Arkell.1956.Jurassic..

6io
GENERAL SURVEY
It is with the beginning of the Callovian that peculiar endemic Boreal
faunas first appear, brought south over the surrounding lands by transgression
spreading from the Arctic Ocean (fig. 99). The first Boreal
elements are specialized Stephanocerataceae, Cranocephalites and Arctocephalites,
soon followed by other peculiar Cadoceratinae and Kosmoceratidae
such as Seymourites. While these were taking possession of
northern Alaska, northern Canada, Greenland and Siberia, and sending
out pioneers perhaps as far south as Japan, there were also synchronous
transgressions taking place in the temperate and tropical world which
caused a great equatorial expansion of the true Macrocephalitidae.
These last are not of Arctic origin as sometimes supposed, for they are
not found in the extreme north except perhaps in Arctic North America,
whereas they abound throughout the temperate and tropical world and
are obviously at home in the Tethys from western Europe to Indonesia
and Madagascar, and round the Pacific. They were gradually replaced
by the Reineckeidae, which have a similar distribution and dominate the
Middle Callovian in regions as far apart as Argentina, Mexico, south
Germany and Cutch, but reached as far north as southern Alaska and
Yorkshire only as rarities.
The Callovian, Lower Oxfordian and Lower Kimeridgian were
characterized by great expansion, enrichment and spread of Boreal genera
such as Kosmoceras, Cadoceras and its derivatives (Quenstedtoceras,
Cardioceras, Amoeboceras), Rasenia and Aulacostephanus. These genera
spread from the north across European Russia and Britain, but stopped
short south of the Caspian and Caucasus and only reached south of the
Alps as occasional stragglers—though Cardioceras went in some strength
down the lower Rhone valley and a single species even reached Portugal.
There was at this time considerable mingling of the Boreal and Pacific
realms, for Kosmoceratids arrived in Peru; and the pelecypod family
Buchiidae was equally abundant in the Boreal and Pacific Oceans, abounding
in California and New Zealand and in Europe reaching south to Dorset
and the Boulonnais in the Kimeridgian (Dutertre, 1926).
This fundamental fact in Jurassic history, the northward spread of
temperate and equatorial faunas in the Lower Jurassic and Bajocian, and
the southward spread of northern faunas in the Callovian and early Upper
Jurassic, has been rightly emphasized by Termier & Termier in their
brilliant review of historical geology (1952). They designate the first
phase as the Tethyan transgression, the second as the Arctic transgression,
and they point out that during the Oxfordian and Kimeridgian coral
reefs were driven southward across Europe by the progressively cooling
waters.
In order to keep distinct two separate phenomena, marine transgression
and faunal migration, the process under discussion, namely the southward
migration and colonization of boreal faunas, will here be referred to as
the Boreal spread. The Boreal spread was not a continuous process. It
is true that by the Kimeridgian coral reefs had retreated south to central
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