Geoscience Reference
In-Depth Information
Biozones underpin the stratigraphic division of Phanero-
zoic time. Commencing 570 Ma ago in the Cambrian
period, this aeon is marked by far greater abundance
of organisms with hard parts. Many subdivisions are
characterized by single taxon biozones, such as the
graptolites (forms of plankton) during the Lower
Palaeozoic era and better-known ammonites (squid-like
molluscs) of the Mesozoic era.
BIOSTRATIGRAPHY
Fossils and biozones
Biostratigraphy and lithostratigraphy share common
principles but the biosphere adds unique features to
the environmental record and reconstruction of the
palaeoecology of organisms, communities, habitats and
environmental interactions. Recognizable fossils lie at the
heart of biostratigraphy, providing potential evidence
of habitat, evolution over geological time scales and
opportunities for isotopic dating. Fossils are the preserved
remains of body parts ( body fossils ) or evidence of former
biological activity or bioturbation such as root channels,
animal tracks and burrows ( trace fossils ). Durable, hard
body parts (external or exoskeletons and internal or
endoskeletons ) are more common as fossils than rarely
preserved soft tissues. The fossil form survives best after
replacement mineralization or contrasting sedimenta-
tion - the substitution of original organic material by
inorganic minerals or infilling of trace fossil spaces
respectively. Often, only a mould of inner or outer surfaces
of the organism survives after removal of original or
replacement body parts. Biogenic material may also
appear as formless organic residues, such as amorphous
as opposed to fibrous peat, and as chemical precipitates
such as carbonate-rich chalk, tufa, travertine and marl or
silicate-rich chert and flint. These make fossil taxonomy
(classification into genus and species) difficult or impos-
sible but their presence and geochemistry are still useful
indicators of former populated (as opposed to sterile)
regions, marine or terrestrial conditions, wet or dry and
hot or cold climates.
Lone fossils are less useful than fossil assemblages which
preserve recognizable communities of organisms. Some
organisms are gregarious by nature and tolerant of a wide
range of environmental conditions whereas other are
extremely fussy about where they live! The latter may
provide more habitat-specific evidence but the former,
widely distributed occurrence combined with survival
over a short-lived span of geologic time, provide the best
indicator or guide fossils. Just as lithostratigraphic facies
represent particular packets of generically related rocks,
biozone and assemblage zones identify packets of rock in
which an individual taxon or assemblage of taxa occurs.
Both embrace the life span of a particular species or taxon,
or a distinct community of many taxa sharing a common
habitat, with stratigraphic boundaries marked by the
appearance of new species. This also carries important
chronostratigraphic implications, since community
succession and species evolution take place over time.
Taphonomy and preservation context
The means by which organisms become preserved as
fossils and the accuracy with which they represent
particular environments - their taphonomy - are subject
to major biases. How much of your garden or our national
flora and fauna, for example, might survive natural
decomposition and burial after ten years or 10 M years
respectively? Organisms which die naturally usually
experience some mechanical break-up and early decom-
position around the time of death. Total disarticulation
may occur during sediment transport before final burial.
Species identification often depends on limited remains.
Sometimes the manner of death in short-lived, high-
energy events may be determined from such death
assemblages. Facies densely packed with coral and shell
fragments, clearly not in their life positions, signify major
storms or tsunamis. Cowering human shapes preserved
in volcanic ash at Pompeii record pyroclastic flows from
the AD 79 eruption of Vesuvius and abraded conifer trunks
embedded in some Alaskan moraines were bulldozed by
rapidly advancing glaciers.
Chemical decomposition removes soft body parts first
and longer-chain hydrocarbons later. Deciduous leaves,
identifiable in leaf litter by species during their first
autumn, are quickly reduced to amorphous humus and
incorporated into topsoil. Main branches and trunks
containing lignin may survive beyond a decade in
temperate latitudes but the process is greatly accelerated
in the humid tropics and slowed down in arctic condi-
tions. Decomposition also decelerates under anaerobic
wetland conditions, and aerobic conditions of extreme
drought, but accelerates in most aerobic environments
through alternate wetting and drying. Wet substrates
with low pH values (high acidity) preferentially dissolve
carbonate material such as bone and shells whilst
preserving soft tissue. This was instrumental in the
survival of prehistoric 'Lindow man' in a Cheshire peat
bog, whose skin, hair, stomach (and contents) were
remarkably well preserved but whose endoskeleton was
not. Outer hard parts, such as chitin exoskeletons in
beetles and other insects, the exine of pollen grains and
 
 
Search WWH ::




Custom Search