Ichnology of Cenozoic marine deposits from Patagonia (Southern Argentina): the role of the modern evolutionary fauna in Neogene infaunal Ecosystems

Neogene deposits from Patagonia are characterized by extremely diverse ichnofaunas, including a great variety of feeding (e. Protovirgularia) and equilibrium/ escape structures (e.g., Scalichnus) occur. Trace fossils in open-marine deposits are abundant and diverse, with extremely complex tiering structures, and most commonly characterized by the presence of the archetypal Cruziana Ichnofacies. On the contrary, restricted, commonly tide-infl uenced, brackish-water environments show low to moderate ichnodiversity, mono-specifi c associations, trace fossils generally of small sizes, and the presence of an impoverished Cruziana-Skolithos Ichnofacies. Irregular echinoid structures are abundant mainly in open-marine environments while those produced by bivalves and crustaceans dominate the open-marine and brackish-water ichno-faunas, refl ecting the peak of the Modern Evolutionary Fauna. Ichnofaunas of modern aspect seem to have been well established in shallow-marine, open environments since the Mesozoic, and this is particularly well exemplifi ed in the Neogene ecosystems from Patagonia, where fi nely tuned climax communities display vertical niche partitioning and a remarkable use of the infaunal ecospace. These observations agree with the trends exhibited by the body-fossil record, which show that for the Late Cenozoic middle-latitude successions, marine paleocommunities include a greater representation of infaunal organisms, especially of deep-burrowers. In addition, local infl uence of nutrient-rich waters brought to the surface during upwelling events along the Patagonian coast may have also contributed to the increasing complexity seen in these Neogene infaunal ecosystems.