Faunas. T1u> fossils contained in a rock for-
mation are in general indicative of the animal
and vegetable life that lived during the period
of time in which that rock was deposited. Suc-
cessive faunas and lloras of variable expression
have succeeded each other on the earth and have
left their remains in the rocks that were forming
during the periods of their existence. The large
divisions of time are distinguislied by the pre-
dominant tyi)es of life, as the Paleozoic era by
invertebrates, the ilesozoio era by reptiles, etc.,
and the smaller ])eriods of time, represented by
the .stages and substages, are characterized by
particular genera and species. Thus the Helder-
bergian .series is identiliable by the brachiopod
genus Gypidula, and a division of that series, the
C'oeymans limestone, is recognizable by the ])res-
ence of (li/jiiihilii (/(ilralu a well-marked s])ecies
that serves tor the identification of that hoiizon
in many parts of America. Such a characteristic
genus or species, which can l)e relied upon for
the recognition of geological horizons or zcmcs, is
called an index fossil, and its use is the result
primarily of careful observation in the field.
This empirical usage of index fossils is that
adopted by geologists who have not been trained
in the methods of paleontological research. There
is a second and more im])ortant usage of index
fossils based ujion a knowledge of the phylogeny
of races of animals and plants. By recognition of
the phyletic ])osition of a fossil the ex])ert t:v-
ontologist can determine within close limits the
relative age of the rock from which the fossil was
obtained.
IxnicENK AND Exotic Faunas. In a single
Iiasin it is sometimes found that a series of
similar faunas, evidently evolved each from its
predecessor in this basin, is eventually succeeded
at a higher horizon by a new fauna totally dif-
ferent ironi those below. The lower faunas are
the indig<'ne faunas, developed or evolved in this
province through a long period of time and con-
sisting of members nicely adapted to their en-
vironment and to each other. The new fauna,
generally associated with a change of sedimenta-
tion, is called an alien or exotic fauna, and it
has invaded this region in consequence of phys-
iographic or climatic changes. It may find
itself in congenial surroundings, and its species
will then multiply and evolve, and if left undis-
turlied it will in turn become the indigene fauna
of the region. But if its new environment hap-
pens to become uncongenial it may sufl'er partial
extinction of its members or suppression of de-
velopment of its individuals, and it will remain
in the region for a short time only, to be suc-
ceeded by another alien fauna or by a returning
party of the original indigene fauna. The migra-
tions of faunas are largely due to changes in the
'facies' conse(|uent upon physiographic and cli-
matic changes.
Facies Diovelopment and Bionomy. Facies is
the combination of physical and biological char-
acteristics exhibited by a geological formation
at a particular point. These are determined at
the present time by climate, depth, tides, nature
of medium, distance from shore, etc. (see Dis-
tribution OF Animals; Ecology; Fauna;
Flora), and just as different types of facies are
being developed in the ocean and on the land at
the present time, so they have been developed
during all past periods of geological history. It
follows then that the deposits formed during any
one period may be represented in dillcrent regions
by littoral, sublittoral, abyssal, corallic, estua-
rine, lacustrine, or terrestrial facies, and each of
the.se facies will have its own distinctive fauna.
Study of the life habits of modern organisms en-
ables us to restore the habits of extinct forms of
life, and we are able to portray with a considerable
degree of correctness the conditions under which
the ancient faunas lived, and consequently also
to picture the physiography of past times. The
large majority of fossiliferous rocks are of ma-
rine origin, and hence a study of modern marine
organisms is essential to a proper understanding
of the bionomic conditions of the past. Fresh-
water and terrestrial deposits are also claiming
more attention than they formerly received; they
with their jjcculiar faunas and floras occur chief-
ly in the Jlesozoic and Cenozoic formations.
Bioxoiiv. Marine organisms are broadly di-
vided into ]ielagic, or those that inhabit the open
sea, and littoral, those that live in the vicinity
of the coasts. According to their modes of life
they are divided into plankton, nekton, benthos.
Plankton includes the majority of pelagic
organisms that are more or less passively drifted
about by the waves and currents of the ocean.
Many organisms are ])lanktonic only during their
larval stages and when adult become attached
and adopt a benthonic life (nieroplankton) . Such
are s|)onges, medusa', annelids, eehinoderms.
brachiopods, bryozoans, most mollusks. and ( 'riis-
taeea. Others are planktonie througlidut their
lives, as Foraminifera. Kadiolaria. Siphonophora,
Ctenopliora, Ch;rtognalh:i, Pteropoda. Hetero-
poda, and some crustaceans. Planktonie organisms
are generally widely distributed and their fossil
remains occur principall.y in the sublittoral and
abyssal facies, though they may be found in the
littoral facies.
Nekto.n includes the organisms that are ac-
tive swimmers independent of storms, currents,
and tides. Here are included fishes, most nia-
lacostraean crustaceans, the dibranchiate cephalo-
pods. and also the marine reptiles, including the
extinct ichthyosaurs, plesiosaurs, Pythonomor-
plia, and the cetaceans.
Be.xtiios includes all forms which live at the
bottom, and of which there are recognized two
sub-groups: the sessile and the vagile benthos.
The benthos includes by far the larger proportion
of marine organisms that are found in a fossil
state. Among them are the marine alga-, some
foraminifers, sponges, hydroids, corals, ecliino-
dernis, « onns, brachiopods, bryozoans, gastropods,
lamellibranchs, cirripedes, tunicates, and many
cephalojiods, such as the orthoceratites and
belemnifes. Radial symmetry is most highly de-
veloped in the sessile benthos (corals and eehino-
derms) .
Littoral Facies includes deposits formed on
the beach and in the shallow sea in the vicinity
of the coast. They consist mostly of terrigenous
materials ranging in size from fine mud, through
sand to coarse conglomerate, and some kinds of
organic deposits. The coarse beds contain few
fossils, but those of finer grain abound in them.
Fossils of the littoral facies comprise remains of
the organisms that inhabited the shallow water
and the region between tides: also those of tha
beach, and others that have drifted in to shore
from the open sea. The majority of the fauna
is made up of benthonic forms and all the ani-
mals and many of the plants have strong cal-
Page:The New International Encyclopædia 1st ed. v. 15.djvu/281
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PALEONTOLOGY.
244
PALEONTOLOGY.
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