ON THE ORIGIN AND EVOLUTION OF TRUE CRABS: INSIGHTS FROM NORTHERN SOUTH AMERICA

The Brachyura, or ‘true’ crabs, are the most derived and diverse group of crustaceans. They are first known from the Mid–Jurassic (~160 Ma.) and rapidly diversified worldwide during the Cretaceous (~146 to 66 Ma.). Since then, crabs have exploded in form and function, resulting in the incredible morphological diversity seen among the more than ~9,000 fossil and extant species. Despite the large disparity of their body plans, various molecular, morphological and spermatological studies have shown that Brachyura is a monophyletic group, sister to Anomura (i.e hermit crabs and allies). Nevertheless, the oldest brachyurans are anatomically very different from the rest of higher crab clades, obscuring their phylogenetic relationships.

Previous and recent field works in Early Cretaceous rocks of Santander, Colombia (Zapatoca, from 1983 to 2012; San Vicente de Chucurí, 2013) have yielded more than 400 specimens of a dozen ancient brachyuran crab species (Luque et al., in progress). Based on synthetic and cladistic classifications, some of these taxa cannot be placed in any currently recognized fossil or extant family. Their combinations of plesiomorphic and synapomorphic traits indicate that the early evolution of brachyuran crabs is more complex than currently envisioned. Among other recent findings are some of the oldest known members of many families and superfamilies of ‘podotreme’ crabs (e.g. Homoloida, Raninoida), and even the earliest record of ‘higher’ true crabs (i.e. Eubrachyura). These findings challenge current spatio-temporal and phylogenetic hypotheses, and suggest a) that several Colombian crab higher taxa have Pantropical distributions but with a high degree of endemism of families and genera, b) a tropical South American origin for many clades previously considered to have originated in higher latitudes, and c) that the Neotropics have acted as a hotspot of diversity for crustaceans through time. This research was partly funded by NSERC CGS-D Scholarship (Canada) and Fondo Corrigan–ACGGP–ARES Grant (Colombia) to JL.