Origin and Nature of the Phytoclast Group

Abstract

The majority of dispersed fossilizing phytoclasts are derived from the ligno-cellulosic tissues of terrestrial macrophytes. Most probably represent fragments of strongly lignified mechanical support and vascular tissues of the secondary xylem (‘wood’) of arborescent gymnosperms and angiosperms (plus the analogous tissues in tree ferns and extinct vascular plants that exhibited secondary growth). The preservation of this material is enhanced by the highly stable, resistant and hydrophobic nature of lignin. Lignin decay is mostly due to ‘mouldering’ by slow-growing lignolytic white-rot fungi (mainly Basidiomycotina) under oxic subaerial conditions (Teichmüller, 1982a, p. 228; Deacon, 1984, p. 152; Rayner and Boddy, 1988, pp. 123, 239, 242; Robinson, 1990; sections 22.1.2, 22.2). These fungi probably derive little energy from the lignin itself, but rather utilize the chemically bonded cellulose associated with it (Deacon, 1984, p. 152). This may be why the action of white-rot fungi does not necessarily result in extensive depolymerization of lignin, but often only the loss of some low molecular weight ‘degradation fragments’ (Given, 1988, p. 19).