Silurian, Downtonian and Dittonian strata from the Welsh Borderland contain a succession of thelodonts (Agnatha). Isolated scales have been identified and a new species, Apalolepis toombsi, is described. Four scale assemblages are recognized: Turinia pagei in Dittonian; Goniporus/Katoporus in U. Red Downton Group; Acanthodians (thelodonts absent) in Holdgate Sandstones Group and Thelodus parvidens in the Silurian—Lower Red Downton Group.
The Thelodus parvidens assemblage occurs above and below the Ludlow Bone Bed, which, on the basis of the thelodonts, does not correspond to a major break in the succession. The incoming of the Turinia pagei assemblage at the beginning of the Dittonian marks a principal faunal change, and this can be correlated with the N. German Beyrichienkalk erratics, and the Polish-Lithuanian and Spitsbergen sequences.
Comparison of the Welsh Borderland thelodont succession with that of Scotland, Europe, Russia and Canada suggests the existence of distinct faunal provinces, influenced by palaeogeography.