Article: A review of brachiopod dominated palaeocommunities from the type Ordovician
Recent studies on Ordovician (mainly Llanvirn to Caradoc) faunas from type and classical localities in Wales and the Welsh Borderland have resulted in the publication of a wealth of data representing approximately 200,000 individual identifications from about 2,000 samples (assemblages) collected through some 10 km of strata (average sample interval 5 m). Contributing authors have named at least thirty variously defined assemblages, associations, sets, communities, and palaeocommunities which are reviewed and subjected to cluster analysis. This reveals eight highly correlated (p > 95%) taxonomic subclusters which also closely reflect palaeogeography, stratigraphic relationships, and fades preferences. These are conveniently characterized, using pre-existing guidelines, as the 'mixed' Dalmanella, Hesperorthis, Lingulella, Onniella, Dalmanella, Bancroftina-Kjaerina, Howellites, and Nicolella palaeocommunities. Other important faunas include inarticulate brachiopod associations dominated by Monobolina, Pseudolingula, and Schizocrania.Palaeoecological and evolutionary considerations indicate a marked contrast between widespread and diverse, biologically accommodated associations from the middle part of the facies (textural) spectrum and localized, physically limited, low-diversity faunas from extremely coarse (shoreface) facies and more widespread low-diversity faunas from very fine (offshore) facies. This pattern differs from some Silurian models but accords with others. Differences in the Welsh Basin during the two periods suggest that comparisons between supposedly analagous palaeocommunities are tenuous and certainly not viable from a taxonomic viewpoint. Diversity, facies preference, palaeogeography, and taxonomic composition are all shown to be useful guides to palaeocommunity evolution; they indicate that high-diversity, pre-Caradoc, Dalmanella-dominated faunas represented the most stable palaeocommunity which evolved in basinal (open-shelf?) rather than basin-margin (shoreface) localities to give rise to associations characterizing the Nicolella palaeocommunity of Caradoc times. In contrast, low-diversity shoreface faunas show more rapid and unpredictable change whilst those from argillaceous offshore facies remain unchanged over long periods.