DIVERSITY AND NICHE PARTITIONING IN PRIMITIVE CRABS (DECAPODA: BRACHYURA)

The least derived crabs, the so-called podotrematous crabs, reached their peak in generic level diversity in the Late Cretaceous. They reached their highest proportion of the decapod fauna at nearly the same time, in the late Early to Late Cretaceous, when they dominated the entire decapod fauna by percentage. The polyphyletic podotreme crabs are comprised of several clades, and each displays distinctive environmental preferences as inferred from the enclosing rock. The earliest group to appear among the podotremes, the Dromiacea, is composed of several families. Nearly all of these demonstrate strong preference for carbonate and coral-rich environments. Although most of these families became extinct by the Eocene, this environmental preference extends into the Holocene for related extant groups. The other major extant clade, Raninoida, exhibits strong evidence for niche partitioning within its included families. Major clades within Raninoida display distinct preferences for either siliciclastic or carbonate environments. On a larger scale, podotreme crabs over Cenozoic time diminished in their percentage of the total array of decapods as the more derived heterotreme crabs increased, eventually comprising the vast majority of the decapod fauna by percentage by Holocene time. During both the Mesozoic and the Cenozoic, podotreme groups appear not to have competed with one another for niche space as each clade was found in a different type of environment. This pattern parallels that seen for the clades of lobsters as well as the one seen for Decapoda on the infraordinal scale. Thus, extinction patterns within Decapoda seem to be predominantly related to factors extrinsic to the individual groups and not to intraspecific competition. This work was funded by NSF EAR 1223206 to Schweitzer and Feldmann.