) which disappear, while the smaller land and
water reptiles survive and retreat southward-- where the mammals
are just as numerous. That assuredly is not the effect of an
invasion of carnivores, even if we could overlook the absence of
such carnivores from the record until after the extinction of the
reptiles in most places.
I have entered somewhat fully into this point, partly because of
its great interest, but partly lest it be thought that I am
merely reproducing a tradition of geological literature without
giving due attention to the criticisms of recent writers. The
plain and common interpretation of the Cretaceous
revolution--that a fall in temperature was its chief devastating
agency--is the only one that brings harmony into all the facts.
The one comprehensive enemy of that vast reptile population was
cold. It was fatal to the adult because he had a three-chambered
heart and no warm coat; it was fatal to the Mesozoic vegetation
on which, directly or indirectly, he fed; it was fatal to his
eggs and young because the mother did not brood over the one or
care for the other. It was fatal to the Pterosaurs, even if they
were warm-blooded, because they had no warm coats and did not
(presumably) hatch their eggs; and it was equally fatal to the
viviparous Ichthyosaurs.
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