant diagenetic processes.