Terrestrial Palaeoecology and Global Change

Chapter 2. Taphonomy
9
Fig. 5. Subcrustation, a preservation form commonly
described as impression but actually resulting from
deposition of a mineral film under the cuticle: a leaf
“impression” of Ushia, a nothofagoid angiosperm
from the Palaeocene of Cisuralia, on a coarse-grained
polymictic sandstone.
residual organic matter. Such films are deposited on the surface of plant remains (incrustations)
or subcutaneously (subcrustations: Krassilov & Makulbekov, 1996; Figs. 5, 6).
Migrations of ions from organic-poor coarse-grained to organic-rich fine-grained components
of sand/silt couplets (Awwiller, 1993) is as common as redistribution of lime over
limestone/marl couplets (Einsele, 1982). Binding of ions by organic remains varies with
the latter’s chemical composition. In a decaying leaf, the relief of mineral incrustation is
defined by differentiation of photosyntetic and mechanical tissues over the pattern of
costal/intercostal zones. A similar correlation of anatomical differences and mineralization
is described for soft-bodied animals (Orr et al., 1998).
Organic remains facilitate preservation of other organic remains by serving as foci
for a concentric accretion of silicates, sulfides or carbonates. Terrestrial tetrapod remains
from redbeds are embedded in nodular casts, the elementary composition of
which differs from that of the surrounding rock (Rogers et al., 2001). In marine deposits,
the fully permineralized megaspores, cones and even small flowers are often
preserved in syderitic concretions with an ammonoid shell as the core (examples in
Krassilov & Zakharov, 1975; Nishida, 1985). Even trace fossils, such as burrows, are
preserved three-dimensionally owing to their trapped organic debris that enhance fossilization.
In sediments enriched with buried organic material, fossilization is thus a
self-accelerating process involving structures that otherwise escape preservation. The
origins of the richest fossil plant/animal localities seem related to the phenomenon of
taphonomic enrichment.
Thus, fossils are not so much a gift of chance preservation as a normal product of
interaction between dead mass and taphonomic environment. Freshly buried organic
remains modify their taphonomic environment in such a way as to enhance their own
fossilization. Structural preservation is owing to migration of ions along the chemical
gradients imposed by the buried organic matter that is dehydrated, macerated, impregnated
or molded by mineral substances. With the advance of palaeontological techniques,
rocks apparently barren of fossils are becoming increasingly rare and, with time, may
come to be considered as exceptional.