Devonian floras; a study of the origin of Cormophyta

Abstract

INTRODUCTION It is now clear that in Devonian times, two terrestrial floras, quite distinct as regards affinity, existed, one in the earlier part, and one in the later portion of the Devonian period. The former Avill here be termed the Psilopliyton flora; it consisted largely, as we hope to show, of Thallophyta belonging, for the most part, to a group now quite extinct which we propose to term the Procormophyta or Propteridophyta. The later flora consisted chiefly, but not entirely, of plants which were obviously Pteridophyta. This assemblage we propose to term the Archaeo-2)teris flora. Our knowledge of both these floras, though still far from complete, has been entirely revolutionised during the last few years by the publication, both at home and abroad, of a series of memoirs, to which we shall presently refer more in detail. These invaluable contributions have necessitated a complete revision of the whole subject, and since no general account of these floras, including these recent advances, at present exists, we propose to commence by a brief enumeration of the characters of their more important genera. We have purposely omitted from our review all the more doubtful types about which little or nothing is known beyond the existence of very obscure or fragmentary examples. We have further in Chapter III of this Ijook confined our attention to a critical summary of the essential features of the morphology and anatomy of the genera belonging to the Psilopliyton flora, reserving for a separate chapter (p. 46) the entire discussion of the question of their affinities and systematic position. Finally in yet a further Chapter we discuss the very important bearing of these new discoveries on the phylogeny of Cormophyta and in particular of the various Pteridophytic lines of descent. The researches to which we particularly refer are firstly Kidston and Lang's^memoir on the Scottish plant Rhyniâ Kidston and Lang (1917). Old Bed Facies Upper Old Red (Cheirolepis, Holoptychius and Asterolepis fauna) i| AGE OF FLORAS 5 The horizon of the plant-bearing beds in the Old Red of Scotland can in most cases be determined by the associated fish faunas. Important specimens, including Psilophyton in a petrified condition^, recently discovered in the Dryden Shales at Rhynie, Aberdeenshire, by Dr Mackie^are, however, exceptional in that the precise horizon has not yet been ascertained. These beds are, however, regarded as not younger than Middle Old Red^, but since this genus is known to range throughout Devonian time, the precise horizon of these beds is immaterial from this point of view. Ireland. The plant-bearing sandstones of the Old Red of the South of Ireland are referred to the Upper Old Red (Upper Devonian) on the evidence of the associated fish remains. England. The few plants known from the Devonian of England come from the type beds of the Upper Devonian (Baggy or Cucullaea beds) in North Devon. Belgium. Two plant-bearing horizons occur in Belgium which on stratigraphical grounds are assigned to the Lower and Upper Devoni...

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