Name: Princeton Chert
Location: British Columbia, North America
Age: Middle Eocene (49 mya)
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Taphonomy


Fossil plant and animal remains are permineralised and preserved in 3-D with internal cellular details which do not usually survive in fossil plant assemblages (Moore et al., 1991). This is indeed a rare type of preservation, and allows detailed anatomical descriptions of many species of vascular plants, and also has enabled the reconstruction of whole plants from just many isolated parts (Cevallo-Ferris et al., 1989; Little and Stockey, 2002). Permineralised plant material is thought to be preserved by the influx of mineral-charged liquid into the cell spaces. When the rock material crystallises, it then encases the plant material in a rock matrix (Stockey, 2001).

An unusual feature of the Princeton Chert is the permineralised preservation of monocotyledons (Erwin and Stockey, 1990). Due to their delicacy, they are not usually preserved as plant fossils. Fruits and seeds (including embryos) of Keratosperma (Araceae) and the petioles of Heleophyton (Alismataceae), as well as stems, with leaves and roots attached, from Ethela (a sedge/rush) and Soleredera (Liliaceae) have also been identified and described (Stockey et al., 1998). The most common and best known monocotyledon of the Princeton Chert however, is Uhlia - a small fern pollen which may have grown near the water's edge (Stockey, 2001). Many features are known from these plants, including stems with attached petioles and roots. These plants are good indicators of a sub-tropical climate (Erwin and Stockey, 1998; Cevallo-Ferris et al., 1995).

 
Figure 7. Illustrated reconstruction of Eorhiza arnoldii - the Princeton Chert's most common Eocene plant species (courtesy of RA Stockey and K Pigg)

Several of the Princeton Chert flora were buried where they grew (Stockey, 2001). The most common of these in situ plants is Eorhiza arnoldii - an aquatic dicotyledon (two embryos; broad-leafed plant) - which occurs in growth position (Figure 7; Stockey and Pigg, 1994). Evidence of growth position is how researchers working on the Princeton Chert deciphered whether plant remains were actually aquatic, or just came in via further, external sources. The extensive stems (rhizomes) of Eorhiza - up to 41 cm in length (Stockey and Pigg, 1994) - have been found with attached leaves and roots, allowing vegetative features to be reconstructed. Up to 13 of these flowers are now known and can be linked to specific fruits and seeds, making Eorhiza one of the best represented Eocene plants. The characters of Eorhiza are unique, and thus provide the basis of a new family of flowering plants (Stockey, 2001).

Aquatic plants are generally well preserved in the fossil record. In lakes and/or ponds, plant parts simply fall to the murky bottom, where decomposition is slow or absent, and are then entombed by further plant parts and mineral sediments (Pigg and Stockey, 1996). Plant fossils, especially pollen and spores, can be important elements for understanding the history of terrestrial ecosystems, and the processes which form them. Indeed, a change in plant species composition points to a change in the landscape (Cevallo-Ferris and Stockey, 1988; Stockey and Pigg, 1991).

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