COLONIZATION OF THE DEEP-MARINE ENVIRONMENT IN THE EARLY PHANEROZOIC: THE ICHNOFAUNAL RECORD

Body fossils in early Phanerozic deep marine environments are volumetrically rare; the majority are allochthonous and cannot contribute to our understanding of the evolution of the deep-marine biosphere over the late Precambrian-early Palaeozoic interval. In marked contrast, trace fossils, biogenic sedimentary structures, rarely survive re-working and are almost always autochthonous; this source of palaeobiological information has been under-used. Temporal changes in the ichnogeneric diversity and ethological structure of the fifty best-studied Cambrian-Carboniferous deep-marine ichnofaunal assemblages were quantified. Assemblages of Cambrian age are distinctive: their diversity is lower, the behaviour patterns pascichnia and agrichnia are relatively unimportant, and 'shallow-marine' ichnotaxa more common, than in subsequent periods.

Limited colonization of the deep marine environment before the Ordovician does not appear to be due to environmental factors, e.g. low oxygen concentrations, or inadequate supplies of organic detritus.

The depth and intensity of bioturbation increases with time in shallow-marine carbonate environments during the early Phanerozoic, and may explain the post Cambrian/early Ordovician decline in the abundance of flat-pebble intraformational conglomerates. Reductions in the abundance, morphological diversity and environmental distribution of stromatolites during the Cambro-Ordovician have been attributed to the early Paleozoic diversification of metazoans. Vertical sediment disruption is limited (except for Skolithos in nearshore facies) in Vendian and Lower Cambrian shallow marine environments, but more developed during the Cambro-Ordovician transition. It appears that complex sand-dwelling, vertically tiered stratified communities did not develop in these environments until the latest Cambrian.

The significant restructuring of deep-marine communities at, or near, the Cambrian-Ordovician boundary is therefore attributed to competition for ecospace and/or resources within shallow-marine environments during the early Phanerozoic, as a result of which ichnotaxa, including examples of pascichnia and agrichnia, were displaced offshore. Suporting this hypothesis the oldest examples of these behaviour categories are occur in shallow marine environments of late Precambrian and Cambrian age.