Arkell.1956.Jurassic..

CHAPTER 27
THE OCEANS
All the occurrences of Jurassic formations chronicled in the preceding
chapters amount to little more than relics of marginal lappings of the sea
around the edges of the continents; the sole exception being the Tethys,
a sea which stretched across southern Asia from west to east along the
band that was destined to emplace the great Tertiary mountain chains.
Despite modern advances in geophysical techniques, we are not in
sight of being able to distinguish and date sedimentary systems hidden
beneath the ocean floors. Accordingly the only way that light can be
thrown at present on the major distribution of land and sea and the extent
of the oceans at any particular period is by extrapolation from study of the
fossil faunas in the fringing sediments deposited at times of transgression.
Those faunas when properly correlated and dated give some idea of the
contemporaneity or otherwise of transgressions in different parts of the
world, and tell us something of the migration routes that were open across
the seas, and so indirectly of the existence and extent of the seas themselves.
Most of the immense literature on the old controversy about the
permanence of the oceans consists of arguments drawn from the distribution
of land and freshwater faunas, recent and fossil. Nearly all of it
(as summarized, for instance, by J. W. Gregory in his two presidential
addresses to the Geological Society of London, 1929 and 1930) is highly
uncritical, based on the assumption that if animals or fossils on different
continents are sufficiently similar they require land bridges to provide
migration routes across the oceans. Too little account was taken of the
immense time factor involved, and the rare chance possibilities of crossings
by adult or larval individuals under the influence of currents or wind
storms, which, when suitably multiplied by such a time factor, become
probabilities. This has been shown ably and convincingly by Simpson
(1952). As he states, 'a possible event, however improbable it may be as
an isolated occurrence, becomes probable if enough time elapses'; and
the degree of probability is increased in proportion to the size of the
population involved as well as to the time available, for both increase
the opportunities.
In contrast to most of the preceding literature, our study is of migrations
of marine organisms. They have the advantage that whereas all land
animals can conceivably cross the sea and populate distant continents,
no means have been suggested whereby marine animals such as ammonites
can cross continents. Even here a caveat must be entered, however.
At the present time both freshwater and marine molluscs and lowlier
organisms can cross wide tracts of land in a very short time, carried on the
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