Evolution, succession and replacement in the type upper caradoc (ordovician) benthic faunas of England

Abstract

Measured sections through rocks of the type upper Caradoc Series show a succession of seven terrigenous shelf facies with decreasing sediment grain size, increasing bioturbation and sediment clay content and decrease in thickness of storm-deposited beds of laminated sand and silt with shelly coquinas. This indicates an environmental gradient of general deepening which is locally interrupted geographically, in the Actonian, with the formation of a thin proximal calcareous sandstone unit. The Laminated Shale Facies which is restricted to offshore areas may indicate upwelling centres, which result in bottom oxygen depletion and thus low benthic densities and diversities. Continuous faunal sampling reveals nine associations directly related to sedimentary facies and dominated by articulate brachiopods, bryozoans, bivalves, gastropods and trilobites. Substrate and sedimentation processes were important facets of the physical environment determining community organisation. Species diversity increases and number of opportunistic species declines into the more distal shelf environments. As far as can be ascertained from sedimentary information, this correlates with decreasing environmentally related stress. Local perturbations occur in the vicinity of the Laminated Shale Facies. Number of characteristic species peaks three times along the environmental gradient and relates directly to three association groupings based on the generalised adaptive modes of the constituent species. The highest concentration of characteristic species occurs in the most stable environment, containing the highest diversity Onniella reuschi — Chonetoidea radiatula Association. Greatest concentration of the ubiquitous and intergrading species is in the proximal environments. Community trophic structuring was relatively simple and dominated by low-level, suspension-feeding groups (mainly brachiopods) throughout. A more complex structure prevailed in offshore associations with infaunal deposit-feeding bivalves, epifaunal deposit-feeding or browsing gastropods and trilobites as well as the suspension feeders. Nearshore communities are viewed in terms of succession along a proximal to distal gradient of decreasing stress. It is suggested that community 1 ( Bancroftina robusta Association), composed of initial colonisers, is followed by community 2 ( Kjaerina typa Association), composed of later successional stage species in a less physically stressed setting. Severe downgrading of communities 2 and 3 should produce 1. Community compositions fluctuate and there appear to be some predictive successional patterns. The most important is the development of a trinucleiid trilobite, Onnia spp., and a palaeotaxodont bivalve mollusc, Nuculites planulatus as initial colonisers in offshore muds, a feature known from other Lower Palaeozoic sequences.