Evolutionary and Environmental Controls on Ediacaran–Cambrian Ichnofaunas  

A compilation of Ediacaran–Cambrian ichnofaunas allows analysis of the role of evolutionary and environmental controls.  Re-evaluation of Ediacaran ichnofaunas indicates very limited diversity and complexity levels, because almost all the ichnogenera that were considered exclusive to the Ediacaran (e.g. Yelovichnus, Palaeopascichnus) are no longer regarded as trace fossils.  However, resting traces of Dickinsonia and Yorgia, and scratch marks of Kimberella, represent recently documented additions to Ediacaran global ichnodiversity.  An increase in complexity is revealed by the terminal Ediacaran with the appearance of branched burrow systems (Treptinids) in shallow-marine deposits.  Ichnological evidence (e.g. the Helminthopsis ichnoguild) supports the notion that benthic communities developed in association with widespread microbial matgrounds.  Ediacaran deep-marine ichnofaunas are even less diverse than their shallow-marine counterparts, and essentially consist of nonspecialized grazing trails (e.g. Helminthoidichnites), indicating exploitation of microbial mats. 

Ichnodiversity levels had changed dramatically by the earliest Cambrian, with the appearance of much more diverse assemblages - particularly in shallow-marine environments.  Relatively diverse ichnofaunas dominated by arthropod trackways (e.g. Diplichnites, Dimorphichnus) and resting traces (Rusophycus), and moderate to large shallow grazing, locomotion and feeding trace fossils (e.g. Psammichnites, Didymaulichnus) of deposit feeders are known worldwide.  This increase in diversity is paralleled by an increase in complexity, with the appearance of more sophisticated feeding strategies as revealed by patterned fodinichnial structures (e.g. Treptichnus pedum, Psammichnites saltensis). The highest ichnodiversity occurred in upper-offshore to shelf environments, ranging from the fair-weather wave base to slightly below storm wave base, revealing the onset of an archetypal Cruziana ichnofacies.  Colonization of high-energy areas, such as subtidal sandbodies and foreshore-shoreface complexes, was slightly delayed until the establishment of vertical dwelling structures (Skolithos, Diplocraterion, Arenicolites) of suspension feeders and passive predators, reflecting the appearance of the Skolithos ichnofacies.  The J-shaped spreite trace fossil Syringomorpha is also present in high-energy shallow-marine deposits, apparently restricted to the early–middle Cambrian.  Integrated ichnological and sedimentological data suggest that colonization was not restricted to fully marine environments, but instead that some arthropods were able to foray into brackish-water embayments. 

Ediacaran-style ecosystems persisted in the deep sea during the Cambrian, with benthic styles essentially linked to the widespread occurrence of matgrounds.  However, Oldhamia flourished in early Cambrian deep-marine environments, experiencing a remarkable behavioural diversification and revealing increased levels of complexity in feeding strategies.  Also, arthropod trackways are relatively common in deep-marine deposits.  The face of the deep started to change significantly during the early Ordovician, with the establishment of complex graphoglyptids and grazing trails leading to the ultimate demise of matground-dominated ecosystems and signalling the arrival of the agronomic revolution to the deep sea.