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The Ichthyosaur
The Ichthyosaur came of the reptile group which we have called
the Diapsids. The Plesiosaur seems to belong to the Synapsid
branch. In the earlier Mesozoic we find partially aquatic
representatives of the line, like the Nothosaur, and in the later
Plesiosaur the adaptation to a marine life is complete. The skin
has lost its scales, and the front limbs are developed into
powerful paddles, sometimes six feet in length. The neck is drawn
out until, in some specimens, it is found to consist of
seventy-six vertebrae: the longest neck in the animal world. It
is now doubted, however, if the neck was very flexible, and, as
the jaws were imperfectly joined, the common picture of the
Plesiosaur darting its snake-like neck in all directions to seize
its prey is probably wrong. It seems to have lived on small food,
and been itself a rich diet to the larger carnivores. We find it
in all the seas of the Mesozoic world, varying in length from six
to forty feet, but it is one of the sluggish and unwieldy forms
that are destined to perish in the coming crisis.
The last, and perhaps the most interesting, of the doomed
monsters of the Mesozoic was the Pterosaur, or "flying reptile."
It is not surprising that in the fierce struggle which is
reflected in the arms and armour of the great reptiles, a branch
of the family escaped into the upper region. We have seen that
there were leaping reptiles with hollow bones, and although the
intermediate forms are missing, there is little doubt that the
Pterosaur developed from one or more of these leaping Deinosaurs.
As it is at first small, when it appears in the early Jurassic
--it is disputed in the late Triassic--it probably came from a
small and agile Deinosaur, hunted by the carnivores, which relied
on its leaping powers for escape. A flapperlike broadening of the
fore limbs would help to lengthen the leap, and we must suppose
that this membrane increased until the animal could sail through
the air, like the flying-fish, and eventually sustain its weight
in the air. The wing is, of course, not a feathery frame, as in
the bird, but a special skin spreading between the fore limb and
the side of the body. In the bat this skin is supported by four
elongated fingers of the hand, but in the Pterosaur the fifth (or
fourth) finger alone--which is enormously elongated and
strengthened--forms its outer frame. It is as if, in flying
experiments, a man were to have a web of silk stretching from his
arm and an extension of his little finger to the side of his
body.
From the small early specimens in the early Jurassic the flying
reptiles grow larger and larger until the time of their
extinction in the stresses of the Chalk upheaval. Small
Pterosaurs continue throughout the period, but from these
bat-like creatures we rise until we come to such dragons as the
American Pteranodon, with a stretch of twenty-two feet between
its extended wings and jaws about four feet long. There were
long-tailed Pterosaurs (Ramphorhyncus), sometimes with a
rudder-like expansion of the end of the tail, and short-tailed
Pterosaurs (Pterodactyl), with compact bodies and keeled breasts,
like the bird. In the earlier part of the period they all have
the heavy jaws and numerous teeth of the reptile, with four or
five well-developed fingers on the front limbs. In the course of
time they lose the teeth--an advantage in the distribution of the
weight of the body while flying--and develop horny beaks. In the
gradual shaping of the breast-bone and head, also, they
illustrate the evolution of the bird-form.
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