|
The Pterosaurs
But the birds were meantime developing from a quite different
stock, and would replace the Pterosaurs at the first change in
the environment. There is ground for thinking that these flying
reptiles were warm-blooded like the birds. Their hollow bones
seem to point to the effective breathing of a warm-blooded
animal, and the great vitality they would need in flying points
toward the same conclusion. Their brain, too, approached that of
the bird, and was much superior to that of the other reptiles.
But they had no warm coats to retain their heat, no clavicle to
give strength to the wing machinery, and, especially in the later
period, they became very weak in the hind limbs (and therefore
weak or slow in starting their flight). The coming selection will
therefore dismiss them from the scene, with the Deinosaurs and
Ammonites, and retain the better organised bird as the lord of
the air.
There remain one or two groups of the Mesozoic reptiles which are
still represented in nature. The turtle-group (Chelonia) makes
its appearance in the Triassic and thrives in the Jurassic. Its
members are extinct and primitive forms of the thick-shelled
reptiles, but true turtles, both of marine and fresh water,
abound before the close of the Mesozoic. The sea-turtles attain
an enormous size. Archelon, one of the primitive types, measured
about twelve feet across the shell. Another was thirteen feet
long and fifteen feet from one outstretched flipper to the other.
In the Chalk period they form more than a third of the reptile
remains in some regions. They are extremely interesting in that
they show, to some extent, the evolution of their characteristic
shell. In some of the larger specimens the ribs have not yet
entirely coalesced.
The Crocodilians also appear in the later Triassic, abound in the
Jurassic, and give way before the later types, the true
Crocodiles, in the Cretaceous. They were marine animals with
naked skin, a head and neck something like that of the
Ichthyosaur, and paddles like those of the Plesiosaur. Their back
limbs, however, were not much changed after their adaptation to
life in the sea, and it is concluded that they visited the land
to lay their eggs. The Teleosaur was a formidable narrow-spouted
reptile, somewhat resembling the crocodiles of the Ganges in the
external form of the jaws. The modern crocodiles, which replaced
this ancient race of sea-crocodiles, have a great advantage over
them in the fact that their nostrils open into the mouth in its
lower depths. They can therefore close their teeth on their prey
under water and breathe through the nose.
Snakes are not found until the close of the Mesozoic, and do not
figure in its characteristic reptile population. We will consider
them later. But there was a large group of reptiles in the later
Mesozoic seas which more or less correspond to the legendary idea
of a sea-serpent. These Dolichosaurs ("long reptiles") appear at
the beginning of the Chalk period, and develop into a group, the
Mososaurians, which must have added considerably to the terrors
of the shore-waters. Their slender scale-covered bodies were
commonly twenty to thirty feet in length. The supreme
representative of the order, the Mososaur, of which about forty
species are known, was sometimes seventy-five feet long. It had
two pairs of paddles--so that the name of sea-serpent is very
imperfectly applicable --and four rows of formidable teeth on the
roof of its mouth. Like the Deinosaurs and Pterosaurs, the order
was doomed to be entirely extinguished after a brief supremacy in
its environment.
|
