The Nature and Affinities of the Enigmatic Silurian–Devonian Genus Pachytheca from Britain

Millimetre-sized coalified spheres named Pachytheca are among the commonest fossils in Upper Silurian and basal Devonian rocks in the Welsh Borderland and, although first recorded in 1839, their affinities remain conjectural. Charcoalified examples in a Lochkovian Lagerstätte provide excellent anatomical evidence for colonial organization comprising a central medulla, full of branching, interweaving filaments surrounding a cortex composed of filaments radiating through a sphere of mucilage and thus confirm generic identity. Extensive scanning electron microscopy demonstrates considerable diversity in organization initially captured by the morphotype concept, extended into development and eventual senescence of the colony, with further speculation on the presence of vegetative reproduction via modification of surface and cortex to facilitate liberation of filaments and asexual reproduction by production of resting spores. Comparisons with other British Pachytheca species, viz. Silurian P. hookeri, and Lower Devonian P. media and P. fasciculata, all silicifications, are hampered by these differences in preservation as well as nomenclatural issues surrounding coalified Silurian P. sphaerica. The latter are resolved by its designation as the type of the genus with its formal diagnosis based on W. H. Lang’s specimens from the Silurian (Downtonian/Pridoli) of the Welsh Borderland complemented by data from the current investigation. Considered the commonest taxon in the Lagerstätte, intraspecific anatomical variation might indicate further subspecific designations, but these would be practically difficult to apply. Four morphotypes are potentially new species or even genera, but evidence is insufficient in terms of number and/or completeness to erect them with confidence. Extensive geological studies on Old Red Sandstone rocks yielded a range of habitats distributed across an extensive river system in a warm, probably seasonal climate. They lead to the conclusion that Pachytheca was part of a wetland community dominated by cryptogamic covers with occasional stands of vascular plants, all subject to periodic drying. Its affinities remain conjectural, but most probably with the green algae – an example, not uncommon in the fossil record, of an organism with no antecedents that proliferates and then disappears, leaving no descendants. The discovery of the almost consistent fungal ensheathing layer leads to the possibility of a unique form of commensalism with Pachytheca the autotroph, a hypothesis that might gain support from future geochemical analyses.