Arkell.1956.Jurassic..

CLASSIFICATION AND CORRELATION J
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On the first two requirements (the most important) they have no serious
rivals. Marine pelecypods have been found in recent years to have much
longer ranges than had sometimes been supposed: single species in Europe
alone often range through four or five stages and many ammonite zones.
This makes them of little value for correlation in spite of the great geographical
extension of some of them, which may exceed that of many
ammonites. It is illuminating (to name an instance personally brought
home to me) to find the limestone excarpment of Jebel Tuwaiq in central
Arabia strewn with some of the same species of pelecypods as may be
picked up in the English Cotswolds—Pholadomya, Homomya, Mactromya,
Ceromyopsis, etc.; but whereas in the Cotswolds the limestones enclosing
them are Bajocian, in the Jebel Tuwaiq they are Upper Bathonian and
Callovian.
Gastropods are sometimes useful locally, where ammonites are rare
or non-existent, but they too have long ranges, some species occurring
through four stages.
Brachiopods are also helpful locally, but over wider areas they are
disqualified because of their colonial habits, heavy dependence on facies
and frequent homoeomorphy.
On the third requirement, independence of facies, most groups of
fossils take low marks and even ammonites are not entirely successful
here. The types of facies in which ammonites are seldom found are coral
reef and current-bedded rocks. This is not the place to enter into the
wide fields of speculation concerning the mode of life of ammonites, but
there is much to be said for the hypothesis of Termier & Termier (1951)
that many bred in loose mud among marine vegetation, which on decaying
could have produced the iron and sulphur that went to make the iron
pyrites in which large numbers of ammonites in clays and shales are so
often preserved.
The clay milieu which was so favourable to many ammonites seems to
have been inimical to others, such as Stephanocerataceae, which are
usually linked with limestone deposits (Arkell, 1952, pp. 83-4). The great
variety of forms and apertural modifications among ammonites strongly
suggests that there were adaptations to almost every kind of niche,
excepting always coral reefs and the distributaries of deltas. In addition
their floating shells were probably carried far and wide after death, like
Nautilus shells at the present day, by winds and currents. Drifted specimens
therefore may come to the rescue of stratigraphers in unlikely places.
As to the fourth requirement of a zonal index fossil, ease of recognition,
ammonites have lately been gaining ground since the introduction of
statistical methods of classification in pelecypods and brachiopods.
Therefore, wherever ammonites are present they take pride of place.
Where they are absent other fossils are brought in for local correlation of
Jurassic rocks just as for the Chalk; but at the best of times such schemes
of classification and correlation are makeshifts and liable to be overthrown
by the discovery of a single ammonite.
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