“Lias” is an English provincial name adopted in geology, and applied to
a formation of limestone, marl, and petrified clay, which forms the base
of the oolite, or immediately underlies that division of secondary
rocks. The lias has been traced throughout a great part of Europe,
forming beds of a thickness varying from 500 to 1000 feet of the
above-mentioned substances, which have been gradually deposited from a
sea of corresponding extent and direction. The lias abounds with marine
shells of extinct species, and with remains of fishes that were clad
with large and hard shining scales. Of the higher or air-breathing
animals of that period, the most characteristic were the
Enaliosauria.
The creatures called Enaliosauria or Sea-lizards (from the Greek
_enalios_, of the sea, and _sauros_, lizard), were vertebrate animals,
or had back bones, breathed the air like land quadrupeds, but were
cold-blooded, or of a low temperature, like crocodiles and other
reptiles. The proof that the Enaliosaurs respired atmospheric air
immediately, and did not breathe water by means of gills like fishes, is
afforded by the absence of the bony framework of the gill apparatus, and
by the presence, position, and structure of the air passages leading
from the nostrils, and also by the bony mechanism of the capacious chest
or thoracic-abdominal cavity: all of which characters have been
demonstrated by their fossil skeletons. With these characters the
Sea-lizards combined the presence of two pairs of limbs shaped like
fins, and adapted for swimming.
The Enaliosauria offer two principal modifications of their anatomical,
and especially their bony, structure, of which the two kinds grouped
together under the respective names of Ichthyosaurus and Plesiosaurus
are the examples.
The Ichthyosaurus.
The genus Ichthyosaurus includes many species: of which three of the
best known and most remarkable have been selected for restoration to
illustrate this most singular of the extinct forms of animal life.
The name (from the Greek _ichthys_, a fish, and _sauros_, a lizard)
indicates the closer affinity of the Ichthyosaur, as compared with the
Plesiosaur, to the class of fishes. The Ichthyosaurs are remarkable for
the shortness of the neck and the equality of the width of the back of
the head with the front of the chest, impressing the observer of the
fossil skeleton with a conviction that the ancient animal must have
resembled the whale tribe and the fishes in the absence of any
intervening constriction or “neck.”
This close approximation in the Ichthyosaurs to the form of the most
strictly aquatic back-boned (vertebrate) animals of the existing
creation is accompanied by an important modification of the surfaces
forming the joints of the back-bone, each of which surfaces is hollow,
leading to the inference that they were originally connected together by
an elastic bag, or “capsule,” filled with fluid—a structure which
prevails in the class of fishes, but not in any of the whale or porpoise
tribe, nor in any, save a few of the very lowest and most fish-like, of
the existing reptiles.
With the above modifications of the head, trunk, and limbs, in relation
to swimming, there co-exist corresponding modifications of the tail. The
bones of this part are much more numerous than in the Plesiosaurs, and
the entire tail is consequently longer; but it does not show any of
those modifications that characterise the bony support of the tail in
fishes. The numerous “caudal vertebræ” of the Ichthyosaurus gradually
decrease in size to the end of the tail, where they assume a compressed
form, or are flattened from side to side, and thus the tail instead of
being short and broad, as in fishes, is lengthened out as in crocodiles.
The very frequent occurrence of a fracture of the tail, about one fourth
of the way from its extremity, in well-preserved and entire fossil
skeletons, is owing to that proportion of the end of the tail having
supported a tail-fin. The only evidence which the fossil skeleton of a
whale would yield of the powerful horizontal tail-fin characteristic of
the living animal, is the depressed or horizontally flattened form of
the bones supporting such fin. It is inferred, therefore, from the
corresponding bones of the Ichthyosaurus being flattened from side to
side, that it possessed a tegumentary tail-fin expanded in the vertical
direction. The shape of a fin composed of such perishable material is of
course conjectural, but from analogies, not necessary here to further
enlarge upon, it was probably like, or nearly like, that which the able
artist engaged in the restoration of the entire form of the animal has
given to it. Thus, in the construction of the principal swimming-organ
of the Ichthyosaurus we may trace, as in other parts of its structure, a
combination of mammalian (beast-like), saurian (lizard-like), and
piscine (fish-like) peculiarities. In its great length and gradual
diminution we perceive its saurian character; the tegumentary nature of
the fin, unsustained by bony fin-rays, bespeaks its affinity to the same
part in the mammalian whales and porpoises; whilst its vertical position
makes it closely resemble the tail-fin of the fish.
The horizontality of the tail-fin of the whale tribe is essentially
connected with their necessities as warm-blooded animals breathing
atmospheric air; without this means of displacing a mass of water in the
vertical direction, the head of the whale could not be brought with the
required rapidity to the surface to respire; but the Ichthyosaurs, not
being warm-blooded, or quick breathers, would not need to bring their
head to the surface so frequently, or so rapidly, as the whale; and,
moreover, a compensation for the want of horizontality of their tail-fin
was provided by the addition of a pair of hind-paddles, which are not
present in the whale tribe. The vertical fin was a more efficient organ
in the rapid cleaving of the liquid element, when the Ichthyosaurs were
in pursuit of their prey, or escaping from an enemy.
That the Ichthyosaurs occasionally sought the shores, crawled on the
strand, and basked in the sunshine, may be inferred from the bony
structure connected with their fore-fins, which does not exist in any
porpoise, dolphin, grampus, or whale; and for want of which, chiefly,
those warm-blooded, air-breathing, marine animals are so helpless when
left high and dry on the sands: the structure in question in the
Ichthyosaur is a strong osseous arch, inverted and spanning across
beneath the chest from one shoulder-joint to the other; and what is most
remarkable in the structure of this “scapular” arch, as it is called,
is, that it closely resembles, in the number, shape, and disposition of
its bones, the same part in the singular aquatic mammalian quadruped of
Australia, called _Ornithorhynchus_, _Platypus_, and Duck-mole. The
Ichthyosaurs, when so visiting the shore, either for sleep, or
procreation, would lie, or crawl prostrate, or with the belly resting or
dragging on the ground.
The most extraordinary feature of the head was the enormous magnitude of
the eye; and from the quantity of light admitted by the expanded pupil
it must have possessed great powers of vision, especially in the dusk.
It is not uncommon to find in front of the orbit (cavity for the eye),
in fossil skulls, a circular series of petrified thin bony plates,
ranged round a central aperture, where the pupil of the eye was placed.
The eyes of many fishes are defended by a bony covering consisting of
two pieces; but a compound circle of overlapping plates is now found
only in the eyes of turtles, tortoises, lizards, and birds. This curious
apparatus of bony plates would aid in protecting the eyeball from the
waves of the sea when the Ichthyosaurus rose to the surface, and from
the pressure of the dense element when it dived to great depths; and
they show, writes Dr. Buckland,[4] “that the enormous eye, of which they
formed the front, was an optical instrument of varied and prodigious
power, enabling the Ichthyosaurus to descry its prey at great or little
distances, in the obscurity of night, and in the depths of the sea.”
Of no extinct reptile are the materials for a complete and exact
restoration more abundant and satisfactory than of the Ichthyosaurus;
they plainly show that its general external figure must have been that
of a huge predatory abdominal fish, with a longer tail, and a smaller
tail-fin: scale-less, moreover, and covered by a smooth, or finely
wrinkled skin analogous to that of the whale tribe.
The mouth was wide, and the jaws long, and armed with numerous pointed
teeth, indicative of a predatory and carnivorous nature in all the
species; but these differed from one another in regard to the relative
strength of the jaws, and the relative size and length of the teeth.
Masses of masticated bones and scales of extinct fishes, that lived in
the same seas and at the same period as the Ichthyosaurus, have been
found under the ribs of fossil specimens, in the situation where the
stomach of the animal was placed; smaller, harder, and more digested
masses, containing also fish-bones and scales have been found, bearing
the impression of the structure of the internal surface of the intestine
of the great predatory sea-lizard. These digested masses are called
“coprolites.”
In tracing the evidences of creative power from the earlier to the later
formations of the earth’s crust, remains of the Ichthyosaurus are first
found in the lower lias, and occur, more or less abundantly, through all
the superincumbent secondary strata up to, and inclusive of, the chalk
formations. They are most numerous in the lias and oolite, and the
largest and most characteristic species have been found in these
formations.
No. 12.—Ichthyosaurus platyodon.
This most gigantic species, so called on account of the crown of the
tooth being more flattened than in other species, and having sharp
edges, as well as a sharp point, was first discovered in the lias of
Lyme Regis, in Dorsetshire. Fossil remains now in the British Museum,
and in the museum of the Geological Society, fully bear out the
dimensions exhibited by the restoration of the animal as seen basking on
the shore between the two specimens of Long-necked Plesiosaurs. The head
of this species is relatively larger in proportion to the trunk, than in
the _Ichthyosaurus communis_ or _Ichthyosaurus tenuirostris_: the lower
jaw is remarkably massive and powerful, and projects backwards beyond
the joint, as far as it does in the crocodile. In the skull of an
individual of this species, preserved in the apartments of the
Geological Society of London, the cavity for the eye, or orbit,
measures, in its long diameter, fourteen inches. The fore and hind
paddles are large and of equal size.
The lias of the valley of Lyme Regis, Dorsetshire, is the chief
grave-yard of the _Ichthyosaurus platyodon_; but its remains are pretty
widely distributed. They have been found in the lias of Glastonbury, of
Bristol, of Scarborough and Whitby, and of Bitton, in Gloucestershire;
some vertebræ, apparently of this species, have likewise been found in
the lias at Ohmden, in Germany.
No. 13.—Ichthyosaurus tenuirostris.
Behind the _Ichthyosaurus platyodon_, is placed the restoration of the
_Ichthyosaurus tenuirostris_, or Slender-snouted Fish-lizard. The most
striking peculiarity of this species is the great length and slenderness
of the jaw-bones, which, in combination with the large eye-sockets and
flattened cranium, give to the entire skull a form which resembles that
of a gigantic snipe or woodcock, with the bill armed with teeth. These
weapons, in the present species, are relatively more numerous, smaller,
and more sharply pointed than in the foregoing, and indicate that the
_Ichthyosaurus tenuirostris_ preyed on a smaller kind of fish. The
fore-paddles are larger than the hind ones. In the museum of the
Philosophical Institution, at Bristol, there is an almost entire
skeleton of the present species which measures thirteen feet in length.
It was discovered in the lias of Lyme Regis. Portions of jaws and other
parts of the skeletons of larger individuals have been found fossil in
the lias near Bristol, at Barrow-on-Soar, in Leicestershire, and at
Stratford-on-Avon. The _Ichthyosaurus tenuirostris_ has also left its
remains in the lias formation at Boll and Amburg, in Wirtemberg,
Germany.
No. 14.—Ichthyosaurus communis.
Of this species, which was the most “common,” when first discovered in
1824, but which has since been surpassed by other species in regard to
the known number of individuals, the head is restored, as protruded from
the water, to the right of the foregoing species.
The _Ichthyosaurus communis_ is characterised by its relatively large
teeth, with expanded, deeply-grooved bases, and round conical furrowed
crowns; the upper jaw contains, on each side, from forty to fifty of
such teeth. The fore-paddles are three times larger than the hind ones.
With respect to the size which it attained, the _Ichthyosaurus communis_
seems only to be second to the _Ichthyosaurus platyodon_. In the museum
of the Earl of Enniskillen, there is a fossil skull of the
_Ichthyosaurus communis_ which measures, in length, two feet nine
inches, indicating an animal of at least twenty feet in length.
Plesiosaurus.
The discovery of this genus forms one of the most important additions
that geology has made to comparative anatomy. Baron Cuvier deemed “its
structure to have been the most singular, and its characters the most
monstrous, that had been yet discovered 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,” writes Dr. Buckland, “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.”
(Op. cit., vol. v. p. 203).
The first remains of this animal were discovered in the lias of Lyme
Regis, about the year 1823, and formed the subject of the paper by the
Rev. Mr. Conybeare (now Dean of Llandaff), and Mr. (now Sir Henry) De la
Beche, in which the genus was established and named Plesiosaurus (from
the Greek words, _plesios_ and _sauros_, signifying “near” or “allied
to,” and “lizard”), because the authors saw that it was more nearly
allied to the lizard than was the Ichthyosaurus from the same formation.
The entire and undisturbed skeletons of several individuals, of
different species, have since been discovered, fully confirming the
sagacious restorations by the original discoverers of the
_Plesiosaurus_. Of these species three have been selected as the
subjects of Mr. Waterhouse Hawkins’s reconstructions and representations
of the living form of the strange reptiles.
No. 15.—Plesiosaurus macrocephalus.
The first of these has been called, from the relatively larger size of
the head, the _Plesiosaurus macrocephalus_ (No. 15), (Gr. _macros_,
long, _cephale_, head). The entire length of the animal, as indicated by
the largest remains, and as given in the restoration, is eighteen feet,
the length of the head being two feet, that of the neck six feet; the
greatest girth of the body yields seven feet.
[Illustration: No. 15. Plesiosaurus macrocephalus.]
Although Baron Cuvier and Dr. Buckland both rightly allude to the
resemblance of the fins or paddles of the Plesiosaur to those of the
whale, yet this most remarkable difference must be borne in mind, that,
whereas the whale tribe have never more than one pair of fins, the
Plesiosaurs have always two pairs, answering to the fore and hind limbs
of land quadrupeds; and the fore-pair of fins, corresponding to those in
the whale, differed by being more firmly articulated, through the medium
of collar-bones (clavicles), and of two other very broad and strong
bones (called coracoids), to the trunk (thorax), whereby they were the
better enabled to move the animal upon dry land.
Remains of the _Plesiosaurus macrocephalus_ have been discovered in the
lias of Lyme Regis, in Dorsetshire, and of Weston, in Somersetshire.
No. 16.—Plesiosaurus dolichodeirus.
Further to the left, on the shore of the Secondary Island, is a
restoration of the _Plesiosaurus dolichodeirus_, or Long-necked
Plesiosaurus (No. 16). The head in this remarkable species is smaller,
and the neck proportionally longer than in the _Plesiosaurus
macrocephalus_. The remains of the Long-necked Plesiosaur have been
found chiefly at Lyme Regis, in Dorsetshire. The well known specimen of
an almost entire skeleton, formerly in the possession of His Grace the
Duke of Buckingham, is now in the British Museum.
No. 17.—Plesiosaurus Hawkinsii.
The most perfect skeletons of the Plesiosaurus are those that have been
wrought out of the lias at Street, near Glastonbury, by Mr. Thomas
Hawkins, F.G.S., and which have been purchased by the trustees of the
British Museum. A restoration is given by Mr. Waterhouse Hawkins, at No.
17, of a species with characters somewhat intermediate between the
Large-headed and Long-necked Plesiosaurs, and which has been called,
after its discoverer, _Plesiosaurus Hawkinsii_.
The Plesiosaurs breathed air like the existing crocodiles and the whale
tribe, and appear to have lived in shallow seas and estuaries. That the
Long-necked Sea-lizard was aquatic is evident from the form of its
paddles; and that it was marine is almost equally so, from the remains
with which its fossils are universally associated; that it may have
occasionally visited the shore, the resemblance of its extremities to
those of a turtle leads 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 adapted the Ichthyosaurus to cut its swift course
through the waves. “May it not, therefore, be concluded that it swam
upon, or near the surface,” asks its accomplished discoverer, “arching
back its long neck like a swan, and occasionally darting it down at the
fish that 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 enabled it to make on every animal fitted for its prey
which came within its reach.”[5]
For the Secondary Island three species of the Plesiosaurus have been
restored, the _Plesiosaurus macrocephalus_, the _Plesiosaurus
dolichodeirus_ (Gr. _dolichos_, long, _deire_, neck), and the
_Plesiosaurus Hawkinsii_. The name “long-necked” was given to the second
of these species before it was known that many other species with long
and slender necks had existed in the seas of the same ancient period:
the third species is named after Mr. Thomas Hawkins, F.G.S., the
gentleman by whose patience, zeal, and skill, the British Museum has
been enriched with so many entire skeletons of these most extraordinary
extinct sea-lizards.
The remains of all these species occur in the lias at Lyme Regis, and at
Street, near Glastonbury; but the _Plesiosaurus Hawkinsii_ is the most
abundant in the latter locality.
[Illustration: {uncaptioned}]