Basal Wenlock shelly biofacies from the Knockgardner Formation (middle
Straiton Group) of the Girvan district are dominated by brachiopods a
nd trilobites assigned, respectively, to the Howellella-Plotochonetes
and the Podowrinella faunas. The Podowrinella fauna is consistently de
veloped across the Midland Valley in Llandovery-Wenlock boundary rocks
but the association is not known outside Scotland. In contrast the Ho
wellella-Protochonetes fauna is more variably developed and restricted
to the western part of the Midland Valley; but the components of this
type of fauna occur in Wenlock and Ludlow rocks elsewhere. In Scotlan
d both associations occur in nearshore, commonly storm-dominated sedim
ents. The contrasting distributional patterns of the mobile and sessil
e benthos may reflect the differing life strategies of both the larval
and adult stages in each group. Lithofacies within the upper Llandove
ry-lower Wenlock Straiton Group are revised within the regressive fram
ework of the region.