Trilobites from the Ordovician Balclatchie and lower Ardwell groups of the Girvan district, Scotland

Abstract

ABSTRACTAll trilobites known from the lower and upper Balclatchie and lower Ardwell groups of the Girvan district are listed and frequencies recorded. Eight new species are described, a number of generic assignments are new, and hitherto undescribed parts are illustrated. Of the 60 named species, 19 occur in the lower and upper Balclatchie and lower Ardwell groups; the upper Balclatchie Group yields the largest number of species. The assemblage is generally indicative of outer shelf conditions of deposition; a deeper water association at certain Balclatchie Group localities consists of 5 species identical with, or closely related to, species from the Liberty Hall facies of the lower Edinburg Formation of Virginia, U.S.A. This deeper water element constitutes a significantly larger proportion of the trilobite assemblages at certain localities than it does at others. A further 8 species from the Balclatchie Group are allied to lower Edinburg Formation forms, thus confirming the evidence of the brachiopods regarding the strong North American affinities of the fauna. The Balclatchie conglomerate is considered to be the result of a catastrophic event accompanied by slumping near the shelf edge causing the instantaneous entombment of the fauna.