EVOLUTION OF PRIMITIVE CRABS: INSIGHTS FROM THE NEOTROPICS

The order Decapoda is one of the most diverse crustacean groups, exhibiting a remarkable morphological disparity. Among decapods, the infraorder Brachyura (‘true’ crabs) is the most derived, and it is comprised of eight main sections or bauplans with only five known from extant representatives. The two oldest brachyuran sections, Dromiacea and Homoloida, are first known from the Early to Middle–Late Jurassic, while the other six (Torynommoida, Etyoida, Raninoida, Cyclodorippoida, Dakoticancroida, Eubrachyura) have their earliest records in the Cretaceous, depicting it as a time of large Brachyura morphological diversification. However, the phylogenetic relationships among ancestral bauplans; i.e. all but Eubrachyura, or ‘higher’ crabs, remain unclear. While Mesozoic brachyurans are well known for high latitudes, occurrences in the equatorial Neotropics are scarcely known, resulting in significant biases when attempting to address their phylogenetic relationships. A new bauplan from the Cretaceous of Colombia displays a unique combination of morphological traits (e.g. pediform dentate mouthparts, fusiform carapace, broad sternum, and large eyes) that, if considered independently, may advocate for different phylogenetic relationships. As a result, inclusion of this ‘chimera’ in a cladistic context deeply affects the topology of the Brachyura tree, collapsing all of the primitive sections into a major polytomy. Majority–rule consensus trees better resolve this polytomy, and place this bauplan as sister taxon to the clade containing all of the Cretaceous–originated sections. Furthermore, the morphological innovation of fusiform carapaces, previously recognized only in some fossil and all extant Raninoida, appears to have evolved independently at least twice among podotreme clades during the Cretaceous, probably triggered by similar ecological pressures or adaptations to a similar lifestyles. In addition, newly discovered raninoidans from the Early Cretaceous of northern South America pull back the oldest confirmed records into the Valanginian (~137 My), and indicate a higher early disparity than previously assumed. The new findings help us gaining a better understanding on the role of the Neotropics on the origin, evolution and diversity of primitive crabs throughout geological time.