10.—Triradioxylon—a New Genus of Lower Carboniferous Petrified Stems and Petioles together with a Review of the Classification of Early Pterophytina

Abstract

SynopsisTriradioxylongen. nov. is erected for petrified stems and petioles in which the primary xylem has a triradiate form and possesses protoxylem at the ends of the arms and in the central region. Secondary xylem is dense and composed of small tracheids having multiseriate bordered pits on all walls. Rays are narrow and high. The cortex has sclerotic nests and radial bands of fibres (sparganum structure).In the type speciesT. primaevumsp. nov. the petioles are borne 5–6 cm apart in a phyllotactic spiral of ⅓ on a stem about 1 cm (or more) diameter. The petiole is swollen at its base (about 8 mm diameter) and very gradually tapers to about 2 mm in the rachis which has not been seen to dichotomise. The length of petiole up to the first pinna may exceed 10 cm. Pinnae arose alternately and themselves branched. The T-shaped petiolar bundle has two protoxylem groups at the end of each arm and the central protoxylem divided into three in the rachis.Two detached rachises which cannot be assigned with certainty toTriradioxylon primaevumare placed in the genusLyginorachisand namedL. whitadderensissp. nov. In these the T-shaped vascular bundle is slightly larger than that in the rachises ofT. primaevumand the ends of the transverse arms are wider and may possess three protoxylem groups.The rachises in bothT.primaevumandL. whitadderensisbear alternate triarch pinna-traces but inL. whitadderensisa pair of sub-opposite monarch pinna-traces is also present.Triradioxylonis classified along withButeoxylonin the family Buteoxylonaceae placed tentatively asincertae sedisin the Pteridosperms although showing some affinity withAneurophytonin the Progymnospermopsida.