Terminal Proterozoic-Early Cambrian ecosystems: Ichnology of the Puncoviscana Formation, northwest Argentina

The Puncoviscana Formation consists of a thick, folded and slightly metamorphosed succession of sandstones, mudstones and conglomerates of terminal Proterozoic to earliest Cambrian age, representing the metasedimentary basement of northwest Argentina. The Puncoviscana ichnofauna is recorded in lowermost Cambrian strata and includes a wide variety of ichnotaxa, such as Archaeonassa fossulata, Circulichnis montanus, Cochlichnus anguineus, Didymaulichnus lyelli, Diplichnites isp., Helminthopsis abeli, Helminthopsis tenuis, Helminthoidichnites tenuis, Multina isp., Nereites saltensis, Oldhamia antiqua, Oldhamia curvata, Oldhamia flabellata, Oldhamia radiata, Oldhamia isp., Palaeophycus tubularis, Saerichnites isp., Treptichnus pollardi and Volkichnium volki. Structures considered in previous studies as graphoglyptids (agrichnia), such as Protopaleodictyon and Squamodictyon, are reinterpreted as wrinkle marks and elephant skin textures. Vertical dwelling structures (domichnia) are absent. Nearly all the ichnofossils in the association are oriented parallel to the bedding plane, displaying restriction to two-dimensional biotopes, and therefore they do not disturb the primary sedimentary fabric. The recognition of trace fossil associations in both shallow- and deep-marine deposits of the Puncoviscana Formation provides valuable information on evolutionary and ecological controls on Early Cambrian marine infaunal communities. Shallow-marine ichnofaunas are dominated by moderate- to large-sized, shallow grazing and feeding traces of deposit feeders. Deep-marine ichnofaunas are dominated by Oldhamia and non-specialised grazing trails and reflect lifestyles related to microbial matgrounds. Early Cambrian deep-marine ichnofaunas are of "Ediacaran aspect", recording persistence of relict communities after the onset of the "Agronomic Revolution" that pervaded shallow-marine ecosystems during those times. Early Cambrian tiering was moderately developed in both shallow- and deep-marine communities. Biogenic structures were emplaced within the uppermost millimetres of the sediment (i.e. micro-tiering), with the exception of burrowers that record post-event colonisation of tempestites and turbidites.