Hepatic characters in the earliest land plants

Abstract

EVIDENCE for terrestrial vegetation in Ordovician and Silurian times, before the advent of vascular plants, comes from palyno-morphs (cryptospores¹), that is, non-dissociating tetrads and dyads^2,3 and cuticles⁴. The lack of a megafossil record for the spore producers is usually attributed to low fossilization potential of vegetative tissues, and this, plus spore type⁵, 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 hepatics⁵. Here we describe a minute coal-ified fossil from the Lower Devonian (Lochkovian: micrornatus-newportensis Spore Biozone) of the Welsh Borderland⁷, which contains obligate, smooth-walled 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 epoch³), 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 Eotracheophyta³). Its anatomy in toto finds no exact parallels in embryophytes, but many of the individual cellular features match those in extant hepatics (liverworts).