Arkell.1956.Jurassic..

614 GENERAL SURVEY
Subneumayria invaded New Zealand, and the same forms of Idoceras
reached Indonesia and Japan. However, as has been remarked above,
the Pacific realm was already differentiated in the Middle Bajocian,
and so this spread of the Idoceras fauna in the western Pacific was only a
reclamation of territory.
The great southern bay off the central Tethys, which extended across the
Arabian sea and down the east side of Africa to Madagascar, developed
several peculiar faunas which warrant the recognition of an Ethiopian
province, though in a wider sense than used by Uhlig and including Cutch,
Baluchistan and Arabia (p. 336). That Uhlig was nevertheless right to
classify this region as a province rather than a realm has been shown by
the discovery of isolated stragglers of nearly all these endemic Ethiopian
genera in other parts of the Tethys as distant as Portugal (Bouleiceras),
Algeria (Ermoceras) and Indonesia (Mayaitidae).
In the eastern Tethys, from Spiti to Indonesia and New Guinea, another
special fauna appeared at the very end of the Jurassic, but it is not yet
certain which of its elements are Tithonian and which Lower Cretaceous.
The collection of these faunas on rigid stratigraphical principles is one
of the most important tasks still waiting to be done.
Summarizing, it appears that during the Jurassic there were only three
faunal realms, which may be most simply named the Tethyan, Pacific
and Boreal. None of the three was differentiated at the time of the Lias,
when one fauna was universal. By progressive differentiation and spread
of the Pacific and Boreal realms during the Middle and Upper Jurassic
the remainder became the Tethyan realm. During Upper Jurassic times
the three realms changed their frontiers considerably in certain regions
at the expense of each other's territory.
The Pacific realm began to be differentiated perhaps in the Toarcian,
but certainly in the Middle Bajocian, at which time there began a general
retreat of ammonites from the periarctic regions. This retreat culminated
in the Middle Bathonian. In the Upper Oxfordian Mexico, Cuba,
California and Indonesia belonged to the Tethyan realm, but in the Lower
Kimeridgian the Pacific realm asserted itself anew and incorporated
Mexico, Japan, Indonesia and New Zealand.
The first indications of the Boreal realm appear in the Lower Callovian,
and during the rest of the Callovian and Lower Oxfordian it spread south
over Europe to the borders of the present Mediterranean and Caspian
seas. It was thrown back during the Upper Oxfordian by a temporary
re-advance of the Tethyan realm, which took equatorial Perisphinctids
and coral reefs to the north of England and Scotland but not into the
Arctic regions. In the Lower Kimeridgian the Boreal spread was renewed
and all the old territory was regained.
Finally, from the Middle Kimeridgian onwards the Boreal realm
shrank and its marginal areas were subdivided by regional uplifts into
the Portlandian and Volgian provinces. Meanwhile the Tethyan
realm continued to exist and locally to extend, as shown by Tithonian
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