New dyad-producing plants from the Lower Devonian (Lochkovian) of the Welsh Borderland

Abstract

As part of an investigation to determine the diversity and disparity of early terrestrial vegetation, hundreds of charcoalified mesofossils have been isolated from Lower Devonian strata, and two new, relatively rare groups of permanent dyad-producers have been identified. The first group comprises fig-shaped sporangia, with distinctive apical dehiscence. These contain palynomorphs, interpreted as permanent dyads, which are not known from coeval spore assemblages. Sporangia are attached to branching stomatiferous axes. They are assigned to Ficoiditheca aenigma. The second group consists of a few poorly preserved elongate sporangia that contain palynomorphs belonging to the dispersed spore genus Chelinohilates, with a prominent murornate sculpture. Semi-thin sections through the in situ examples indicate that they are permanent dyads with a common, sculptured outer wall. The valvate construction of the sporangial wall and the fundamental structure of the palynomorphs suggest a possible affinity with recently described Partitatheca. Both groups exhibit a combination of tracheophytic and bryophytic characteristics and hence represent early embryophytes quite distinct from tracheophytes. In particular they are united as permanent dyad-producers, a group that includes in situ laevigate examples known from the dispersed spore record as far back as the Ordovician, where their ultrastructure suggests hepatic affinity. © 2012 The Linnean Society of London.