THE GREAT SEA-LIZARDS AND THEIR ALLIES.
"The wonders of geology exercise every faculty of the
mind--reason, memory, imagination; and though we cannot put
our fossils to the question, it is something to be so aroused
as to be made to put the questions to one's self."--Hugh
Miller.
The fish-lizards, described in our last chapter, were not the only
predaceous monsters that haunted the seas of the great Mesozoic age,
or era. We must now say a few words about certain contemporary
creatures that shared with them the spoils of those old seas, so
teeming with life. And first among these--as being more fully
known--come the long-necked sea-lizards, or Plesiosaurs.
The Plesiosaurus was first discovered in the Lias rocks of Lyme-Regis,
in the year 1821. It was christened by the above name, and introduced
to the scientific world by the Rev. Mr. Conybeare (afterwards Dean of
Llandaff) and Mr. (afterwards Sir Henry) de la Beche. They gave it
this name in order to distinguish it from the Ichthyosaurus, and to
record the fact that it was more nearly allied to the lizard than the
latter.[9] Conybeare, with the assistance of De la Beche, first
described it in a now-classic paper read before the Geological Society
of London, and published in the _Transactions_ of that Society in the
year 1821. In a later paper (1824) he gave a restoration of the
entire skeleton of Plesiosaurus dolichodeirus; and the accuracy of
that restoration is still universally acknowledged. This fine specimen
was in the possession of the Duke of Buckingham, who kindly placed it
at the disposal of Dr. Buckland, for a time, that it might be properly
described and investigated.
[9] The name is derived from two Greek words--_plesios_, near, or
allied to, and _sauros_, a lizard.
A glance at our illustration, Plate III., will show that this strange
creature was not inaptly compared at the time to a snake threaded
through the body of a turtle.
Dr. Buckland truly observes that the discovery of this genus forms one
of the most important additions that geology has made to comparative
anatomy. "It is of the Plesiosaurus," says that graphic author, in his
_Bridgewater Treatise_, "that Cuvier asserts the structure to have
been the most heteroclite, and its characters altogether the most
monstrous that have been yet found amid the ruins of a former world.
To the head of a lizard it united the teeth of a crocodile; a neck of
enormous length, resembling the body of a serpent; a trunk and tail
having the proportions of an ordinary quadruped; the ribs of a
chameleon, and the paddles of a whale! Such are the strange
combinations of form and structure in the Plesiosaurus--a genus, the
remains of which, after interment for thousands of years amidst the
wreck of millions of extinct inhabitants of the ancient earth, are at
length recalled to light by the researches of the geologist, and
submitted to our examination in nearly as perfect a state as the bones
of species that are now existing upon the earth."
Perhaps the best way in which we can gain a clear idea of the general
characters of a long-necked sea-lizard, as we may call our
Plesiosaurus, is by comparing it with the fish-lizard, described in
the last chapter. Its long neck and small head are the most
conspicuous features. Then we notice the short tail. But if we compare
the paddles of these two extinct forms of life, we notice at once
certain important differences. In the fish-lizard the bone of the arm
was very short, while all the bones of the fore-arm and fingers were
modified into little many-sided bodies, and so articulated together as
to make the whole limb, or paddle, a solid yet flexible structure. In
the long-necked sea-lizard, however, we find a long arm-bone with a
club-like shape; and the two bones of the fore-arm are seen to be
longer than in the fish-lizard. But a still greater difference shows
itself in the bones of the finger, as we look at a fossilised skeleton
(or a drawing of one); for the fingers are long and slender, like
those of ordinary reptiles.
There are only five fingers, and each finger is quite distinct from
the others. This is the reason why the Plesiosaur was considered to
depart less from the type of an ordinary reptile, and so received its
name. Other remarkable differences present themselves in the shoulders
and haunches, but these need not be considered here. The species shown
in Fig. 8 had rather a large head. It is obvious that such a long
slender neck as these creatures had could not have supported a large
head, like that of the fish-lizard. Consequently, we find a striking
contrast in the skulls of the two forms. That of the Plesiosaur was
short and stout, and therefore such as could easily be supported, as
well as rapidly moved about by the long slender neck. Thus we find
another simple illustration of the "law of correlation," alluded to on
p. 6. The teeth were set in distinct sockets, as they are in
crocodiles, to which animals there are also points of resemblance, in
the backbone, ribs, and skull. Fig. 7 shows three different types of
lower jaws of Plesiosaurs. The one marked C belongs to Plesiosaurus
dolichodeirus, the species represented in our plate. There were no
bony plates in the eye. Professor Owen thinks that they were
long-lived. The skin was probably smooth, like that of a porpoise.
[Illustration: Plate III.
PTERODACTYLS. LONG-NECKED SEA-LIZARD. CUTTLE-FISH OR BELEMNITE.
_Plesiosaurus dolichodeirus._
Length 22 feet.]
[Illustration: Fig. 7.--Mandibles of Fish-lizards. A, _Peloneustes
philarchus_ (Seeley); from the Oxford Clay. B, _Thaumatosaurus
indicus_ (Lydekker); Upper Jurassic of India. C, _Plesiosaurus
aolichodirus_ (Conybeare); from the Lower Lias, Lyme Regis.]
The visitor to the geological collection at South Kensington will find
a splendid series of the fossilised remains of long-necked
sea-lizards. They were mostly obtained from the Lias formation of
Street in Somersetshire, Lyme-Regis in Dorset, and Whitby in
Yorkshire. Those from the Lias are mostly small, about eight to ten
feet in length. But in the rocks of the Cretaceous period, which was
later, are found larger specimens. There is a cast of a very fine
specimen from the Upper Lias on the wall of the east corridor (No. 3
on Plan) of the geological galleries at South Kensington, which is
twenty-two feet long. But some of the Cretaceous forms, both in Europe
and America, attained a length of forty feet, and had vertebræ six
inches in diameter. The bodies of the vertebræ, or "cup-bones," are
either flat or slightly concave, showing that the backbone as a whole
was less flexible than in the fish-lizards.
[Illustration: Fig. 8.--_Plesiosaurus macrocephalus._]
It may be mentioned here that Mr. Smith Woodward, of the Natural
History Museum, recently showed the writer a fossil Plesiosaur that is
being set up in the formatore's shop, in the same manner that a recent
skeleton might be. In this, and many other ways, the guardians of the
national treasure-house are endeavouring to make the collection
intelligible and interesting to the general public. Specimens of
extinct animals thus set up, give one a much better idea than when the
bones are all lying huddled together on a slab of rock. But it is not
always possible to get the bones entirely out of their rocky bed, or
matrix. Great credit is due to Mr. Alfred N. Leeds, of Eyebury, who
has disinterred the separate bones of many distinct skeletons of
Plesiosaurs from Oxford Clay strata near Peterborough.
It will be remembered that the long and powerful tail of the
fish-lizard was its principal organ of propulsion through the water;
and that, consequently, the paddles only played a secondary part. They
were small, but amply large enough for the work they had to perform.
But our long-necked sea-lizards possessed very short tails. What,
then, was the consequence? Obviously that the paddles had all the more
work to do. They were the chief swimming organs. The vertebræ of this
short tail show that it probably was highly flexible, and could move
rapidly from side to side; but, for all that, its use as a propeller
would not be of much importance. We see now why the paddles are so
long and powerful, like two pairs of great oars, one pair on each side
of the body. In a fossil skeleton you will notice the flattened shape
of the arm-bone (or humerus), and of the thigh-bone (or femur). This
gave breadth to the paddles, and made them more efficient as swimming
organs. They give no indication of having carried even such imperfect
claws as those of turtles and seals, and therefore we may conclude
that the Plesiosaur was far more at home in the water than on land,
and it seems probable that progression on land was impossible.
The tail was probably useful as a rudder, to steer the animal when
swimming on the surface, and to elevate or depress it in ascending and
descending through the water. Like the fish-lizard, this creature was
an air-breather, and therefore was obliged occasionally to visit the
surface for fresh supplies of air. But probably it possessed the power
of compressing air within its lungs, so that the frequency of its
visits to the surface would not be very great.
From the long neck and head, situated so far away from the paddles, as
well as for other reasons, it may be concluded that this creature was
a rapid swimmer, as was the Ichthyosaurus. Although of considerable
size, it probably had to seek its food, as well as its safety, chiefly
by artifice and concealment. The fish-lizard, its contemporary, must
have been a formidable rival and a dangerous enemy, whom to attack
would be unadvisable.
Speaking of the habits of the long-necked sea-lizard, Mr. Conybeare,
in his second paper, already alluded to, says, "That it was aquatic,
is evident from the form of its paddles; that it was marine, is almost
equally so, from the remains with which it is universally associated;
that it may occasionally have visited the shore, the resemblance of
its extremities to those of the turtle may lead us to conjecture; its
motion, however, must have been very awkward on land; its long neck
must have impeded its progress through the water, presenting a
striking contrast to the organisation which so admirably fits the
Ichthyosaurus to cut through the waves.
"May it not therefore be concluded (since, in addition to these
circumstances, its respiration must have required frequent access of
air) that it swam upon or near the surface, arching back its long neck
like the swan, occasionally darting it down at the fish which happened
to float within its reach? It may, perhaps, have lurked in shoal-water
along the coast, concealed among the sea-weed, and, raising its
nostrils to a level with the surface from a considerable depth, may
have found a secure retreat from the assaults of dangerous enemies;
while the length and flexibility of its neck may have compensated for
the want of strength in its jaws and its incapacity for swift motion
through the water, by the suddenness and agility of the attack which
they enabled it to make on every animal fitted for its prey, which
came within its extensive sweep."
More than twenty species of long-necked sea-lizards are known to
geologists.
Professor Owen, in his great work on _British Fossil Reptiles_, when
describing the huge Plesiosaurus dolichodeirus from Dorset, suggests
that the carcase of this monster, after it sank to the bottom of the
sea, was preyed upon by some carnivorous animal (perhaps sharks). It
seems, he says, as if a bite of the neck had pulled out of place the
eighth to the twelfth vertebræ. Those at the base of the neck are
scattered and dispersed as if through more "tugging and riving." So
with regard to its body, probably some hungry creature had a grip of
the spine near the middle of the back, and pulled all the succeeding
vertebræ in the region of the hind limbs. Thus we get a little glimpse
of scenes of violence that took place at the bottom of the bright
sunny seas of the period when the clays and limestones of the Lias
rocks were being deposited in the region of Lyme-Regis.
As time went on, these curious reptiles increased in size, until, in
the period when our English chalk was being formed (Cretaceous
period), they reached their highest point (see p. 147). After that
they became extinct--whether slowly or somewhat suddenly we cannot
tell.
Until more is known of the ancient life of the earth, it will not be
possible to say with certainty what were the nearest relations of the
long-necked sea-lizards. They first appear in the strata of the New
Red Sandstone, which is below the Lias. Certain little reptiles, about
three feet long, from the former rocks, known as Neusticosaurus and
Lariosaurus, seem to be rather closely related to the creatures we are
now considering, and to connect them with another group, namely, the
Pliosaurs. They were partly terrestrial and partly aquatic; but it is
not easy to say whether their limbs had been converted into true
paddles or not. At any rate, there is every reason to believe that the
long-necked sea-lizards were descended from an earlier form of land
reptile. They gradually underwent considerable modifications, in order
to adapt themselves to an aquatic life. We noticed that the same
conclusion has been arrived at with regard to the fish-lizards. Both
these extinct groups, therefore, present an interesting analogy to
whales, which are now considered to have been derived, by a like
series of changes, from mammals that once walked the earth.
The Plesiosaur presents, on the one hand, points of resemblance to
turtles and lizards,--on the other hand, to crocodiles, whales, and,
according to some authorities, even the strange Ornithorhynchus. But
it will be very long before its ancestry can be made known. In the
mean time, we must put it in a place somewhere near the fish-lizards,
and leave posterity to complete what has at present only been begun.
It must, however, be borne in mind that some of the above resemblances
are purely accidental, and not such as point to relationship. Because
their flippers are like those of a whale, it does not mean that
Plesiosaurs are related to modern whales. It only means that similar
habits tend to produce accidental resemblances--just as the whales and
porpoises, in their turn, resemble fishes. To make torpedoes go
rapidly through the water, inventors have given them a fish-like
shape;--in the same way the early forms of mammals, from which whales
are descended, gradually adapted themselves to a life in the water,
and so became modified to some extent to the shapes of fishes.
The Pliosaurs, above mentioned, are evidently relations, but with
short necks instead of long ones. They had enormous heads and thick
necks. Fine specimens of their huge jaws, paddle-bones, etc., may be
seen at the end of the reptile-gallery at Cromwell Road. One of the
skulls exhibited there is nearly six feet long, while a hind paddle
measures upwards of six and a half feet in length, of which
thirty-seven inches is taken up by the thigh-bone alone. The teeth at
the end of the jaws are truly enormous. One tooth, from a deposit
known as the Kimmeridge Clay, is nearly a foot long from the tip of
the crown to the base of the root. In some, the two jaw-bones of the
lower jaw are partly united, as in the sperm-whale or cachalot.
Creatures so armed must have been very destructive.