Arkell.1956.Jurassic..

6l2 GENERAL SURVEY
Europe, but in the Upper Oxfordian they extended in force northwards
to Yorkshire where none had been before, and perhaps even to northern
Scotland. * This was long after the first and greatest southward spread of
Boreal ammonites, which had reached the fringe of the present Mediterranean
in the Callovian and Lower Oxfordian. The Upper Oxfordian
in fact represents a temporary reversal of the Boreal spread and a return
of Tethyan faunas ; for the abundant large Perisphinctidae of the English
Upper Oxfordian are a Tethyan race which does not occur in the Arctic;
some are identical at specific level with those in equatorial East Africa
and also closely related to forms that lived on and even south of the
equator in Indonesia and northern Chile. In Europe they are common
in the Mediterranean area, south of the southern limit of Cardioceratidae
(fig. 100).
In the Lower Kimeridgian the Boreal spread was renewed, but in the
Middle Kimeridgian it stopped. Thereafter in northern Europe and
Russia old stocks became extinct, no fresh ones appeared, and specialization
of the single surviving stock, the Perisphinctaceae, set in. In England,
the Boulonnais and Arctic America (Greenland and Canada) these gave
rise to the Dorsoplanitinae which culminated in the giant Portlandian
Perisphinctids, the largest Jurassic ammonites. There is still difference
of opinion whether these are to be correlated with the similar giants of
the Blakei Zone of the Moscow basin, and until that fauna has been
monographed the last word cannot be said. In any case, during the
Lower Portlandian there was still free communication across Pomerania
and Poland between England and Russia, but in the Moscow basin a
highly specialized and peculiar form of Perisphinctids, the Virgatitinae, had
already reached its acme in Virgatites (pi. 45, figs. 1, 2). With the end of
the Portlandian (assuming that to coincide with the end of the Lower
Volgian) ammonites became extinct in NW. Europe, Greenland and
North America, while another specialized, smooth, degenerate stock,
the Craspeditidae, was evolved in the Moscow basin and adjoining
Arctic regions (pi. 46). These presumably lived at the time of the
Purbeckian. Their descendants (Subcraspedites fauna) returned to colonize
Yorkshire and Greenland with the early Cretaceous transgression.
During this closing phase of regression and specialization in the north,
the Tethyan fauna continued to thrive and evolve actively over all the rest
of the world. From the Beckeri Zone of the Middle Kimeridgian onwards
there were produced many and varied experiments in a number of
ammonite stocks. The Perisphinctids evolved on separate and productive
lines, producing the world-wide Berriasellidae with their numerous
offshoots such as the Himalayitinae and Neocomitinae; the Simoceratids
and Aspidoceratids also produced further novelties; and the Oppeliaceae
abounded in vast numbers and infinite variety, augmented by fresh
developments from the Haploceratids. There is no basis for comparison
* If the reef-building corals in the Lower Kimeridgian boulder beds (Arkell, 1933,
p. 476) are derived from erosion of Oxfordian beds on the upthrown side of the fault.
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