The Trace-Fossil Record of Organism–Matground Interactions in Space and Time

Organism–matground interactions reflect two somewhat interrelated aspects: (1) the environmental restriction of microbial mats through geologic time and (2) the evolutionary changes in benthic faunas. The history of such interactions may be subdivided into six phases: (1) Ediacaran, (2) Cambrian, (3) Ordovician, (4) Silurian to Permian, (5) Early Triassic, and (6) Middle Triassic to Holocene. Widespread matgrounds in both shallow- and deep-marine deposits during the Ediacaran provided substrates that were available for benthic colonization and the development of various interactions. The most abundant ichnofossils in Ediacaran rocks are very simple grazing trails (Helminthopsis ichnoguild), representing grazing of organic matter concentrated within microbial mats below a thin veneer of sediment. In shallow-marine environments, interactions were also evidenced by the mollusk-like Kimberella and associated scratch marks (Radulichnus) preserved on microbial mats. Interactions are also indicated for vendozoans, as reflected by serially repeated resting traces of Dickinsonia and the related genus Yorgia preserved on biomats. By the latest Ediacaran, simple burrow systems (treptinids) also occur in association with matgrounds. The replacement of matgrounds by mixgrounds was arguably the most significant change at the ecosystem scale in the history of marine life. By the Early Cambrian, branched burrow systems became more complex and common, resulting in increasing disruption of matgrounds in nearshore and offshore settings. While matgrounds were widespread in supratidal and upper- to middle-intertidal environments during most of the early Paleozoic, lower-intertidal deposits were already intensely bioturbated by the late Early Cambrian. The diachronic nature of the Agronomic Revolution is evident in the deep sea, where microbial matground ecosystems persisted during most, if not all, of the Cambrian. In addition to the Helminthopsis ichnoguild, Cambrian deep-marine ichnofaunas also consist of arthropod trackways and sophisticated feeding strategies represented by different Oldhamia ichnospecies, revealing complex architectural designs by undermat miners. In contrast, in deep-marine Lower Ordovician deposits, microbial textures are rare and patchy and typically not associated with trace fossils. Biomats persisted into the late Paleozoic in the innermost, freshwater region of estuarine systems, as well as in fluvio-lacustrine deposits, glacial lakes, and fjords. Ichnofaunas dominated by very shallow tier structures, such as arthropod trackways and grazing trails, locally associated with matgrounds, were common in these deposits. The widespread development of matgrounds after the end-Permian mass extinction sets the stage for the reappearance of feeding strategies linked to the exploitation of biomats. However, subsequent faunal recovery and deep and pervasive bioturbation resulting from the establishment of the Modern evolutionary fauna led to increased restriction of microbial mats. Analysis of ichnofaunas in matgrounds provides evidence of the temporal and environmental restriction of biomats and allows a better understanding of animal–matground interactions, as well as of preservational biases in the trace-fossil record.