THE TRACE-FOSSIL RECORD OF MAJOR EVOLUTIONARY RADIATIONS (Invited Presentation)

Recognition and characterization of evolutionary radiations have been essentially based on the analysis of the body-fossil record. Compilation of ichnogeneric occurrences through geologic time suggests that the main evolutionary radiations in the marine biosphere, the Cambrian Explosion (CE), the Great Ordovician Biodiversification Event (GOBE) and the Mesozoic Marine Revolution (MMR), are evident form an analysis of the trace-fossil record also. The CE is characterized by a 433 % increase in ichnodiversity during the Terraneuvian (12 and 64 ichnogenera in the Ediacaran and Terraneuvian, respectively). This increase in ichnodiversity essentially reflects innovations in styles of animal-substrate interactions in softgrounds (i.e. bioturbation structures). In addition to diversification, the CE is associated with a remarkable increase in sediment disturbance and ecosystem engineering. The GOBE displays 77 % increase in ichnodiversity (82 ichnogenera in the Furongian and 145 ichnogenera by the Late Ordovician). The GOBE is associated with colonization of both soft and hard substrates, the latter reflecting evolutionary innovations in bioerosion structures. The MMR is more protracted showing ichnodiversity increases in the Early Jurassic (9%; 164 and 179 ichnogenera in the Late Triassic and Early Jurassic, respectively) and Late Cretaceous (19%; 188 and 223 ichnogenera in the Early and Late Cretaceous, respectively). From the perspective of colonization of the infaunal ecospace, the MMR reflects a progressive increase initially in the number of ichnoguilds and subsequently in the number of ichnotaxa per ichnoguild, revealing the evolution of a finely tuned infauna. Ichnology is providing valuable insights to reconstruct diversity trajectories and to assess major ecologic changes during evolutionary radiations.