The earliest turrilepadid: a machaeridian from the Lower Ordovician of the Northwest Highlands

摘要

A machaeridian sclerite from a thrombolite bed in the Sangomore Formation (Durness Group: Tremadoc) is the first to be described from NW Scotland. The large size, thickness and coarsely spaced rugae indicate the sclerite is that of a turrilepadid, but its morphology is distinct from any taxon described previously. Lacking a longitudinal fold and with three major inflections present along the accreting margin, it is interpreted as an outer right sclerite. It is the oldest turrilepadid yet recorded and shows that all three major groups of the problematical Machaeridia were present in the early Ordovician.