Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales
and Scotland became progressively more disparate despite the narrowing
of the ocean. We compare faunal distributions in continuous sections
from opposite sides of Iapetus, at Whitland in South Wales and Hartfel
l Score in the Southern Uplands of Scotland. A comprehensive graptolit
e range-chart is given for each. The graptolite assemblages from the c
lingani Biozone are subdivided into a lower Ensigraptus caudatus Subzo
ne and upper Dicellograptus morrisi Subzone at both localities, though
the faunas differ in detail. Higher in the sequence, the distinctive
Scottish linearis Biozone fauna is not recognizable at Whitland, its p
resumed equivalent being a fauna dominated by variable morphotypes of
the genus Normalograptus. This suggests that environmental gradients (
depth and/or temperature) were changing more rapidly than the geograph
y. Significantly, the subsequent inception of limestone deposition at
Whitland was approximately contemporaneous with widespread replacement
of black mudstones by oxic, bioturbated sediments elsewhere in the We
lsh Basin and in Scotland. This was possibly a response to an early ph
ase of cooling prior to the end-Ordovician glaciation. In Wales the cl
aimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofac
ies variation.