OFR 151.pdf - CRC LEME

SECTION 2 (THE NATURE OF FOSSIL EVIDENCE)
Fossils are the traces of once-living organisms that have been buried by natural processes and
subsequently preserved in whole or in part.
This definition covers skeletal and cell wall material of any size, whether chemically
unaltered, reduced to mineral carbon, replaced by other minerals such as silica, calcite,
limonite and pyrite or reduced to impressions (casts and moulds), as well as excreted material,
and tracks, trails and borings. Microanalytical and geochemical techniques allow some fossil
organisms to be identified from organic residues (organic trace fossils). Examples include
marine algae such as acritarchs and dinoflagellates, and C4 grasses, which possess distinctive
biochemical ‘signatures’.
As an adjective, ‘fossil’ is used more widely, to cover any entity of perceived geological
antiquity whether or not the object is extant. Examples include fossil ice wedges
(sedimentary casts of ice wedges) and ‘living fossils’ such as Wollemi Pine (Macphail et al.
1995).
2.1 Taphonomy
Taphonomy encompasses the post-mortem history of the organisms. Taphonomic processes
include the decomposition phase after death (necrophysis), the sedimentological history of
fossil remains (biostratinomy), and chemical and physical changes in the fossil between burial
and collection (diagenesis).
2.1.1 Fossil assemblages
With few exceptions, the individual accumulations of fossils (assemblages) represent
geologically instantaneous records of past life. Stratified sequences, which cover much
longer periods of geological time, may provide a time series record of past life.
The value of a fossil assemblage as proxy-climatic evidence depends on the degree to which
the individual fossils can be associated directly to past environments or indirectly via living
relatives whose ecological relationships are known. Preservation and geological age are
important constraints. In most instances, the palaeoclimatic inferences are qualitative, e.g.
warmer and wetter or, if quantitative, are expressed as a range (climatic envelope). Geologic
evidence such as glendonites and the ratios of naturally occurring isotopes can provide more
or less precise quantitative evidence of past climates.
2.1.2 Palaeobotany
Palaeobotany is the study of plant remains. The discipline may or may not be distinguished
from palaeontology, which can encompass animal remains only (colloquial usage) or all
fossils (dictionary definition/industry usage) according to context.
Palaeobotanical evidence is subdivided into two main classes according to the size of the
remains: macrofossils (chiefly stems, foliage, flowers and fruits) and microfossils (chiefly
algal cysts, spores and pollen grains). The two forms of evidence are complementary in that
they usually reflect different elements in the palaeovegetation. It is important to note that
good preservation of plant macrofossils is no guarantee that microfossils of the same plants
will have been preserved or can be identified to the same taxonomic level (and vice versa).
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