Abstract
Seed plants of the lowermost Carboniferous time (Tournaisian) display important differences in morphology and habit with at least two size classes. On one side are plants of modest stature with protostelic stems, manoxylic wood and large leaves, interpreted as calamopityan, buteoxylalean and lyginopteridalean seed ferns, and on the other side are arborescent plants, with trunks up to 2 metres in diameter, of which the systematic position remains unclear. We summarize present information on these trees characterized by a thick development of generally dense wood, a broad eustele consisting of a large number of discrete primary xylem strands, short internodes and deciduous medium-sized fronds. New data have been recently obtained on Pitus, the best known member of this group, on Eristophyton, Bilignea, Stanwoodia, Aporoxylon and on several new taxa which exhibit a broad circular parenchymatous pith surrounded by numerous sympodial xylem strands, but which differ in secondary xylem and leaf trace features. Of particular interest is a new plant that has leaf traces originating as a double strand and a petiole base of the Kalymma-type, therefore showing characteristics of calamopityan seed ferns but being quite distinct in features of the stele, secondary xylem and phloem. Emphasis is placed on the evolutionary dynamics of this important diversification of arborescent plants which included: i) a rather abrupt increase in overall diameter, ii) increase in primary xylem size, with regard to other contemporaneous early seed plants; iii) very different types of wood, instead of a single dense/pycnoxylic wood as generally assumed; iv) distinct periderm types; v) deciduous leaves. The last feature may be interpreted as an innovation, related to the tree habit, within the lignophytes. The origin and systematic position of these arborescent plants remain problematical.