Research paper New plant taxa from the Lower Devonian (Lochkovian) of the Welsh Borderland, with a hypothesis on the relationship between hilate and trilete spore producers J.L. Morris a, ⁎, D. Edwards a , J.B. Richardson b , L. Axe a , K.L. Davies a a School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff, CF10 3AT, UK b Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK abstract article info Article history: Received 30 March 2011 Received in revised form 14 June 2011 Accepted 27 June 2011 Available online xxxx Keywords: Lower Devonian mesofossils cryptospores trilete spores meiosis early embryophytes A detailed investigation of charcoalified Lochkovian mesofossils, undertaken to circumscribe diversity and disparity in early land vegetation, has revealed a number of discoidal spore masses, enveloped by a featureless cuticular layer. The specimens are interpreted as incompletely preserved terminal sporangia. Based on haptotypic features of the spores, two groups of sporangia are recognised: the first contains alete sculptured monads, the separated products of dyads that are assigned to the dispersed cryptospore genus Cymbohilates Richardson; the second contains laevigate and apiculate trilete monads, the products of tetrads that are assigned to the dispersed genera Ambitisporites Hoffmeister, Streelispora (Chaloner and Streel) Richardson and Lister, and Aneurospora Streel. A new genus, Lenticulatheca, with four new species, L. magna, L. mesodeca, L. variabilis, and L. allenii, has been erected for the cryptospore producers, distinction being based on species and varieties of Cymbohilates. The trilete spore producers, which were far more numerous, have been assigned to a second new genus, Paracooksonia, to emphasise similarities with Cooksonia Lang, both in gross morphology of the spore mass and in situ spore genera. Paracooksonia apiculispora is erected for specimens with in situ apiculate spores and Paracooksonia ambitispora is erected for laevigate in situ spores. Similarities in the bilayered exospore and sculpture, together with those of gross morphology have led to the hypothesis that the two new genera are closely related, the major distinction arising from changes in the timing of cytokinesis during meiosis that resulted in either dyads or tetrads. Information on missing vegetative features is essential to elucidating unequivocally the proximity to the tracheophytes, but similarities with the Cooksonia complex suggest that the two new genera should be considered rhyniophytoids. © 2011 Elsevier B.V. All rights reserved. 1. Introduction One of the most intriguing aspects of evidence relating to the early history of terrestrial plants is the presence of dyads, together with more familiar tetrahedral tetrads, in palynomorph assemblages from Ordovi- cian to Lower Devonian rocks (e.g. Richardson, 1992; Rubinstein et al., 2010; Steemans and Wellman, 2004; Strother and Traverse, 1979; Wellman, 1996). It was the description of ultrastructure in dyads, not tetrads, that contributed the most convincing fossil evidence to the hypothesis that the earliest embryophytes had hepatic affinity (Taylor, 2000), although many workers have emphasised that provided by the permanent tetrads as in certain extant hepatics (e.g. Gray, 1985). Ambiguity on the affinities of dyads and alete monads led one of us to coin the term cryptospores (Richardson, 1985; Richardson et al., 1984). Further discussions on their nature have led to a plethora of definitions for such palynomorphs (e.g. Steemans, 2000; Strother, 1991; Strother and Beck, 2000) that would certainly benefit from knowledge of the parent plants. To this end we have been screening thousands of the earliest of such fossils (mesofossils) available to us, albeit from relatively recent Lochkovian strata, in an attempt to find in situ cryptospores. Success to date has centred on laevigate hilate monads, most assigned to the Laevolancis complex (Wellman et al., 1998a), permanent laevigate dyads (Wellman et al., 1998b), and smooth and sculptured permanent tetrads (Edwards et al., 1999). Here we report on mesofossils containing sculptured hilate monads, the separated products of dyads that are assignable to members of the Cymbohilates complex (Richardson, 1996a). At the outset we should emphasise that the erection of new taxa on a very limited number of specimens is a practice we should not normally wish to follow, but are of the opinion that the importance of these specimens supersedes such reservations in this case. By contrast, a second group of terminal sporangia of similar shape and peripheral ‘tissue’, but bearing trilete spores with similar construction and sculpture to the hilate monads, contains numerous representatives and has allowed analysis of variation in sculpture and features of the proximal surface. 2. Geological setting The fluvial rocks containing the mesofossils were collected from a stream section on the north side of Brown Clee Hill, Shropshire (Edwards, 1996). They occur in the lower and middle parts of the Ditton Formation Review of Palaeobotany and Palynology 167 (2011) 51–81 ⁎ Corresponding author. Tel.: + 44 2920876213. E-mail address: morrisj2@cf.ac.uk (J.L. Morris). 0034-6667/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2011.06.007 Contents lists available at ScienceDirect Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo