HEPATIC CHARACTERS IN THE EARLIEST LAND PLANTS

EVIDENCE for terrestrial vegetation in Ordovician and Silurian times, before the advent of vascular plants, comes from palynomorphs (cryptospores(1)), that is, non-dissociating tetrads and dyads(2,3) and cuticles(4). The lack of a megafossil record for the spore producers is usually attributed to low fossilization potential of vegetative tissues, and this, plus spore type(5), contributes to the hypothesis that the plants were embryophytes/archegoniates at a bryophyte level of organization(3,5,6) and perhaps most similar in organization to modern hepatics(5). Here we describe a minute coalified fossil from the Lower Devonian (Lochkovian: micrornatus-newportensis Spore Biozone) of the Welsh Borderland(7), which contains obligate, smooth-waded tetrahedral tetrads, similar (by scanning electron microscopy) to those first recorded in the Ordovician. To our knowledge, these are the first data on the gross morphology and tissues of the plants that comprised the earliest embryophyte land flora (Gray's Eoembryophytic epoch(3)), albeit obtained from a relict Devonian example fossilized at a time when the composition of dispersed spore and megafossil assemblages suggests that tracheophytes and tracheophyte-like plants (rhyniophytoids) had generally begun to dominate land vegetation (Gray's Eotracheophyta(3)). Its anatomy in toto finds no exact parallels in embryophytes, but many of the individual cellular features match those in extant hepatics (liverworts).