The late Silurian – Early Devonian adaptive radiation of vascular land plants: palynological evidence from the Anglo-Welsh Basin, U.K.

The classic Anglo-Welsh Basin sequence yields a rich assemblage of plant macrofossils, mesofossils and spores. The terrestrial basin straddles the Silurian – Devonian boundary and is well placed to investigate vegetation change during a transformative episode of plant evolution. Between the late Silurian and Early Devonian, tracheophytes (vascular plants) (± represented by trilete spores) were beginning to radiate and dominate variously less derived, ancestral embryophytes (cryptospores), having wide ranging ramifications for biogeochemical cycles, geomorphology and the atmosphere. Here, the palynological record of Ludlow to mid Lochkovian (late Silurian – Early Devonian) aged rocks from the Welsh borderlands and south Wales, encompassing the uppermost Palaeozoic Marine Welsh Basin and the Lower ‘Old Red Sandstone’ of the Anglo-Welsh Basin is examined. Temporal changes in the abundance of spore species and ornament types is documented, and species ranges are assessed. 183 species in 42 genera of trilete spores and cryptospores are catalogued. A gradual proliferation of trilete spore and cryptospore species through the sequence is observed, and a major radiation is seen for both between the earliest and early Lochkovian. Morphological diversity (disparity) shows a similar pattern and is driven by several key trilete spore and cryptospore genera. The spatial diversity and disparity of coeval assemblages are assessed, and ‘pockets’ of specialised vegetation seem to occur by the early Lochkovian. The drivers of this radiation are uncertain, but it may have been facilitated by a shift in climate and change of environment, with facies change also playing a role in the final observed patterns. The radiation of spore species allows the preliminary construction of spore assemblage biozones around, and defining, the Silurian – Devonian boundary in the Anglo-Welsh Basin. The source plants of several dispersed spores are investigated, giving insights into their morphology and affinities, and glimpses of cryptogamic communities and plant-animal interaction are also recorded.