HIZEMODENDRON, GEN-NOV, A PSEUDOHERBACEOUS SEGREGATE OF LEPIDODENDRON (PENNSYLVANIAN) - PHYLOGENETIC CONTEXT FOR EVOLUTIONARY CHANGES IN LYCOPSID GROWTH ARCHITECTURE

The classic coal-swamp lycopsid genus Lepidodendron (Lepidodendrales) is transformed from apparent paraphyly to monophyly by segregating 'L.' serratum as a new genus, Hizemodendron, currently containing only H. serratum. All potential organs of H. serratum have been correlated, with varying degrees of confidence, to yield a conceptual whole-plant. Secondary tissues were probably confined to the rhizomorph and putatively short stem, which generated a repeatedly branched leaf- and cone-bearing crown. This pseudoherbaceous habit resulted in a scrambling, recumbent growth form that allowed the development of dense ground-cover in some Carboniferous coal-swamp forests of Euramerica. Truly prostrate growth (and accompanying bilateral symmetry of axes) was precluded by ontogenetic constraints, notably determinate growth and the centralized rhizomorphic rootstock. Hizemodendron and Lepidodendron are very similar in reproductive characters but differ in several vegetative characters. Hizemodendron serratum is the only recorded non-tree in the apomorphic (monosporangiate-coned) portion of the lepidodendralean clade, implying that it originated from an arboreous, Lepidodendron-like ancestor. If so, Hizemodendron retained the ancestral reproductive organs and bauplan, but decreased in body size and acquired a new, pseudoherbaceous growth habit that prompted other vegetative modifications. The pseudoherbaceous habit is iterative within the arborescent lycopsids, also occurring in Oxroadia and Paurodendron; it probably reflects precocious apical dichotomy. This in turn may be caused by mutation of genes regulating early development, which offers a range of heterochronic mechanisms for repeated saltational macro-evolution within the lycopsids.