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The first evidence for the emergence of land plants (embryophytes) consists of mid-Ordovician spore tetrads (∼476 Myr old)1,2. The identity of the early plants that produced these spores is unclear; they are sometimes claimed to be liverworts3,4, but there are no associated megafossils, and similar spores can be produced by a diversity of plants2. Indeed, the earliest unequivocal megafossils of land plants consist of early vascular plants and various plants of uncertain affinity1. Different phylogenetic analyses have identified liverworts, hornworts and bryophytes as each being the first lineage of land plants1,2,5,6,7,8,9,10,11,12,13; the consensus of these conflicting topologies yields an unresolved polychotomy at the base of land plants. Here we survey 352 diverse land plants and find that three mitochondrial group II introns are present, with occasional losses, in mosses, hornworts and all major lineages of vascular plants, but are entirely absent from liverworts, green algae and all other eukaryotes. These results indicate that liverworts are the earliest land plants, with the three introns having been acquired in a common ancestor of all other land plants, and have important implications concerning the early stages of plant evolution.

The origin and early evolution of land plants in the mid-Palaeozoic era, between about 480 and 360 million years ago, was an important event in the history of life, with far-reaching consequences for the evolution of terrestrial organisms and global environments. A recent surge of interest, catalysed by palaeobotanical discoveries and advances in the systematics of living plants, provides a revised perspective on the evolution of early land plants and suggests new directions for future research.

The earliest fossil evidence for land plants comes from microscopic dispersed spores. These microfossils are abundant and widely distributed in sediments, and the earliest generally accepted reports are from rocks of mid-Ordovician age (Llanvirn, 475 million years ago). Although distribution, morphology and ultrastructure of the spores indicate that they are derived from terrestrial plants, possibly early relatives of the bryophytes, this interpretation remains controversial as there is little in the way of direct evidence for the parent plants. An additional complicating factor is that there is a significant hiatus between the appearance of the first dispersed spores and fossils of relatively complete land plants (megafossils): spores predate the earliest megafossils (Late Silurian, 425 million year ago) by some 50 million years. Here we report the description of spore-containing plant fragments from Ordovician rocks of Oman. These fossils provide direct evidence for the nature of the spore-producing plants. They confirm that the earliest spores developed in large numbers within sporangia, providing strong evidence that they are the fossilized remains of bona fide land plants. Furthermore, analysis of spore wall ultrastructure supports liverwort affinities.

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 dyads2,3 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 organization3,5,6 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).