CRABS IN AMBER REVEAL AN EARLY COLONIZATION OF FRESHWATER DURING THE CRETACEOUS

Transitions from marine to non-marine habitats are infrequent in most metazoan groups, largely due to different physical and physiological requirements in saltwater and freshwater, competition with previously established residents, and exposure to new predators. True crabs, or Eubrachyura, are among the animal groups that have conquered land, brackish water and freshwater multiple times independently, although much more recently than other arthropod groups. Molecular phylogenies suggest that the first freshwater and terrestrial crabs diverged from their closest marine relatives during the Early Cretaceous or before (~125 Ma). Direct evidence of their colonization of non-marine environments is sparse, however, because the few fossils known from such environments are largely comprised of isolated carapaces or claws of latest Cretaceous to Quaternary age (~73 Ma to Holocene).

Here we present a new true crab preserved in Cretaceous amber (~99 Ma, Cenomanian). This is the oldest occurrence of a true crab in amber and one of the oldest crown group eubrachyurans known. Micro-CT digital reconstructions reveal that antennae, large compound eyes, mouthparts with multiple fine hairs, and even gills are preserved. Our phylogenetic analysis reveals a crown eubrachyuran with a unique mixture of primitive and advanced characters. The fossil appears to have been trapped in a brackish or freshwater setting near a coastal to fluvio-estuarine environment, bridging the ~50 Ma gap between the predicted molecular divergence of non-marine crabs and their younger fossil record, while providing a reliable calibration point for molecular divergence time estimates for crown true crabs.