The Cambrian revolutions: Trace-fossil record, timing, links and geobiological impact

Several concepts pertaining to the dramatic changes that occurred during the Cambrian have been proposed, namely the Agronomic Revolution (AR), the Cambrian Substrate Revolution (CSR) and the Cambrian Information Revolution (CIR). The original concept of the AR refers to the replacement of Precambrian-type substrates (“matgrounds”) by Phanerozoic-type ones (“mixgrounds”). The CSR highlights the evolutionary and ecological effects of Cambrian substrate changes on epifaunal sessile metazoans. The CIR involves the distribution of signals in an environment that an organism can potentially respond to, underscoring an increased complexity and heterogeneity of marine ecosystems, which may have played a major role as a driving force of further evolutionary change during the Cambrian. A systematic review of the Ediacaran-Cambrian ichnofossil record allows to provide some temporal resolution for these Cambrian revolutions and to explore the synergies among these evolutionary breakthroughs and possible geobiological outcomes. The AR is not coincident with the Ediacaran-Cambrian boundary, but rather it is manifested by Cambrian Age 2. Because the CSR focuses on the response of the epifauna to newly evolved mixgrounds and the late Cambrian appearance of hardgrounds, it may be understood, at least in part, as a consequence of the AR. In contrast, the CIR involved the appearance of sophisticated, innovative feeding strategies that allowed benthic organisms to successfully exploit resources in an increasingly more heterogeneous sea bottom. The occurrence in the Fortunian of highly patterned grazing trails reflects the establishment of efficient navigational devices. Although the CIR signals the appearance of novel body plans and novel ways of animal-substrate interactions, strategies involving matground exploitation were still widespread in the Fortunian. More dramatic changes took place in Cambrian Age 2, as revealed by the onset of the AR and later the effects on epifaunal sessile organisms and communities during the CSR, signalling to the transition to shallow-marine ecosystems of modern aspect.