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Summary Sedimentology and Stratigraphy Chapter 20
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Samenvatting Sedimentology and Stratigraphy Gary Nichols Chapter 20: Biostratigraphy
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Chapter 20: Biostratigraphy“The occurrence of fossils in beds of sedimentary rocks provided the basis for correlation of strata and
the concept of a stratigraphic column when the science of geology was still young. The fundamental
importance of biostratigraphy has not diminished through time, but has merely been complemented
by other stratigraphic techniques discussed in preceding and following chapters. The evolution of
organisms through time and the formation of new species provided the basis for the recognition of
periods in the history of the Earth on the basis of the fossils that are contained within strata. In this
way Earth history can be divided up into major units that are now known to represent hundreds of
millions of years, some of which are familiarly known as ‘the age of fish’, ‘the age of reptiles’ and so
on, because of the types of fossils found. Fossils also provide high-resolution stratigraphic tools that
allow recognition of time slices of only tens to hundreds of thousands of years that are important for
building up a detailed picture of events through time. Correlation between biostratigraphic units and
the geological time scale therefore provides the temporal framework for the analysis of successions of
sedimentary rocks.”
20.1 Fossils and stratigraphy
Fossils are important indicators of processes and environments of deposition. The study of fossils has
also provided fundamental information about the evolution of life on Earth. Some look familiar to
modern species, and other are totally different. Cataloguing the fossils found in sedimentary rocks
provided the first clues about the passage of geological time. The rocks that contained a particular
fossil type were often the same lithology, but, crucially for the development of stratigraphy,
sometimes the same fossil type was found in a different rock type.
There were patterns in de distribution of fossils. Certain types of organism were found to be
dominant groups of strata. This led to the erection of the scheme of systems that were initially
grouped into deposits formed in 3 eras: ‘ancient life’ -> Paleozoic, ‘middle life’ -> Mesozoic, ‘recent
life’ -> Cenozoic. Those periods were pure speculation, until radiometric techniques developed. The
occurrence of some fossils in specific stratigraphic units was simply an observation at this stage: an
explanation for the distribution of the fossils in the stratigraphic record came once ideas of the
evolution of life were developed.
20.2 Classification of organisms
20.2.1 Species
Species: groups of interbreeding organisms that are reproductively isolated from other such groups.
The concept of species is fundamental to the classification of organisms. Today we can also provide
information about the genetic characteristics of organisms. Similarities or differences in genetic
make-up make it possible to rigorously define species and determine the relationship between them.
Genetic provided a basis for a hierarchical system of classification.
Such a classification system existed long before Linnaean system:
Kingdom
Phylum General term for the ranks is a taxon
Class (plural taxa). Fundamental taxon rank is
Order
the species. Species together for Genus,
Family Linnaean
genus together form a family enz.
Genus hierarchy
Species
, This can be used for fossils but does not work well, due to the degraded DNA material in fossils. They
use morphology differences for fossils. The soft parts are only in extraordinary circumstances
preserved, which leaves the hard parts. The hard-part morphology is not very reliable because there
are many similar-looking organisms that are genetically distinct and at the same time some species
show considerable variations in form. So, there is always an element of doubt.
To define species, besides information about the associated fauna and the paleoenvironmental
habitat, the following are used.
Holotype: a single representative specimen against which other potential representatives of
the species can be compared.
Morphometrics: statistics about the shape and size of the organism.
20.2.2 Other ranks in taxonomy
Subspecies and races are distinct sets which show common characteristics that set them apart, but
which can still be considered as part of the same species.
Variations due to geographical separation of the sets Note: the first letter of the genus name is
leading to the development of different characteristics. capitalized, while the species is always in
lower case, and italics are used in printed
Genus (plural genera) is a group of species that are
text
closely related, and when an organism is named it is
given a genus as well as a species. (Homo sapiens is
name for human species). Only by biostratigraphy the species is identified, otherwise its identified to
the genus.
Major Phyla can be compared with modern representatives of these subsets if the main kingdoms.
Some classes, many orders and a large number of families, have no modern equivalents. As the
similarities to modern organisms become fewer, the problems of classification become greater as the
significance of morphological differences is less well understood. Linnaean hierarchy is therefore in a
constant state of flux as new fossil discoveries are made that shed light on the probable relationships
between fossil organisms.
20.3 Evolutionary trends
Emerge and diversification of organisms, as signs of decline and extinction. After a particular type of
organism has developed a new feature to become more ‘advanced’, later changes do not result in a
return to the more ‘primitive’ form. The concept of evolutionary trends therefore provides us with a
way of interpreting the fossil content of rocks in terms of biological changes through time. Provides a
means of correlating rocks and determining their relative ages by the fossils that they contain.
20.3.1 Population fragmentation and phyletic transformation
Many modern species of plants and animals show regional variation in their form. Geographical
isolation can cause characteristically changes making them distinctly different. Results in mutations
that are advantageous to the isolated population, better adapted to their habitat.
Speciation: development of isolated group with sufficiently different characteristics from the rest of
the population for it to be considered a new species. This population fragmentation process results
in an increase number of species. Which is generally balanced by the extinction. This happens when a
species population is no longer well adapted to changing environmental conditions and/or
competition from other organisms leads to terminal decline in numbers.
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