Monograph: Sporomorph Assemblages from the 'Lower Old Red Sandstone' of Lorne, Scotland

The basal strata of the' Lower Old Red Sandstone' in the Lorne area of the Grampian Highlands of Scotland, have yielded plant microfossil assemblages comprising sporomorphs (cryptospores and miospores) and plant fragments (tubular structures and cuticles). The sporomorphs are described systematically, and one genus (Abditusdyadus), 19 species (Artemopyra laevigata, A. robusta, Chelinohilates lornensis, C. sinuosus, Cymbohilates amplus, Cy. histricosus, Cy. microgranulatus, Hispanaediscus? irregularis, Qualisaspora kidstonii, Abditusdyadus histosus, Ab. chalazus, Ab. laevigatus, Retusotriletes maccullocki, Dibolisporites ardchoircii, cf. Amicosporites discus, cf. Am. macconochiei, cf. Am. symesii, Aneurospora geikiei and An. hispidica), two varieties (c. sinuosus var. sinuosus and C. sinuosus var. angustus) and one combination Velatitetras (Nodospora) retimembrana are proposed as new. The sporomorph assemblages are all similar and suggest an earliest Lochkovian (earliest Devonian) age. The new biostratigraphical information indicates that the basal strata exposed in the Lorne area on the island of Kerrera and on the mainland at Oban are of similar age. Furthermore, the sporomorph assemblages occur below the Lorne lavas which have been dated radiometrically; combined, these provide a geochronological tie point of value for age determination of the Silurian- Devonian boundary. The plant microfossil assemblages are interpreted as having accumulated in continental, probably lacustrine, environments. This paper describes the first well preserved land plant microfossil assemblages of earliest Devonian age recovered from non-marine deposits and provides an important insight into the nature of terrestrial vegetation at this time. There are a high abundance and diversity of cryptospores, suggesting that cryptospore-producing plants were thriving and co-existed with miosporeproducing plants, which were presumably of vascular status. The presence of rare envelope-enclosed cryptospores is intriguing as similar spores have only rarely been reported from post early Silurian strata. Small scale variation in the distribution of vegetation, possibly associated with differences in environment or palaeogeography, is suggested by the presence of Dibolisporites and the paucity of patinate miospores and Synorisporites spp.