Tetrads in sporangia and spore masses from the Upper Silurian and Lower Devonian of the Welsh Borderland

Abstract

Prior to the mid-Silurian, evidence for the earliest embryophytes comes from dispersed spores, particularly permanent tetrads, there being no fossils showing gross morphology or anatomy of the producers. The fragmentary coalified mesofossils described here from the uppermost Silurian (Pridoli) and basal Devonian (Lochkovian) of the Welsh Borderland contain tetrads assigned to Tetrahedraletes, Velatitetras and Cheilotetras. These spores together with examples from spore masses have been examined by scanning and transmission electron microscopy and display diversity in ultrastructure of the exospore and envelope. Tetrads have been found, together with a putative elater, in the forking apex of an axial Lochkovian fossil, named Grisellatheca salopensis gen. et sp. nov., that anatomically, apart from spore characters, reveals no unequivocal evidence for hepatic affinity. The remaining fossils are equally as uninformative as regards affinity. Tetrads with ornamented envelopes are recorded in an isolated discoidal sporangium and in the bases of incomplete sporangia borne terminally on a bifurcating axis. Both ornament and ultrastructure suggest that the spores belong to quite distinct species within Velatitetras. Tetrahedraletes is recorded in an incomplete sporangium subtended by a forking axis, in which no cellular detail is preserved. Naked unfused tetrads also assigned to Tetrahedraletes are recorded in spore masses from both localities and again exospore ultrastructure demonstrates diversity. A final Lochkovian sporangium contains naked tetrads with sporadic Papiculate ornament and shows a unique trilayered exospore. Comparisons of exospore ultrastructure in these tetrads, which it is argued are mature and dispersed as such, provide no unequivocal evidence for affinities, be they tracheophyte or bryophyte. The bifurcating sporophytes are evidence against similarities with extant bryophytes. It is concluded that these very fragmentary fossils derive from small plants comprising relict populations of the vegetation that flourished on land in turfs through the greater part of the Ordovician and early Silurian, but that was gradually replaced by the tracheophytes.