Timing of infaunalization in shallow-marine early Paleozoic communities in Gondwanan settings: discriminating evolutionary and paleogeographic controls

Ichnofabric analysis of upper-offshore facies in a series of lower Paleozoic siliciclastic shallow-marine units of northwest Argentina revealed ichnofaunal changes that may record faunal turnovers at the macroevolutionary level. Comparative study reveals a pronounced difference between Furongian to lower Tremadocian and upper Tremadocian to Caradocian upper-offshore deposits, particularly in the fairweather suite of trace fossils. Furongian to lower Tremadocian upper-offshore deposits are dominated by shallow-tier structures, mostly cruzianids produced by trilobites. The bioturbation index of these deposits is typically 0 to 1, with trace fossils commonly emplaced parallel to bedding with little disturbance of the primary fabric. In contrast, upper Tremadocian to Caradocian upper-offshore deposits display more intense bioturbation (bioturbation index commonly between 2 and 4, but locally 5). The most significant innovation is the appearance of a mid-tier component represented by the ichnospecies Trichophycus venosus, which cross-cuts cruzianids and vermiform trace fossils emplaced at shallow depths. The early to late Tremadocian transition is regarded as a turning point for exploitation of the infaunal ecospace in high- to intermediate-latitude siliciclastic basins of Gondwana. Interestingly, the Trichophycus ichnofabric seems to have been present since the Early Cambrian in other paleocontinents, such as Baltica and Laurentia. Our study underscores the importance of comparing ecologic trends among different continents to further understand controls and timing of evolutionary innovations during major faunal radiations.