Arkell.1956.Jurassic..

598 GENERAL SURVEY
When in the light of these reflections we examine the Jurassic faunas of
the surrounding geosynclinal seas without bias, we find almost (but not
quite) all the evidence equivocal. The presumption of a complex history
is borne out by the diversity of phenomena in the different stages of the
Jurassic. For instance, in the Callovian, Oxfordian and Kimeridgian
there is a marked boreal influence far down the American side: Seymourites
and Cardioceras extend from Alaska down to the Western Interior of the
United States, and Buchia (Aucella) even to Mexico; and on the Asiatic
side Seymourites is said to extend down to Japan. In the Middle Bajocian,
on the other hand, a southern fauna essentially similar to that in Argentina
and Indonesia extends through Canada to Alaska. This is the same
arrangement as is found in Europe and western Asia, where Lower and
Middle Jurassic faunas at home in the Tethys extend to the edge of the
Arctic Ocean, and Upper Jurassic faunas at home in the north extend
to the northern edge of the Tethys (Chapter 28).
In attempting to draw conclusions from comparisons across the Pacific
it has to be borne in mind that Indonesia and the Gulf of Mexico are
antipodal, so that any genera or species common to opposite sides of the
Pacific would have had no farther journey if they migfated round the
opposite hemisphere. Most of the assemblages on both sides are, in fact,
more like Old World faunas of the same age than like their contemporary
assemblages on the opposite side of the Pacific. This fact invalidates
both the instances on which Gregory based his two landlocked, elongated
Pacific seas—the 'North Pacific Liassic sea' and 'Spiti-Chile' Upper
Jurassic sea (1930, p. xci). The Sinemurian ammonites in British Columbia
are identical at specific level with those in NW. Europe; the Liassic
ammonites of Japan are mainly Toarcian and Upper Pliensbachian and
so not comparable with most of those in British Columbia, but in any case
their relationships are closest with those of the Tethys, chiefly in southern
Europe. As to the Tithonian assemblage of the 'Spiti-Chile sea', it is
world-wide. So also are the Hettangian ammonites (not considered in
Gregory's map), which are represented by an essentially similar assemblage,
the Psiloceras fauna, in the South Island, New Zealand, in New Caledonia,
Indonesia and Indo-China, and also in both North and South America.
All the forms are comparable with European ammonites, more or less
closely.
The same difficulty invalidates the arguments for a west-east connexion
across the Pacific from the Himalayas and Indonesia to South America
in the Callovian, stressed by Jaworski and others (see p. 580). This
was based on misidentifications. When the ammonites are examined
critically it is found that all the species common to Asia and South America
also occur in Europe. On the other hand, there are a number of peculiar
South American Macrocephalitids (e.g. Xenocephalites) which have since
been found in North America and even Greenland and Alaska, but not in
Asia, and these demand free migration north-and-south along the east
side of the Pacific.
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