The uppermost Ordovician (Hirnantian) trilobites of Girvan, SW Scotland with a review of coeval trilobite faunas

Abstract

ABSTRACTThe uppermost Ordovician (Hirnantian) trilobite fauna of the High Mains Formation in the Craighead Inlier near Girvan comprises species of Flexicalymene, Achatella, Hemiarges, Isotelus and an indeterminate proetid. All are illustrated and the first three including Hemiarges extremus sp. nov. are described. This is one of the most diverse Hirnantian trilobite assemblages known. The beginning of the Hirnantian marked a profound global decrease in trilobite diversity and an overall increase in endemicity with many genera persisting only as local relicts. Many faunas include a dalmanitid (Mucronaspis or Dalmanitina) and the absence of this family suggests a shallow shelf environment for the High Mains fauna which comprises relicts of earlier lineages in the area. Some genera have yet to be discovered in the Hirnantian, but are recorded from both older and younger strata. Like many of the known Hirnantian taxa, they must have survived in localised faunas before becoming widespread again in the Silurian. Limited evidence suggests that the spread of cold/deep water trilobites, notably Mucronaspis / Dalmanitina, on to low latitude shelves preceded the major trilobite extinction at the Rawtheyan-Hirnantian boundary, but this would still be within the timescale suggested by other workers for a rapid onset of the late Ordovician glaciation and the consequent ecological disruption.