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The Stegosaur
The Stegosaur is one of the most singular and most familiar
representatives of the group in the Jurassic. It ran to a length
of thirty feet, and had a row of bony plates, from two to three
feet in height, standing up vertically along the ridge of its
back, while its tail was armed with formidable spikes. The
Scleidosaur, an earlier and smaller (twelve-foot) specimen, also
had spines and bony plates to protect it. The Polacanthus and
Ankylosaur developed a most effective armour-plating over the
rear. As we regard their powerful armour, we seem to see the
fierce-toothed Theropods springing from the rear upon the
poor-mouthed vegetarians. The carnivores selected the
vegetarians, and fitted them to survive. Before the end of the
Mesozoic, in fact, the Ornithopods became aggressive as well as
armoured. The Triceratops had not only an enormous skull with a
great ridged collar round the neck, but a sharp beak, a stout
horn on the nose, and two large and sharp horns on the top of the
head. We will see something later of the development of horns.
The skulls of members of the Ceratops family sometimes measured
eight feet from the snout to the ridge of the collar. They were,
however, sluggish and stupid monsters, with smaller brains even
than the Sauropods.
Such, in broad outline, was the singular and powerful family of
the Mesozoic Deinosaurs. Further geological research in all parts
of the world will, no doubt, increase our knowledge of them,
until we can fully understand them as a great family throwing out
special branches to meet the different conditions of the crowded
Jurassic age. Even now they afford a most interesting page in the
story of evolution, and their total disappearance from the face
of the earth in the next geological period will not be
unintelligible. We turn from them to the remaining orders of the
Jurassic reptiles.
In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are
the typical representatives of that extinct race. The two
animals, however, belong to very different branches of the
reptile world, and are by no means the most formidable of the
Mesozoic reptiles. Many orders of the land reptiles sent a branch
into the waters in an age which, we saw, was predominantly one of
water-surface. The Ichthyosauria ("fish-reptiles") and
Thalattosauria ("sea-reptiles") invaded the waters at their first
expansion in the later Triassic. The latter groups soon became
extinct, but the former continued for some millions of years, and
became remarkably adapted to marine life, like the whale at a
later period.
The Ichthyosaur of the Jurassic is a remarkably fish-like animal.
Its long tapering frame--sometimes forty feet in length, but
generally less than half that length--ends in a dip of the
vertebral column and an expansion of the flesh into a strong
tail-fin. The terminal bones of the limbs depart more and more
from the quadruped type, until at last they are merely rows of
circular bony plates embedded in the broad paddle into which the
limb has been converted. The head is drawn out, sometimes to a
length of five feet, and the long narrow jaws are set with two
formidable rows of teeth; one specimen has about two hundred
teeth. In some genera the teeth degenerate in the course of time,
but this merely indicates a change of diet. One fossilised
Ichthyosaur of the weaker-toothed variety has been found with the
remains of two hundred Belemnites in its stomach. It is a flash
of light on the fierce struggle and carnage which some recent
writers have vainly striven to attenuate. The eyes, again, which
may in the larger animals be fifteen inches in diameter, are
protected by a circle of radiating bony plates. In fine, the
discovery of young developed skeletons inside the adult frames
has taught us that the Ichthgosaur had become viviparous, like
the mammal. Cutting its last connection with the land, on which
it originated it ceased to lay eggs, and developed the young
within its body.
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