RECENT ADVANCES IN DECAPOD STUDIES: THE BRACHYURA OR TRUE CRABS

Knowledge of the fossil Brachyura, or true crabs, has increased dramatically in the past three decades. Many new families, genera, and species, and even superfamilies and sections, have been added to the group, based upon evidence from the fossil record. The earliest known member of the group, thought to have been Mississippian in age, is now constrained to the early Jurassic based upon the best available evidence. By the Middle Jurassic, at least five families were represented, far more than were previously reported. The Late Jurassic is now known to be a time of explosive evolution within the Brachyura, in which 12 families are represented. Numerous ranges have been extended into the Cretaceous from the Paleogene or out of the Cretaceous into the Paleogene. Our analyses have recovered other patterns. The primitive crabs, the so-called Podotremata, are not monophyletic and are referable to 9 sections instead of 4. The Brachyura in general occupy a much broader morphospace than previously recognized, making the previously held ideas about the relationships between the gonopore position, pereiopods, abdomen, and dorsal carapace ornamentation obsolete. Adaptations such as paddle-like appendages on the fifth pereiopod seem to have contributed to the diversification of lineages within the Portunoidea. The diversification of various lineages within the Brachyura may been driven by adaptations to environmental conditions, not to adaptations to predation. Comparison of the fossil record of extant and extinct taxa within extant lineages indicates varied fossil records among the heterotreme, or more derived Brachyura. Some have excellent fossil records, indicating similar diversity in the past as in modern oceans. Others have apparently poor fossil records and/or extremely robust extant records. Investigation of these patterns is ongoing. Research supported by NSF EF 0531670 to Feldmann and Schweitzer.