The Princeton chert: Evidence for in situ aquatic plants

The Middle Eocene Princeton chert from southern British Columbia represents one of the richest known assemblages of permineralized Tertiary plants. Affinities with modern aquatic angiosperms, anatomical modifications for the aquatic habit and associated fresh water faunal elements support the interpretation of some components of the Princeton chert as in situ aquatic plants. Among these are fossil plants with affinities to the extent Nymphaeaceae (Allenbya), Araceae (Keratosperma), Alismataceae (Heleophyton), Cyperaceae/Juncaceae (Ethela) and Lythraceae (Decodon). Anatomical modifications include aerenchyma in vegetative tissues (Eorhiza, Dennstaedtiopsis, Heleophyton and Uhlia), the thin-walled tracheary elements without prominent secondary wall thickenings and the presence of protoxylem lacunae surrounded by a ring of cells with thickened inner walls (Heleophyton). Seeds that share morphological features with extant aquatics are characterized by a palisade layer, operculum, external mucilage, small amounts of endosperm and abundant perisperm. Associated faunal elements include turtle bones in the peat matrix and freshwater fish at the top of the section. In situ preservation of these aquatic forms is supported by the presence of rooted axes, the large number of plant organs of the same type and preservation of complete flowers, delicate tissues and organic connections allowing for whole plant reconstruction.