Abstract

Dr H. C. Squirrell welcomed the detailed account of the thelodonts, a group long-neglected by British geologists despite their value in both local and worldwide correlation. He asked the author if the sudden appearance of Turinia pagei could be of use in establishing the Downtonian-Dittonian junction in the Welsh Borders, the precise position of which has yet to be agreed. The speaker questioned the statement that the fauna of the Ludlow Bone Bed is essentially no different from the earlier Silurian assemblages. Fig. 6 shows that ten species of thelodonts have been obtained from the Ludlow Bone Bed, but only five of these are found in the Ludlow and Wenlock rocks below. There appears to be a significant difference between the faunas found below and above the base of the bone bed. He also questioned the validity of moving the Silurian-Devonian boundary up to the base of the ‘Psammosteus’ Limestone Group solely on the evidence of the thelodont faunas. As there is a faunal change at the base of the Ludlow Bone Bed, it would seem more logical to retain the boundary there, thus keeping the mainly fish-bearing rocks of the Down-tonian within the Devonian.

The Author replied: The position of the incoming of the Turinia assemblage agrees well with the boundary chosen by the Geological Survey to denote the Downton-Ditton boundary in the Brown Glee region (Greig et al. 1968). This, as most boundaries in this succession, including the Ludlow bone bed, is a facies boundary.

The fauna of

First Page Preview

First page PDF preview
You do not currently have access to this article.