Abstract

The age of some Lower Old Red Sandstone sediments (Arbuthnott Group) is reassessed and confirmed as Lower Devonian (lower Gedinnian) on the basis of a reinvestigation of spore assemblages. The data upon which this age assessment is based are provided by recent palynological researches in the Welsh Borderland, the type area of the Gedinnian (Belgium), and the graptolite-bearing parastratotype for the Silurian-Devonian boundary (Podolia). Samples from the Arbuthnott Group at Wormit (Fife), associated with rocks dated as 407± 6 Ma by Thirlwall (1983) and regarded by him as Silurian, have yielded miospore assemblages of early but not earliest Devonian when compared with the Podolian Silurian-Devonian sequence. Samples throughout the sequence of the Arbuthnott Group as exposed in Angus and Fife have yielded miospores. These assemblages, occurring at levels above and below the Wormit samples, are all of lower Gedinnian age. The whole of the spore-bearing succession of the Arbuthnott Group, which is about 1,800 m thick, belongs to the micrornatus-newportensis Zone (lower and middle subzones) and is equivalent to part of the lower Ditton Group of Brown Clee Hill (Shropshire) of 20 to 30 m in thickness. There is no critical palaeontological evidence for age in the strata associated with the Arbuthnott Group, neither in the underlying Dunnottar and Crawton Groups nor in the overlying Garvock Group so the exact position of the Silurian-Devonian and Gedinnian-Siegenian boundaries is uncertain. A new spore Anteturma, namely Cryptosporites, is proposed. Qualisaspora fragilis gen. et sp. nov. is described herein and appears to have potential as an index fossil for the lower Dittonian of the British Isles. Some regional differences also occur as ?Dibolisporites sp. A and ?Samarisporites sp. A occur in beds dated as lower Gedinnian in Sotland whereas spores of similar sculpture and structure are present higher in the Anglo-Welsh sequence in beds dated as lower Siegenian.