Note on the Occurrence of Dayia navicula (J. de C. Sowerby) in the Lower Ludlow Rocks of Shropshire

Abstract

In the type area the Ludlow Rocks generally have been divided into Lower Ludlow Shales, Aymestry Limestone, Dayia Shales and Whitcliffe Flags in upward sequence. The Dayia Shales are characterized by the presence of enormous numbers of Dayia navicula (J. de C. Sowerby). This preponderance of D. navicula in the shales immediately above the Aymestry Limestone has caused a tendency to regard beds in other localities containing this fossil as being on the same stratigraphical horizon in spite of the character of the accompanying fauna. In two recent papers on the Ludlow Rocks of the Welsh Borderland (Straw, 1937, and Earp, 1938) it has been shown that D. navicula ranges through at least 3,000 feet of strata, occurring commonly throughout this great thickness and outlasting more than one change of fauna. Although, in this area, the brachiopod ranges from the zone of Monograptus nilssoni into the Upper Ludlow it has not hitherto been recorded below the Aymestry Limestone in Shropshire. This gap in our knowledge is now filled by the discovery of specimens in Lower Ludlow Shales exposed in a small quarry 40 yards north-east of Stokewood Cottage, which is on the west side of the railway line a little over a mile south of Craven Arms. The quarry shows about 15 feet of nodular shales with thin limestone seams. The commonest fossils are Chonetes laevigata (J. de C. Sowerby), C. minima (J. de C. Sowerby), and Stropheodonta filosa (J. de C. Sowerby) which occur in large numbers on some of the bedding surfaces. Other fossils are Stropheodonta euglypha (Dalman), Delthyris sp., Orthoceras sp., Dalmanites sp., and a plectambonitid. Dayia navicula seems to be confined to a thin layer on the north side of the quarry. Graptolites referable to Monograptus cf. chimaera occur fairly commonly. About 400 yards in a south-easterly direction another small quarry exposes Conchidium Limestone which is about 170 feet stratigraphically above the beds in the first quarry.