Page 82 - Introduction to Paleobiology and The Fossil Record

TAPHONOMY AND THE QUALITY OF THE FOSSIL RECORD 69

(a) (b)

(c)

(d)
Figure 3.8 Different modes of plant preservation. (a) Permineralization, a silicified plant stem from the

Rhynie Chert (Early Devonian, Scotland) (× 50). (b) Coalified compression, leaves of Annularia from

the Late Carboniferous, Wales (× 0.7). (c) Authigenic preservation, a mold of Lepidostrobus from the
Late Carboniferous, Wales (× 0.5). (d) Direct preservation of a microscopic fossilized diatom in the
original silica (scale bar, 20 μm). (a, courtesy of Dianne Edwards; b, c, courtesy of Chris Cleal; d,
courtesy of David Ryves.)

modified by microbial decay, which releases

phosphate ions into the sediment. These may Plant preservation
combine with calcium ions to form apatite, We deal with plant preservation separately
and this can entirely replace dissolved calcare- because some modes are different from those
ous shells. In other cases, the microbial pro- seen for fossil animals. Plant parts are usually
cesses enable soft tissues, and entirely preserved as compression fossils in fi ne-
soft-bodied organisms, to be replaced by grained clastic sediments, such as mudstone,
phosphate. Coprolites, fossil dung, may also siltstone or fine sandstone, although three-

be phosphatized. In these cases, apatite has dimensional preservation may occur in excep-
been liberated from the organisms themselves, tional situations. There are four main modes
and from surrounding concentrations of of plant preservation (Schopf 1975): cellular
organic matter, and the replacement destroys permineralization, coalifi ed compression,
most, or all, of the original skeletal authigenic preservation and hard-part preser-
structures. vation (Fig. 3.8).