THE ROLE OF THE NEOTROPICS IN THE EVOLUTION OF TRUE CRABS

The evolutionary origins of highly diverse groups are often obscure because early fossils tend to be rare or fragmentary, and geographic centers of diversification are poorly known. True crabs, or Brachyura, are a speciose and anatomically diverse group of aquatic and terrestrial crustaceans with a good fossil record thanks to their mineralized bodies, their abundance, and the broad geographic and environmental ranges they inhabit. Although the tropics hold much of the world’s current crab biodiversity, the timing and pathways of their diversification are still uncertain. Various studies have suggested that several lineages originated in high latitudes and later migrated to lower, tropical latitudes where they established and diversified. The recent discovery of new Lagerstätte and fossiliferous localities, and a detailed re-examination of the brachyuran fossil record from the tropical Americas, have revealed the presence of more than 32 superfamilies, 70 families, 200 genera, and 420 species of marine, terrestrial, and freshwater crabs. Several of these taxa represent the oldest members of their groups’ worldwide, considerably extending their stratigraphic and paleogeographic ranges into the tropical Americas and challenging the paradigm of a high latitude origin. Furthermore, the exquisite preservation in various taxa of soft tissues and a range of ontogenetic stages, provide glimpses of the interplay between environment and development on the evolution of novel forms and functions, the paedomorphic retention of larval traits in adulthood, the origins of aptations for swimming and digging, and the repeated convergence towards ‘carcinized’ and ‘decarcinized’ body forms since at least the Early and Late Cretaceous, highlighting the possible role of the Neotropics on the evolution of true crabs throughout geological time.