TAPHONOMY OF THE NORTH AMERICAN DECAPODS

Abstract

ABSTRACT The geological record of the decapod crustaceans is sparse and fragmentary. Biologic, taphonomic, and collection factors bias the record, enhancing or diminishing it. These biasing factors must be relatively well understood before maximum use can be made of paleontologic samples. Biologic factors that are important are population density and mobility, preservation potential of habitats, differential calcification, and molting. The geologic record of shrimp is exceedingly deficient; that of mud shrimp, crabs, and lobsters is better, but still sparse, fragmentary, and strongly biased. Taphonomic variables affecting the preservation and interpretation of decapods include burial conditions and positioning of the remains for buried-alive decapods, corpses, and molts. Remains react differently and retain evidence of burial conditions, whether immediate or delayed. Exhumed and reburied remains often present evidence of abrasion, being bored, or settled by epizoans. Burrows enhance preservation potential, and such trace fossils can be assigned to construction by decapods, but only occasionally to a specific taxon. Characteristic preservation positions of decapods include those buried alive (Traumatized Position), those buried as corpses (Normal Position), molts of crabs and lobsters (Open Molt Position), or disarticulated remains. Evidence to distinguish molts, corpses, and buried-alive crabs is often circumstantial or lacking. Diagenesis is directly related to burial conditions, rate and depth of burial, and the presence of scavengers and deposit feeders. A general mode of formation of phosphatic concretions is envisioned which involves death, burial, decomposition, infilling, pelletization and bioturbation by worms (concentrating phosphate in fecal pellets), and precipitation of phosphatic concretions. Benthic nutrient enrichment by plankton kill resulting in a rapid increase in detritus feeders may have initiated a rapid population increase in decapods able to exploit the new food resource. The decapod-worm system would form a taphonomic feedback system insuring and allowing exploitation as well as preservation over an interval of time. Other modes of preservation include little altered remains in fine clastics, preservation as allochems in limestone, preservation as nuclei in calcite nodules and concretions, and in hematite concretions. Collections of decapods tend to be small and widely scattered, but contain virtually all remains found in allied geologic and paleontologic work. Most species-level taxa are represented by only a few, fragmental specimens. Approximately 200 decapod taxa are known from the North American Cretaceous. Because of the usual scarcity and small, fragmentary sample size of most taxa, decapod-rich or decapod-dominated fossil assemblages are exceedingly important, since they alone preserve large numbers of diverse decapod taxa.