AN EXCEPTIONALLY PRESERVED ICHNOFAUNA FROM THE EDIACARAN-CAMBRIAN TRANSITION OF THE CORUMBA GROUP OF MATO GROSSO DO SUL, BRAZIL

Dated molecular phylogenies suggest that the major divergences within animals occurred in the Ediacaran or possibly earlier. However, the affinities of Ediacaran body fossils are often equivocal or controversial. Trace fossils provide an important alternate line of evidence, since they document animal behavior and animal-substrate interactions that are largely independent of the body fossil record. The first appearance of complex, vertically penetrating trace fossils is considered to have been globally synchronous, and had important implications for the geosphere, with the matground-dominated ecologies typical of the Neoproterozoic giving way to the mixground ecologies of the Phanerozoic. We report an exceptionally well preserved ichnofauna from the Ediacaran-?Cambrian Corumba Group of Mato Grosso do Sul, western Brazil. These trace fossils are preserved in three dimensions as framboidal pyrite and carbonate void fills in marls of the Guaicurus Formation. The presence of diagenetic pyrite burrow fills suggests that the trace fossils were open immediately prior to burial. The density contrast between the trace fossils and surrounding matrix allows them to be visualized using microCT, revealing complex, intense bioturbation. Burrows as small as 150μm in diameter are reconstructed, and show diverse behaviors including branching and short oblique shafts between layers with net-like galleries. The small size and unusual taphonomy suggest that these trace fossils would typically have a low preservation potential, and therefore raise the question of whether the first appearance of the Treptichnus ichnological assemblage can confidently identify the origin of complex animal behaviors.