Reconstructing the Lower Devonian (Lochkovian) vegetation from the Anglo-Welsh Basin: Two spore masses containing Emphanisporites McGregor spores

Abstract

In situ spores have gone some way towards harmonising the prominent disparity between the Early Devonian dispersed spore and megafossil records, greatly advancing but often challenging our understanding of early vegetation. Here, we investigate an elongate and a discoidal spore mass, yielding Emphanisporites epicautus Richardson and Lister and Emphanisporites sp. respectively from the early (not earliest) Lochkovian (Lower micrornatus-newportensis spore assemblage biozone) of the Ross-Tewkesbury Spur (M50) motorway section in the Anglo-Welsh Basin, UK. We explore their morphology and spore wall ultrastructure using SEM and TEM. A paucity of useful phylogenetic characters precludes formal identification or description of the parent plants but a relationship to the rhyniophytes is hypothesised. A dearth of vascular tissues, however, necessitates their placement amongst the rhyniophytoids. Both the sporangial morphology and spore wall ultrastructure differs between the specimens, distancing them from each other and from other Emphanisporites species. While similarities exist, no unequivocal relationships with contemporaneous or extant taxa, or indeed lineages, can be made using sporangial morphology or spore wall architecture. These differences lend further support to deliberations that the ‘emphanoid’ condition was a consequence of convergent evolution. Using the dispersed spore record we explore the paleoecology of the plants, which points towards them being minor components of the vegetation, restricted to areas away from river catchment. This interpretation is redolent of the middle Lochkovian cf. Horneophyton sp. (E. cf. micrornatus parent plant) from North Brown Clee Hill, but that plant may have been restricted to a more specialised niche. What characterised the niches of these plants is uncertain, but they may have been ephemerally water stressed, perhaps hinting at a moisture sensing function for the ‘emphanoid’ spore structure.