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Paleontology studies the fossilized remains of plants and animals including traces
of their activity. The main method of paleontological investigation is a
comparison of fossils with both living organisms and other fossil forms.
Such a comparative analysis of structure in fossils provides data for
their systematic, evolutionary, ecological and other interpretations.
The principal foundation for all these interpretations is a presumption that
all laws of nature operated in the past just as they do now.
Since the principal subjects of investigation are fossil plants and animals,
paleontology belongs obviously to the biological sciences.
At the same time, paleontology is quite different from any other branch of biology.
The main cause of this difference is a profound natural connection between paleontology
and geology. The more important peculiarities of paleontology are the following:
Paleontological records
In paleontology, the most difficult problem is the incompleteness of the paleontological
chronicle. In other words, the animals and plants preserved as fossils are a small part of
the abundance of life that existed thousands and millions years ago. When studying
modern animals and plants, a researcher can investigate a complete specimen, or even
many specimens of the same species. That is not usually the case in paleontology.
Because soft tissues could fossilize only in very specific conditions those occurred
extremely rarely, so everything we know about extinct animals and plants must be
deduced from their skeletal remains. Unfortunately, a paleontologist usually has to deal
with more or less incomplete fossil specimens. In the paleontology of terrestrial
vertebrates, for example, descriptions of new species are very frequently based on scant
detached skeletal fragments or even on a single bone or a single tooth. So every finding
of a complete fossil skeleton is extremely valuable. Generally, the more complete the
fossil specimen, the more complete is our understanding of the fossil form. Also, complete
specimens are much more spectacular and interesting as museum exhibits.
Stratigraphic distribution
Distribution of modern plants and animals usually refers to geographic locality.
In contrast to modern organisms, geography and time are both equally important in
determining the distribution of fossils. Indeed, time may be even more important.
Certain fossils correspond to specific geological strata that had appeared in certain
time and took up a quite definite position in relation to other geological units.
The order of formation and bedding of rocks is studied by a branch of geology called
stratigraphy. The stratigraphic distribution of fossils is determined by the placement of
geological strata in which they have been found. The method of geological stratification
based on the distribution of fossils is called biostratigraphy.
Geological time
All paleontological events, such as the duration of existence of species and genera are
estimated at many hundred thousands and million years. Hence, paleontology operates
with periods of time that correspond to geological events, like marine regressions and
transgressions, or formation and disappearance of mountains.
That is a reason why geological time is used in both geology and paleontology.
The geological time may be absolute, or relative. The former is astronomical time which can be determined by means of special radioisotope analysis of rocks and fossils.
In contrast to the absolute geological time, relative time is a sequence of conventional
geochronological intervals. Each of these intervals possesses its proper name and is
distinguished by certain paleontological and geological features. Periods of the most
essential biosphere transformations in the history of the Earth are recognized to be
conditional boundaries between geochronological eras. On the whole, the sequence of
geochronological intervals with their limits designated in absolute astronomic time
composes a geological time scale.

Figure: The geological time scale used in the catalogue.

When determining the geographic distribution of fossil organisms, we must remember
that the shape, size, and placement of continents were altering during all their history.
These changes occured very slowly, just as the earth continues to shift now, practically
unnoticeable during a human life. However such slow changes are extremely substantial
in the scale of gelological time. In particular, the arrangement of continents transformed
essentially during the period of the Earth history represented by the current Exposition.
There were three main land masses in the Early Permian. North America composed a unit
with the northern part of Western Europe. The other northern continent consisted of
Eastern Europe and the most part of Asia. South America, Africa, Australia, and
southern parts of Asia composed a single southern continent, colled Gondwana.
Then, from the Late Permian through the Early Triassic, Western Europe was separated
from North America, and the rest of Europe and all of Asia united with the great southern
continent. When the seas retreated to their lowest point in the late Triassic, all the land
plates were joined into a single supercontinent, called Pangaea.
The position of continents in the Late Jurassic was similar generally to that in the Late
Triassic with one exception: an equatorial sea separated North America and South
America. A separation of the northern and southern continental landmasses has
resulted in the appearance of extended equatorial seaway.
In the Early Cretaceous, North America had a contact with South America,
and then the latter separated from the northern continents again.
In the Late Cretaceous North America was connected to Asia.
From the Late Cretaceous through the beginning of the Cenozoic Era the oceans and seas
were at their highest level. They separated all the continents, with the possible exception
of South America and Antarctica. It means, that at the time when dinosaurs became
extinct and mammals began their Cenozoic expansion, the land consisted of several large
isolated continents and numerous islands.

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