Arkell.1956.Jurassic..

THE OCEANS 601
Of these peculiar endemic elements only the Mayaitids have been
found in Indonesia, which was in any case known to have been in communication
with the Himalayas and Baluchistan by way of the Tethys.
If there had been an alternative direct route for migration across the Indian
Ocean a much greater community of faunas might have been expected.
Conversely, the Westralian gulf in the Bajocian was populated by a
prolific assemblage of Stephanoceratids, of which the most characteristic
were the genera already mentioned, Pseudotoites and Zemistephanus.
Although these migrated to Canada, Alaska and Argentina, and Pseudotoites
at least is found in the Moluccas, they are known nowhere else in the Old
World. The Bajocian of Western Australia is not of the right zone to
contain Ermoceras (early Upper Bajocian) but there are in the Old World,
including East Africa, Arabia and Persia, plenty of occurrences of Middle
Bajocian faunas in which Pseudotoites and Zemistephanus and the peculiar
Australian Fontannesiae might be expected if they could have crossed the
Indian Ocean as they could the Pacific.
In this connexion it is worth recalling that when the specialized
Indonesian Triassic pelecypod genus Indopecten was found in Persia
and Oman it was argued that, being such a large and conspicuous form,
it could hardly have escaped detection in the Himalayan Trias if that had
been its route of migration, and that a direct route across the Indian Ocean
was therefore more likely; this involving the conclusion that Lemuria
had already broken down by the Upper Triassic (Douglas, 1929, p. 631).
Within a few years of publication of this conclusion, however, Indopecten
was announced from the Attock district in the western Himalayas (Cox,
1935. P- 3)-
SOUTH ATLANTIC OCEAN
SQuth of the points where the Atlas Mountains in Morocco and the
Caribbean Coast Range of Venezuela and North Range of Trinidad strike
so spectacularly out to sea, no Jurassic rocks are known on the Atlantic
side of either Africa or South America. This has often been taken to
imply that in Jurassic and earlier times the South Atlantic did not exist,
an explanation which may be correct. Nevertheless, it is not necessarily
correct, and all interested in the question should read the symposium
on the subject published by the American Museum of Natural History
(Mayr, editor, 1952). It should be borne in mind that the ring of Jurassic
rocks encircling the Pacific, which produces so strong a contrast with the
South Atlantic, is on the site of a vast system of geosynclines, which has
no parallel on the margins of any other ocean; and that but for the orogenic
buckling and uplift of the geosynclines, which elevated marginal Pacific
sediments high into the air, the existence of these sediments might not
be known.
According to Kober (1928) an orogenic belt, and by implication a preceding
geosyncline, runs down the middle of the Atlantic parallel to the
coasts and is the cause of the Mid-Atlantic ridge, now almost completely
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