Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran

Abstract

Grazing trails found in the rhythmite beds attributed to the Ediacaran Tacuarí Formation (northern Uruguay) had been identified as the oldest evidence of bilaterians in the world. However, a diverse and abundant invertebrate, mostly arthropod, trace fossil assemblage is found associated with Upper Paleozoic palynomorphs in these same beds at the original localities from where the oldest bilaterian burrows were reported. These ichnoassemblages include Arborichnus repetita, Crescentichnus tesiltus, Cruziana problematica, Diplichnites aff. gouldi, Diplopodichnus biformis, Gluckstadtella cooperi, Helminthoidichnites tenuis, Kingella aff. natalensis, Maculichna varia, Rusophycus isp., Treptichnus pollardi, and Umfolozia sinuosa. The palynological assemblages yielded 89 species composed of 33 spores, 49 pollen grains, four chlorophycean algae, a fungal spore, an acritarch, and an indeterminate species. These occurrences point to a Phanerozoic record. The dominant botanical groups recorded are Lycophyta, Cordaitean, Coniferalean, Glossopteridalean, Corystospermaceae/ Peltaspermaceae, Pteridophyta, algal (Botryococcus, Tetraporina, Brazilea, Quadrisporites), and other groups (Deusilites tenuistriatus, Portalites gondwanensis). The high ichnodiversity, the prevalence of unquestionable arthropod biogenic structures, along with the abundance of ichnotaxa suggests a Carboniferous–Permian age. Therefore, the ichnoassemblages indicate that these strata are not Ediacaran in age and belong to the San Gregorio Formation, which represents the Upper Paleozoic Gondwanan glacial deposits in Uruguay. The ichnofauna is equivalent to others that are well known in deposits of the Late Paleozoic Ice Age from Brazil, Argentina, and South Africa. Furthermore, the new palynologic data from the trace-fossil bearing strata show that they can be correlated to the latest Carboniferous–early Cisuralian Cristatisporites inconstans-Vittatina saccata (IS) Zone of Paraná Basin in Uruguay.