SPECIES OF DECAPODA IN THE FOSSIL RECORD

The past 20 years have seen a renaissance in the study of fossil decapod crustaceans. About 600 species have been recognized during that time. However, definition and recognition of species in the fossil record is difficult for a group that is often represented by species known from only one specimen!

The record of decapods is different in many regards from other groups. The vagaries of disarticulation and preservation of the largely proteinaceous cuticle make decapods less likely to be fossilized than some other groups. Most species are recognized from only one locality. Many species are represented by only one specimen, meaning that range of variation within these species is unknown. At the other extreme, some localities have yielded tens of thousands of specimens, with multiple species of the same genus. How do we know that that these species are not members of a single species with a large range of variation?

Our work comparing extant and fossil specimens suggests that determining identifications of Decapoda in the fossil record is straightforward at the family and genus level, with a high level of confidence. This is possible because many characters used by biologists to classify animals at these levels are often preservable in fossils, including aspects of the orbits, carapace groove patterns, appendages, sternum, pleon, and even the genital openings. Even when these are not preserved, it is often possible to find proxy characters of the dorsal carapace that track these ventral or genital characters. However, extant species are often defined based upon features of the antennae, antennal scales, genital articles, and other soft or tiny parts that are rarely, if ever, preserved. The presence of cryptic species, those that look identical but are differentiated by quite divergent DNA, as well as wide variation in the geographic ranges of extant species, further complicates the recognition of decapod species. Proxy characters can help—the arrangement of spines on the carapace, for example—and this is what is typically used to define fossil species. In actual practice, recognition of extinct species of decapods is based upon morphological criteria as well as geographic and stratigraphic bases. Thus, synthetic studies on distribution, evolution, ecology, and biogeography are typically conducted at the generic level.