Age of the basal ‘Lower Old Red Sandstone’ Stonehaven Group of Scotland: The oldest reported air-breathing land animal is Silurian (late Wenlock) in age

Abstract

The basal upper Silurian-Lower Devonian ‘Lower Old Red Sandstone’ deposits of the Midland Valley of Scotland contain several important fossil biotas, including that from the Cowie Harbour Fish Bed. This biota is of great significance because it represents one of the oldest known examples of a fossilised terrestrial ecosystem and includes the oldest reported air-breathing land animal (the myriapod Pneumodesmus newmani Wilson & Anderson 2004). Based on biostratigraphical evidence from dispersed spores this biota is dated as late Wenlock (late Silurian) in age. Recently, however, this age constraint was challenged, based on 238 U- 296 Pb radiometric analysis of zircons, and it was proposed that these deposits are much younger (Early Devonian: Lochkovian). This proposal has serious implications regarding: (i) the composition and nature of early terrestrial ecosystems; (ii) the geological setting with respect to the timing of terrane accretion and the onset of ‘Lower Old Red Sandstone’ sedimentation. We report on newly discovered dispersed spore assemblages and additional zircon data that confirm a late Wenlock (late Silurian) age. This age designation establishes the importance of the biota of the Cowie Harbour Fish Bed and is more compatible with both its biotic composition and the regional geological setting.