THE AGE OF REPTILES
(_Continued._)
The Triassic period in its later stages was very like the earlier
period of the era which followed it, and the reptiles which were
characteristic of the close of the first were continued in some cases
with only slight differentiation in the second. The Plesiosaurs and
Ichthyosaurs are associated with the Trias, and we may therefore
describe them now. Though some of these large aquatic creatures must
have measured thirty feet from snout to tail, they do not equal in
size the great aquatic mammals of to-day--the whales. In life the
Plesiosaur had a body like the hull of a submarine with four great
paddles attached--the fore and the hind legs. It had a long neck like
a gigantic swan, and an elongated head provided with powerful jaws
armed with numerous pointed teeth. It probably could swim under water
as well as on the surface, and when floating could snap small lizards
from the land. The paddles have a definite structure like legs, with
five toes, wrist or ankle, forearm or foreleg, and upper arm or
thigh. A great number of these Plesiosaurs have been found in the
Lias formation of the south of England; and slabs containing whole
skeletons have frequently been obtained. They and two similarly
embedded and flattened skeletons of different kinds of Ichthyosaurs
may be seen in quantity on the wall of the gallery of fossil reptiles
in the Natural History Museum at South Kensington.
The Ichthyosaurs were much more fish-like or whale-like in form than
the Plesiosaurs. "They were, indeed," says Sir E. Ray Lankester,
"singularly like the porpoises and grampuses among living whales
and stand in the same relation to land-living reptiles that the
porpoises do to land-living mammals. Their fish-like appearance and
fins are not primitive characters and do not indicate any closer
blood relationship to fishes than that possessed by other reptiles.
They are the offspring of four-legged terrestrial reptiles which
have become specially modified and adapted to submarine life." Like
many whales, they had a fin on the back devoid of bony support.
The Ichthyosaur had a ring of bony plates supporting the eyeball
(as birds also have), and these are often preserved in the fossil
specimens.
At the end of the Triassic period some strata were laid down which
have been called "Beds of passage." We have seen that the Triassic
strata were probably deposited, altogether or in part, in extensive
salt lakes or inland seas. At the close of the Triassic period the
waters of the ocean were admitted to these areas by the sinking of
the land at some point or other of their margins. With the sea-water
came many living things--fishes, shells, etc.--and the very scanty
life of the Triassic lake was replaced by an abundance of marine
life. These beds were called the Rhætic beds because they were first
found in the old Roman province of Rhætia, which occupied an Alpine
district between Bavaria and Lombardy. Here they were thickest,
3000 feet of limestones and shales; but they have since been found
either thicker or thinner everywhere in England, and in the United
States, as well as in other parts of Europe wherever we can find the
Lias lying on the Trias. They are especially interesting, because
they contain the teeth of the earliest known traces of the highest
division of the animal kingdom--the mammals. These early mammals
belonged to the lowest of all the mammalian tribes--the Marsupials,
or pouched animals, now so common in Australia. The little banded
ant-eater of South America, which lives upon insects and is about
the size of a rat, is probably something like the first mammal, the
_Microlestes_, in habit and appearance.
Let us now return, however, to the reptiles of the Jurassic period.
It is so called from the Jura Mountains which occupy the north-west
of Switzerland, separating that country from France. They are
composed of a thick series of clays, shales, and limestones, to
which, in 1829, the name Jurassic was given by the French geologist
Brogniart. It was soon found, however, that the lower rocks of this
period were very different from the upper. The lower rocks were very
shaly and clayey with thinnish layers of limestone. These were called
Lias. The name Lias is derived from "layers"--pronounced broadly by
the Somerset quarrymen as "lyers"--a very suitable name for the lower
beds of the Lias especially, since the alternation of thin beds of
limestone and of shale gives to the rock a banded or ribbon-like
appearance, which may well cause the workmen to describe it as
occurring in "lyers."
To the upper Jurassic beds, which contained much more limestone and
also occasional beds of sandstone, the name of Oolite was given. The
Oolitic strata have a special interest for English geologists, for
it was in them that William Smith, the west of England surveyor,
first made out (about the year 1790) the _order of succession_ of
the strata, and by this was led to his great discovery that "strata
could be identified by their organic remains," that is by their
fossils. He noticed that some of the limestone beds of the strata
we are about to describe consisted of small rounded grains, which
made them resemble the roe of a fish--indeed, they were called
"roestone" by the workmen. Hence Smith--when seeking a name for
this set of strata--bethought himself of the term "Oolite," which
means "egg-stone" (Gr. _oon_, an egg, and _lithos_, a stone). Where
the grains are very large the limestone is called "pea-grit" or
_pisolite_ (Lat. _pisum_, a pea). Some beds which contain numerous
and irregularly shaped fragments of shells, corals, etc., are called
_rag-stones_.
[Illustration: Plesiosaurs
Different species ranged from ten to forty feet in length.]
The Jurassic strata of Great Britain were sediments laid down in
warm seas surrounding an archipelago of which Dartmoor, Wales, and
Cumberland formed some of the islands. The whole of Western Europe
was sinking and had sunk; and the waters of the open ocean were
admitted into and mingled with the salt mineral waters of the great
Triassic lakes. The change was at first very like what would happen
at the present day if the coast of Palestine were depressed, so
that the waters of the Mediterranean flowed into the Dead Sea. The
few fish of the salt lakes were killed; and as the land continued
to sink, the sea at last flowed all over Central and West England,
bringing with it an abundance of marine life. But the reptiles were
far from being finished with; and the progress of the small mammals
was extremely slow.
First, as to the reptiles. The whale-shaped Ichthyosaurs continued to
develop in the seas, and grew larger and larger till some of which
we have found traces reached a length of forty feet. The long-necked
Plesiosaurs also advanced from strength to strength, and some types
grew larger. But by this time new breeds were developing, with
shorter necks and larger heads (and consequently larger brain-power),
which had a better chance of surviving in the struggle for existence
than the unwieldy and slow-witted reptiles which preceded them.
The Ichthyosaurs became more and more fish-like, and some of them
developed the habit of breeding at sea instead of having to return
to the land to deposit their eggs, as do the sea-going turtles and
crocodiles. Descended from quite a different stock, the Plesiosaurs
adapted themselves to sea life in their own fashion. Instead of
adopting the flowing lines of a fish, the body took on a form more
like that of a turtle, while the lengthened neck gave rise to the
description applied to him since that they had the "body of a turtle
strung on a snake." At their longest their necks had as many as
seventy-six bones, or vertebræ, which is more than any other animal
living or extinct ever possessed. A smaller order of crocodiles
appeared and flourished for a time; and the ancestors of the sea
turtles, which were to enjoy so long a reign, began to make their
first appearance.
Among the land animals the Dinosaurs[16] (or the "fearful" saurians)
attained remarkable size and diversity, and their dominant species
were easily lords of the reptile horde. They developed not only as
flesh-eating monsters, but also in vegetable-eating species. Of the
flesh-eaters the _Ceratosaurus_ was the most terrific. It was only
seventeen feet long, but when standing on its powerful hind legs it
could have looked in at most first-floor windows, and it used its
cruel fore limbs for seizing and holding prey. Imagine a kangaroo
with the teeth of a crocodile, the size of an elephant, and the
ferocity of a tiger and you will have a fair idea of what you would
have met in a _Ceratosaurus_.
[Footnote 16: From Gr. "_deinos_," fearful.]
The vegetarian Dinosaurs first became known in this system, but
their development was so extraordinary that they soon outranked
the flesh-eaters both in size and diversity. Among these the
_Brontosaurus_ attained the extraordinary length of sixty feet,
and possibly more. It walked on its four legs, and is one of the
largest known of all land animals. This enormous creature in spite
of all its size and bulk was yet rather weak than strong. Its
general organisation was unwieldy; the head was very small, and
the brain hardly bigger than a walnut. The task of providing food
for such a body must have been a severe tax on so small a head.
The inconvenience of its bulkiness was perhaps reduced by living
in and about water; but from the excellent preservation of some of
the skeletons it has been thought that its life was often ended by
sinking in some quicksand or shoal, from which its own massiveness
forbade that the _Brontosaurus_ should extricate itself.
[Illustration:
Ornitholestes Diplodoci Carnegiei
From skeletons found in Jurassic strata in Wyoming, U.S.A.
(These reptiles attained a length of about 80 feet.)]
Not greatly removed in habit or appearance from the _Brontosaurus_
was the _Diplodocus_, a magnificent specimen of which has been set
up in Pittsburg, and a fine replica, owing to the generosity of Mr.
Andrew Carnegie, in the Natural History Museum. The _Diplodocus_,
a harmless placid beast, was over eighty feet from the tip of his
snout to the end of his enormous tail. It has been calculated that
impulses travel along the nerves to the brain at the rate of about
twelve yards a second. The rate may have been less in the case of
the sluggish _Diplodocus_, but in any case it would evidently take
at least two seconds for a nerve impulse to travel the length of
this reptile; so that if any enemy attacked him at the end of his
tail it would be two seconds before the _Diplodocus_ would realise
the fact, and perhaps four seconds before he could begin to turn
round to defend himself. Even larger than these was the tremendous
_Brachiosaurus_, who weighed as much as a steam-engine and whose
thigh-bone was nearly eight feet high. These were the largest
reptiles ever known, and may be taken as reaching the point when
bulk becomes a burden, and as signalising an approach to the limit
of evolution in the line of size. Less bulky than these were the
_Stegosaurs_, which were also four-footed. They were curiously
armoured, and formed a group of very remarkable creatures found in
England and Western America. While they were less gigantic than
some of those we have just described, they found compensation
in protective plates, spines, and similar modes of defence. A
_Stegosaurus_ found in Wyoming was probably the most hideous to look
upon; but like his relatives he had an extraordinarily small head
and brain, and was a sluggish creature depending on his ugliness and
armour for protection. Very likely this small size of the brain of
great extinct reptiles had to do with the fact of their ceasing to
exist. Animals with bigger and ever-increasing brains outdid them in
the struggle for existence.
It has already been noted that the crowding of the land may have
led some reptiles to take to the sea. The same influence may have
led others to take to the air and thereby escape the monsters of
the swamps, jungles, and forests. Whatever the cause, the most
striking and wonderful feature of this period was the development
of flying reptiles. They had just been seen in the Trias. In the
Jurassic they appeared fully developed. They doubtless sprang from
some agile hollow-boned saurian, more or less akin to the slender
leaping Dinosaurs. Between the ponderous Brontosaurs and the airy
Pterodactyls was the most striking of contrasts. At first these
bird-like reptiles were small, but later their wings had a spread
of as much as twenty feet, veritable flying dragons. They were not
adorned with feathers, but like bats had leathery membranes stretched
from the fore limbs to the body and to the hind limbs. Their heads
were bird-like, and their jaws at first were set with teeth. They had
true powers of flight, as is shown by the discovery of their remains
in places where they must have been far out at sea when they sank
and were buried. Later Pterodactyls had no teeth, and were, perhaps,
milder in habits.
[Illustration: Archæopteryx (the earliest known fossil bird), and
Compsognathus (a small Dinosaur)]
It seems natural to pass from the fossil reptiles to the birds. But
as a matter of fact the birds are not very closely related to the
Pterodactyls, and seem to have been descended from some other very
special form of reptiles, so peculiar as to be considered a distinct
class. It may actually have been descended from those reptiles among
the Dinosaurs which walked on their hind legs and had only three toes
to the foot. The first bird found belongs to Jurassic times; and its
skeleton, found in some slate remains at Solenhofen in Bavaria, is
now to be seen in the Natural History Museum. There is another one
in Berlin. This bird, called the _Archæopteryx_, was of the size of
a large pigeon, had a short head apparently without a beak, and its
jaws were armed with teeth. Whereas living birds have the fingers
of their "hands" tied together in their wings, this bird has three
distinct fingers at the corner of its wings, each armed with a claw.
Its legs were like those of living birds, and it had four toes. Its
tail was unlike that of any living bird, and like that of a lizard.
Whereas the bony part of the tail of living birds is very short and
bears the tail feathers set across it fanwise, the _Archæopteryx_ had
a long bony tail made up of many bones, and the feathers were set in
a series one behind the other till the tail looked like the leaf of
a date palm in shape. Strange as this little creature appears it was
a genuine bird, for it had these feathers well developed, as the two
fossil specimens showed. There are two sets of feathers forming the
wings, and the thighs were also covered with feathers.