Arkell.1956.Jurassic..

CHAPTER 28
MARINE REALMS AND CLIMATE
FAUNAL REALMS AND PROVINCES
The existence of marked faunal realms or provinces in the oceans and
seas at the present day led long ago to a quest for evidence of similar
phenomena in the Mesozoic. The present differentiation of marine
faunas is a complex resultant of climatic and geographic factors. Where
circumstances are favourable, southern or tropical faunas may grade
imperceptibly into temperate or northern faunas, but in other circumstances
the transition from one extreme to another may be astonishingly
abrupt. For instance, the Indian Ocean fauna of corals and large, brightly
coloured echinoids and molluscs, including the giant clam, occupies the
whole Red Sea up to the head of the Gulf of Suez, whereas on the opposite
side of the 100-mile Suez isthmus the Mediterranean fauna holds sway.
Very little mixture has taken place even during almost a hundred years
since the canal has been cut. Without knowing the geographical lay-out,
no geologist finding these two faunas in strata a hundred miles apart
would dream of correlating them. Moreover, if the needful small subsidence
of the isthmus allowed one fauna to spread over the territory of
the other and oust it, the two faunas would be found in sequence in the
territory of the one that had been vanquished, and there would be nothing
in the territory of the victor fauna that could be correlated with the
vanquished fauna, which would therefore inevitably be considered older.
Such considerations as these seem to be overlooked by those palaeontologists
who insist on the universality of all ammonite assemblages
and on unlimited incompleteness of the geological record and still hope
to find that, for instance, the Upper Volgian faunas of Russia mark a
time interval distinct from the Tithonian and Purbeckian.
The marine realms and provinces of the Jurassic and Lower Cretaceous
were exhaustively discussed by Uhlig (1911) in one of the most inspiring
and masterly contributions ever made to Mesozoic stratigraphy. In this
work, completed a few days before his death and embodying a lifetime's
experience of stratigraphy and fossil faunas, Uhlig revised previous
schemes put forward by Neumayr, Nikitin, Haug and others, and set out
his results on a map of the world. He recognized four realms, which he
subdivided into provinces, and a separate province (Japan) not attached
to any realm. They were as follows :—
1. Boreal realm, with its subdivision the 'North-Andine' province.
The latter was unhappily named, since it comprised western North
America with the whole of Alaska and reached no farther south than the
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