70 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Plant fossils preserved by cellular permin-
eralization, or petrifaction, may show superb
microscopic detail of the tissues (Fig. 3.8a),
but the organic material has gone. The plant
material was invaded throughout by minerals
in solution such as silicates, carbonates and
iron compounds that precipitated to fi ll all
spaces and replaced some tissues. Examples
of cellular permineralization are seen in the
Devonian Rhynie Chert and the Triassic wood
of the Petrifi ed Forest, Arizona. The most
studied examples of permineralized plant
tissues are from coal balls. Coal balls are
irregular masses, often ball-shaped, of con-
centrated organic plant debris in a carbonate
mass, that are commonly found in Carbonif-
erous rocks in association with seams of bitu-
minous coal. Huge collections of coal balls
have been made in North America and Europe,
and cross-sections of the tissues can reveal
astounding detail.
The second common kind of plant preser-
vation is coalifi ed compression, produced
when masses of plant material lose their
soluble components and are compressed by
accumulated sediments. The non-volatile
residues form a black coaly material, made
from broken leaves, stems and roots, and with
rarer fl owers, fruits, seeds, cones, spores and
pollen grains. Coalifi ed compressions may be
found within commercially workable coal
beds, or as isolated coalifi ed fi lms impressed
on siltstones and fi ne sandstones (Fig.
3.8b).
The third mode of plant preservation,
authigenic preservation or cementation,
involves casting and molding. Iron or carbon-
ate minerals become cemented around the
plant part and the internal structure com-
monly degrades. The cemented minerals
produce a faithful cast of the external and
internal faces of the plant specimen, and the
intervening space may be fi lled with further
minerals, producing a perfect replica, or mold,
of the original stem or fruit. Some of the best
examples of authigenic preservation of plants
are ironstone concretions, such as those from
Mazon Creek in Illinois and from the South
Wales coalfi elds (Fig. 3.8c).
The fourth typical mode of plant preserva-
tion is the direct preservation of hard parts.
Some microscopic plants in particular have
mineralized tissues in life that survived
unchanged as fossils. Examples are coralline
algae, with calcareous skeletons, and diatoms,
with their silicifi ed cell walls.
QUALITY OF THE FOSSIL RECORD
Incompleteness of the record
From the earliest days of their subject, pale-
ontologists have been concerned about the
incompleteness of the fossil record. Charles
Darwin famously wrote about the “imperfec-
tion of the geological record” in his On the
Origin of Species in 1859; he clearly under-
stood that there are numerous biological and
geological reasons why every organism cannot
be preserved, nor even a small sample of every
species. In a classic paper in 1972, David
Raup explained all the factors that make the
fossil record incomplete; these can be thought
of as a series of fi lters that stand between
an organism and its fi nal preservation as a
fossil:
1 Anatomic fi lters: organisms are likely to
be preserved only if they have hard parts,
a skeleton of some kind. Entirely soft-
bodied organisms, such as worms and jel-
lyfi sh, are only preserved in rare cases.
2 Biological fi lters: behavior and population
size matter. Common organisms such as
rats are more likely to be fossilized than
rare ones such as pandas. Rats also live
for a shorter time than pandas, so more
of them die, and more can become poten-
tial fossils.
3 Ecological fi lters: where an organism lives
matters. Animals that live in shallow seas,
or plants that live around lakes and rivers,
are more likely to be buried under sedi-
ment than, for example, fl ying animals or
creatures that live away from water.
4 Sedimentary fi lters: some environments
are typically sites of deposition, and organ-
isms are more likely to be buried there. So,
a mountainside or a beach is a site of
erosion, and nothing generally survives
from these sites in the rock record, whereas
a shallow lagoon or a lake is more typi-
cally a site of deposition.
5 Preservation fi lters: once the organism is
buried in sediment, the chemical condi-
tions must be right for the hard parts to
survive. If acidic waters run through the
sediment grains, all trace of fl esh and