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

Prior to the mid-Silurian, evidence for the earliest embryophytes comes fro m dispersed spores, particularly permanent tetrads, there being no fossils showing gross morphology or anatomy of the producers. The fragmentary coali fied mesofossils described here from the uppermost Silurian (Pridoli) and b asal Devonian (Lochkovian) of the Welsh Borderland contain tetrads assigned to Tetrahedraletes, Velatitetras and Cheilotetras. These spores together w ith examples from spore masses have been examined by scanning and transmiss ion electron microscopy and display diversity in ultrastructure of the exos pore and envelope. Tetrads have been found, together with a putative elater , in the forking apex of an axial Lochkovian fossil, named Grisellatheca sa lopensis 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 ornam ent and ultrastructure suggest that the spores belong to quite distinct spe cies within Velatitetras. Tetrahedraletes is recorded in an incomplete spor angium subtended by a forking axis, in which no cellular detail is preserve d. Naked unfused tetrads also assigned to Tetrahedraletes are recorded in s pore masses from both localities and again exospore ultrastructure demonstr ates diversity. A final Lochkovian sporangium contains naked tetrads with s poradic ?apiculate ornament and shows a unique trilayered exospore. Compari sons of exospore ultrastructure in these tetrads, which it is argued are ma ture and dispersed as such, provide no unequivocal evidence for affinities, be they tracheophyte or bryophyte. The bifurcating sporophytes are evidenc e against similarities with extant bryophytes. It is concluded that these v ery fragmentary fossils derive from small plants comprising relict populati ons 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. (C) 1999 The Linnean Society of London.