The lower Devonian flora of the Senni Beds of Monmouthshire and Breconshire

Abstract

New fossil localities, of which the most important is Llanover Quarry, are recorded from the Senni Beds of the Lower Old Red Sandstone of Monmouthshire and Breconshire. All the plants so far known from the Senni Beds occur at Llanover and a number are at present only known in Britain from this locality. The plants distinguished and described, in addition to some remains incertae sedis, are: Drepanophycus spinaeformis, cf. Psilophyton princeps, Dawsonites arcuatus, Gosslingia breconensis, Zosterophyllum cf. australianum, Z. llanoveranum n.sp., Zosterophyllum sp., Cooksonia sp., Sporogonites exuberans, Sciadophyton steinmanni, Taeniocrada sp., Prototaxites sp., Nematothallus sp., Pachytheca sp. The remains of Drepanophycus include shoots bearing sporangia and have H-shaped branching in what was probably the lower region of the plant. The remains of a spiny plant of Psilophyton princeps type have a hitherto undescribed fructification in organic connexion with the vegetative shoots. This fructification is quite unlike Dawsonites, of which typical examples also occur. The numerous specimens of Gosslingia have shown that the sporangia were not borne on special fertile branches, as was originally supposed, but on the margins of the regular dorsiventral branch system. A distinction of two subgenera within the genus Zosterophyllum is suggested. Z. cf. australianum belongs to Eu-zosterophyllum with radial spikes. A new species, Z. llanoveranum, has dorsiventral secund spikes of sporangia of the Zosterophyllum type and is placed in the subgenus Platy-zosterophyllum. A second smaller species belonging to this subgenus is also present. Instructive remains of Cooksonia continue this type of plant from the Downtonian to the Senni Beds. Sporogonites, Sciadophyton and Taeniocrada are recorded for the first time from British rocks. Prototaxites and Nematothallus occur along with the vascular land plants, some of the pieces of the former being of large size. Pachytheca is represented in several exposures. The interesting composition of the flora, which is of non-marine and probably terrestrial habitat, is discussed. The flora is of late Lower Devonian age, probably corresponding to the Siegenian of the continental succession. Comparisons are made with similar floras from Scotland and elsewhere.