Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland

Abstract

ABSTRACTA diverse, but sparsely distributed silicified fauna of over 30 taxa has been recovered from 7·5 tonnes of acid-etched Lower Ordovician Dounans Limestone from the Highland Border Complex, near Aberfoyle. The 13 trilobite taxa obtained include 3 new formally named species:Distazeris adoceta, Punka aetholiciocorusandIschyrotoma stubblefieldi.Other elements, representingCarolinites, Illaenus, Goniotelina, Sycophantia, Kawina, Heliomeroides, Strotactinus, Ectenonotus, Ceratocephalaand an indeterminate bathyurelline are described under open nomenclature although at least 4 are also probably new and 2 more may be conspecific with previously described species. The brachiopods include representatives ofArchaeorthis, Nothorthis, Orthidium, ?Camerella, Idiostrophia, Orthambonitesand 4 other indeterminate stocks. Gastropods tentatively assigned toMurchisonia, Subulites, Straparollina, MacluritesandCyrtodiscusare described, as are bryozoans, an orthocone, crinoids, andIncertae sedis.This profoundly North American fauna is Late Canadian ( = mid Arenig) in age and equates with the Cassinian Stage: it is key evidence in showing that the Highland Border rocks are not part of the Dalradian Supergroup whose earliest deformations and metamorphism predate the Ordovician. The field relationships of the limestone, together with evidence from derived clasts in Middle Ordovician and Devonian sequences near Girvan in SW Scotland and at Stonehaven in eastern Scotland suggest that it forms part of a widespread sub-Old Red Sandstone carbonate sequence of Early Ordovician age beneath the northern Midland Valley.