EVOLUTIONARY PALEOECOLOGY OF EARLY PHANEROZOIC DEEP-MARINE ENVIRONMENTS AS REVEALED BY THE ICHNOFAUNAL (TRACE FOSSIL) RECORD

Ichnological studies form an important, and growing, contribution to our understanding of the evolutionary paleoecology of the Earth's biosphere during the latest Precambrian to earliest Phanerozoic interval. Attention has focused on shallow-marine environments: the rapid change in their paleoecology close to the Precambrian-Cambrian boundary, the product of changing patterns of bioturbation, has been described as the Agronomic or Cambrian Substrate Revolution, and was followed by longer term increases in the intensity and depth to which sediments were bioturbated. The evolutionary palaeoecology of coeval deep-marine environments has been less studied, although as the dominant autochthonous component their ichnofaunas and ichnofabrics have the potential to contribute similarly.

There are significant changes in both the diversity of ichnotaxa (i.e. discrete, taxonomically identifiable, trace fossils) and the ethological structure of deep-marine trace fossil assemblages between the Cambrian and younger periods. The first extensive colonization of the deep-marine environment, accompanied by greater development of systematic deposit feeding trace fossils (pascichnia) and infaunal open burrow networks (agrichnia), occurred during the late Cambrian to early Ordovician interval. This pronounced restructuring of the ecology of deep-marine environments, and the establishment of the archetypical Nereites ichnofacies, can best be modelled as the result of increasing competition for ecospace and/or resources in shallow-marine environments during the earliest Phanerozoic, as a result of which certain behaviour patterns were displaced offshore. Supporting this hypothesis pascichnia and agrichnia occur in late Precambrian and Cambrian, but rarely younger, shallow- marine environments. Alternative suggestions, that limited colonization of the deep-marine environment before the Ordovician reflects low oxygen concentrations or inadequate supplies of organic detritus, are rejected. The restructuring of deep-marine communities between the Cambrian and Ordovician is thus ultimately a delayed response to the major ecological changes in shallow-marine environments initiated at the Proterozoic-Phanerozoic boundary and continued subsequently.