A reappraisal of the Dinantian floras at Oxroad Bay, East Lothian, Scotland. 1. Floristics and the development of whole-plant concepts

Abstract

ABSTRACTRe-investigation of species-rich late Tournaisian floras at Oxroad Bay reveals eight plant-bearing exposures that represent four distinct stratigraphic levels. The allochthonous megafossil assemblages vary in degrees of disarticulation, modes of preservation and taxonomic composition. Within-site stratigraphic distributions are presented for 43 anatomically-preserved organ-species; of these, 12 are new to science, 18 have not previously been recorded at the locality, 21 are illustrated from the locality for the first time and 19 are currently considered endemic. Another six have been recorded in error from unprovenanced water-worn blocks. Nineteen adpressed organ-species are also described. Methods and concepts of whole-plant reconstruction are reviewed. At least 11 whole-plant species occur at Oxroad Bay. The lycopsids comprise an enigmatic plant with anAsteroxylon-like stele and two morphologically- and ecologically-distinct species ofOxroadia. A new species ofProtocalamitesdemonstrates the earliest evidence of heterospory and tuberoid stem-bases in the sphenopsid clade.CladoxylonandStauropterisrepresent the pteropsids. At least five lyginopterid pteridosperm species are present. They include both trees (Eristophyton, Bilignea) and shrubs/pseudoherbs (e.g.Tetrastichiaand theTriradioxylon–Calathopteris–Oxroadopteris–‘Tristichia’ plexus), suggesting a range of life strategies. Their diversity has complicated attempts to reconstruct whole-plants, though attachment is demonstrated between several ovules and cupules, and several ovules and pollen-organs are correlated using shared microspores. Elements of particular evolutionary significance include the ‘megacupule’Calathospermumand a structurally similar pollen-organ aggregate, a bisexualPullarithecacupule, and a complete ontogenetic sequence ofHydraspermaovules. Comparable Dinantian floras are reviewed.