Page 83 - Introduction to Paleobiology and The Fossil Record
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70 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Plant fossils preserved by cellular permin- algae, with calcareous skeletons, and diatoms,
eralization, or petrifaction, may show superb with their silicifi ed cell walls.
microscopic detail of the tissues (Fig. 3.8a),
but the organic material has gone. The plant
material was invaded throughout by minerals QUALITY OF THE FOSSIL RECORD
in solution such as silicates, carbonates and Incompleteness of the record
iron compounds that precipitated to fi ll all From the earliest days of their subject, pale-
spaces and replaced some tissues. Examples ontologists have been concerned about the
of cellular permineralization are seen in the incompleteness of the fossil record. Charles
Devonian Rhynie Chert and the Triassic wood Darwin famously wrote about the “imperfec-
of the Petrified Forest, Arizona. The most tion of the geological record” in his On the
studied examples of permineralized plant Origin of Species in 1859; he clearly under-
tissues are from coal balls. Coal balls are stood that there are numerous biological and
irregular masses, often ball-shaped, of con- geological reasons why every organism cannot
centrated organic plant debris in a carbonate be preserved, nor even a small sample of every
mass, that are commonly found in Carbonif- species. In a classic paper in 1972, David
erous rocks in association with seams of bitu- Raup explained all the factors that make the
minous coal. Huge collections of coal balls fossil record incomplete; these can be thought
have been made in North America and Europe, of as a series of filters that stand between
and cross-sections of the tissues can reveal an organism and its final preservation as a
astounding detail. fossil:
The second common kind of plant preser-
vation is coalifi ed compression, produced
when masses of plant material lose their 1 Anatomic fi lters: organisms are likely to
soluble components and are compressed by be preserved only if they have hard parts,
accumulated sediments. The non-volatile a skeleton of some kind. Entirely soft-
residues form a black coaly material, made bodied organisms, such as worms and jel-
from broken leaves, stems and roots, and with lyfish, are only preserved in rare cases.
rarer flowers, fruits, seeds, cones, spores and 2 Biological fi lters: behavior and population
pollen grains. Coalified compressions may be size matter. Common organisms such as
found within commercially workable coal rats are more likely to be fossilized than
beds, or as isolated coalifi ed fi lms impressed rare ones such as pandas. Rats also live
on siltstones and fi ne sandstones (Fig. for a shorter time than pandas, so more
3.8b). of them die, and more can become poten-
The third mode of plant preservation, tial fossils.
authigenic preservation or cementation, 3 Ecological fi lters: where an organism lives
involves casting and molding. Iron or carbon- matters. Animals that live in shallow seas,
ate minerals become cemented around the or plants that live around lakes and rivers,
plant part and the internal structure com- are more likely to be buried under sedi-
monly degrades. The cemented minerals ment than, for example, fl ying animals or
produce a faithful cast of the external and creatures that live away from water.
internal faces of the plant specimen, and the 4 Sedimentary fi lters: some environments
intervening space may be filled with further are typically sites of deposition, and organ-
minerals, producing a perfect replica, or mold, isms are more likely to be buried there. So,
of the original stem or fruit. Some of the best a mountainside or a beach is a site of
examples of authigenic preservation of plants erosion, and nothing generally survives
are ironstone concretions, such as those from from these sites in the rock record, whereas
Mazon Creek in Illinois and from the South a shallow lagoon or a lake is more typi-
Wales coalfi elds (Fig. 3.8c). cally a site of deposition.
The fourth typical mode of plant preserva- 5 Preservation fi lters: once the organism is
tion is the direct preservation of hard parts. buried in sediment, the chemical condi-
Some microscopic plants in particular have tions must be right for the hard parts to
mineralized tissues in life that survived survive. If acidic waters run through the
unchanged as fossils. Examples are coralline sediment grains, all trace of fl esh and
