Early Colonization of the Deep Sea: Ichnologic Evidence of Deep-marine Benthic Ecology from the Early Cambrian of Northwest Argentina

Tiering and ichnoguild analysis provide valuable evidence on marine benthic paleoecology. This paper provides the first detailed study on the ecology of a Cambrian deep-marine ichnofauna by means of the analysis of its ichnoguilds and tiering structure. Endobenthic tiering in this Early Cambrian deep-sea community was moderately developed. Biogenic structures were emplaced mostly in the uppermost millimeters of the substrate, with the exception of post-depositional burrows preserved on the base of turbidite sandstones. Four ichnoguilds are defined. The Palaeophycus ichnoguild includes semi-permanent, shallow-tier, suspension feeder structures produced by vagile vermiform organisms. The Oldhamia ichnoguild consists of semi-permanent, very shallow-tier, undermat-miner structures produced by stationary vermiform organisms. The Helminthopsis ichnoguild comprises transitory, near-surface to very shallow-tier, mat-grazer structures produced by vagile vermiform animals. The Diplichnites ichnoguild consists of transitory, surface structures produced by vagile arthropods. Comparisons with coeval ichnofaunas indicate that these ichnoguilds are widespread in Vendian–Cambrian deep-marine biotas. This analysis suggests that Cambrian deep-water communities were remarkably different from younger examples and stresses the importance of microbial mats on the ecology of Neoproterozoic–Cambrian ecosystems. Feeding strategies associated with microbial mats persisted in the deep sea after the agronomic revolution that took place in shallow-water environments, suggesting a gradual closure of a taphonomic window during the Proterozoic–Cambrian transition. The presence of post-depositional burrows of the Palaeophycus ichnoguild at the base of turbidite sandstones indicates a higher burrowing depth than those envisaged by current models.