Silicified monocotyledons from the Middle Eocene Princeton chert (Allenby Formation) of British Columbia, Canada

Abstract

The Middle Eocene Princeton chert flora contains the most diverse collection of silicified monocotyledons recovered from the early Tertiary of western North America. Palm vegetative organs are the most common elements and are represented by five stems up to 9 cm wide with attached petiole bases and roots, plus numerous additional isolated petioles, midribs and laminae. Comparisons with extant palms reveal that fossil stem and leaf anatomy is most similar to two coryphoid genera, Rhapidophyllum and Brahea. However, the fossils differ from these genera in several characters and probably represent a new coryphoid taxon. Two small herbaceous monocot stems, one with attached roots, the other with attached roots and remnants of sheathing leaves, each show a pith surrounded by a vascular cylinder composed of several cycles of bundles. The two stems differ in bundle morphology, vessel characters and root anatomy, thus allowing their recognition as distinct types. In addition, a small petiole, Heleophyton helobieoides Erwin and Stockey, shows aerenchymatous ground tissue with numerous minute bundles arranged in five series, a pattern similar to petioles of the extant genus Echinodorus (Alismataceae), but differs in possessing bundle structure like that found in Butomus (Butomaceae). Vascular bundles display a protoxylem lacuna surrounded by a ring of cells with thickened inner tangential walls, 1–3 thin-walled metaxylem elements and a relatively large problem strand. Presence of these structural features suggests Heleophyton grew either submerged or was emergent. Heleophyton also provides further support for the interpretation that the Princeton chert locality represents an ancient lacustrine ecosystem, a site ideal for the study of early Tertiary monocotyledons.