ICHTHYOSAURIA
Early in the eighteenth century a curious work in the Latin language
was published by a famous physician and naturalist—a professor in the
University of Altorf by the name of Scheuchzer—entitled _Querulae
Piscium_, or “Complaints of the Fishes.” The work was illustrated by
many expensively engraved figures of various fossil remains, including
one of some vertebrae which the author referred to as “the accursed
race destroyed by the flood”! The history of the finding of these
famous bones is recorded by Cuvier as follows:
Scheuchzer, while walking one day with his friend Langhans
in the vicinity of Altorf, a village and university of
Nuremburg, went to the vicinity of the gallows to make some
researches. Langhans, who had entered the inclosure of
the gallows, found a piece of limestone containing eight
dorsal vertebrae, of a black color and shining. Seized,
says Scheuchzer, with a panic terror, Langhans threw the
fragment of limestone beyond the wall of the inclosure, and
Scheuchzer, picking it up, preserved two of the vertebrae
which he believed to be human, and which he figured in his
book, _Piscium Querulae_.
About the same time another observer by the name of Baier discovered
other and similar vertebrae in the vicinity of Altorf which he
described and figured as those of a fish; and there was much earnest
contention between Scheuchzer and Baier, as also between their friends,
as to their supposed nature. Scheuchzer’s figure was often cited as
indubitable evidence of the destruction of mankind by a universal
flood, and it was not until nearly a century later that Cuvier showed
that the bones were really those of a marine reptile.
[Illustration: FIG. 51.—Restoration of _Ichthyosaurus_ with young, by
Charles R. Knight. (By permission of the American Museum of Natural
History.)]
It must be recollected, in extenuation of so extraordinary a blunder
on the part of so learned a man as was Scheuchzer, who, as a physician
and professor, one would think ought to have been able to distinguish
between vertebrae so different as are those of an ichthyosaur and a
man, that, during all of the eighteenth century and well into the
nineteenth, the belief was prevalent that all fossils were the relics
of animals and plants that had perished in the great biblical flood.
The science of geology was yet in its infancy, and there was no known
record, other than the biblical one, of any great inundation of the
earth’s surface which might account for the remains of sea-animals
in rocks remote from the seas. This belief, so long held by even
the wisest and most learned of scholars, so long welcomed by the
theologians as proof of the literal accuracy of the Bible, was one
of which Scheuchzer was quite convinced. His _Piscium Querulae_ was
largely a fantastic discussion of the supposed great world-catastrophe,
the Noachian Deluge, by which the fishes had been destroyed and long
imprisoned in the rocks through no sin of their own.
It was the same author who, in a subsequent work, described and figured
the fossil skeleton of a large salamander which he believed to be that
of a child destroyed in the flood, and which he called “Homo diluvii
testis.” In this specimen, which was discovered in the Tertiary rocks
of Oeningen, and which is still preserved among the historically as
well as scientifically famous fossils of the museum at Haarlem under
the name _Andrias Scheuchzeri_, Scheuchzer thought that he detected,
not only the skeleton of a child, but even its brain, liver, muscles,
etc.! His engraving of this “Witness of the Flood,” the “sorrowful
skeleton of an old sinner drowned in the Flood,” as also that of
the ichthyosaur vertebrae of Altorf, were afterward printed in the
famous “Copper Bible” as positive proof of the literal accuracy of the
biblical record.
Earlier than the publication of these figures by Baier and Scheuchzer,
at the very close of the seventeenth century, a Welsh naturalist
by the name of Lluyd, in a large and beautifully illustrated work,
figured—perhaps for the first time—remains of ichthyosaurs, which he
believed to be those of fishes. But Lluyd accounted for these and all
other fossil remains by a very different theory from that of Scheuchzer
and the theologians—a theory which at one time had many adherents among
scholars. He believed that the spawn of fishes or the eggs of other
creatures had been carried up from the seas and lands in moist vapors
into the clouds, whence they had descended in rain, penetrating the
earth to give origin to the fossils; in other words, he believed that
all fossils grew in the earth from germs of the living animals that
inhabited the land and seas. Certainly the old philosophers were hard
driven to make facts agree with theories!
Remains of ichthyosaurs, abundant as they were and are in many deposits
in England and Germany, attracted very little attention from the
naturalists of the eighteenth century after the time of Scheuchzer and
Baier, and nothing more was written about them until 1814, when Sir
Everard Home, an English comparative anatomist, in an extensive series
of large and finely illustrated, though rather discursive, works,
described and figured a number of good specimens. To the animal the
remains of which he rather vaguely and imperfectly described, he gave
in 1819 the name _Proteosaurus_, in the belief that it was allied to
the living _Proteus_, a salamander.
In 1821 the curator of mineralogy of the British Museum—Koenig by
name—after a more critical study of other remains, reached the
conclusion that these animals were intermediate between the fishes
and the reptiles, and gave to them the generic name _Ichthyosaurus_,
meaning fish-reptile, a name by which the chief forms have ever since
been known. Within the next few years many specimens of ichthyosaurs
were carefully and fully described by Conybeare, Cuvier, Owen, and
others of England, France, and Germany, making very clear all the
more important details of their skeletal structure. Blaineville, in
1835, thought that the ichthyosaurs constituted a distinct class of
vertebrates equivalent to all other reptiles, the birds, and the
mammals, which he called Ichthyosauria, the first appearance in
literature of the name by which the order is properly known. Five years
later, however, the famous English anatomist and paleontologist, the
late Sir Richard Owen, united the ichthyosaurs with the plesiosaurs as
a single order of reptiles, to which he gave the name Enaliosauria,
meaning sea-reptiles, a name which has long been current in textbooks
and general works on natural history. Moreover, Owen rather arbitrarily
changed Blaineville’s name Ichthyosauria to Ichthyopterygia, a name
which is often, though incorrectly, used to designate this order of
reptiles. These briefly given and perhaps dry details will make clear
how necessary is that rule of priority upon which naturalists so often
insist. When anyone may change the names of organisms at will there
will be no stability and no uniformity, because there is no one to
decide, and the prestige of a great name, like that of Owen, will carry
authority till someone else with greater authority appears. Whether or
not the name _Proteosaurus_, first given to any member of this order,
should take precedence over the later _Ichthyosaurus_ is still in
doubt, since Home gave no specific name to his species, and the very
particular purists of modern times have decided that a genus is not
named unless the species is also! We moderns sometimes are inclined to
impose very stringent conditions upon the older naturalists; let us
hope that we shall be treated more leniently by the future naturalists!
It will lead us too far astray to follow in detail the history of
the further discoveries of the ichthyosaurs during the early part
of the nineteenth century. It may briefly be said, only, that no
other group of extinct backboned animals excited more interest among
scientific men. One incident will suffice. More than sixty years ago,
an interesting deduction as to the living form of the ichthyosaurs was
made by Sir Richard Owen. He observed that many of the known skeletons,
as they were found in their rocky matrix, had a remarkable dislocation
of the vertebrae at a certain place near the end of the tail, and,
although such an appendage was quite unknown in other reptiles either
living or extinct, concluded that the living animals had a terminal,
horizontal, fleshy fin, very much like that of the whales and
sirenians. Sure enough, discoveries made forty years later disclosed
impressions in the rocks, not only of a large caudal fin, but also of
a dorsal fin, as well as outlines of the flesh-covered paddles. The
dislocation of the vertebrae at the place where the fleshy fin joined
the more slender tail was due to the action of currents of water, or
simple gravitation, upon a thin vertical fin and not, as Owen supposed,
to the twisting of the terminal part as it fell to a horizontal
position after partial decomposition of the soft parts.
About twenty-five years ago, Professor E. Fraas, the present director
of the Stuttgart Museum, described and figured very fully, not only
specimens showing impressions of the fins and paddles, but also others
of well-preserved and very complete skeletons of different species
of ichthyosaurs from the Jurassic deposits of Würtemberg, in which
remains of these animals occur in great profusion. His researches,
and those of several authors since then, supplementing and confirming
or disproving those of the many observers made during the preceding
seventy years, have finally determined almost perfectly the complete
structure of the more typical ichthyosaurs, enabling us to infer not a
little as to their habits and distribution in the old Jurassic oceans.
Within the past few years the discoveries of Professor J. C. Merriam of
California have likewise added greatly to our knowledge of the earlier
ichthyosaurs. It may now truthfully be said that of no group of extinct
reptiles do we have a more complete and satisfactory knowledge than of
the ichthyosaurs.
[Illustration: FIG. 52.—_Ichthyosaurus quadricissus._ Photograph of
specimen in Senckenberg museum, from Dr. Dreverman.]
Nevertheless we have yet very much more to learn about the order
Ichthyosauria as a whole—whence they came and how they originated;
what their nearest kin were among other reptiles; and especially, more
about the connecting links between them and terrestrial reptiles. They
have, as an order, so isolated a position, are so widely separated from
all other reptiles in structure, that they have long been a puzzle to
paleontologists. Like the whales and other cetaceans among mammals,
we know the ichthyosaurs well in the plenitude of their power and the
fulness of their development, but have yet only an imperfect knowledge
of their earlier history, and none whatever of their earliest. However,
as will be seen farther on, the recent discoveries by Merriam have
shed much light on some of the stages of their evolution. So nearly
perfectly were all the later ichthyosaurs adapted to their life in the
water that it was believed by nearly all paleontologists until about
a score of years ago that they had descended directly from fishes.
But this belief has been quite abandoned by all, not only because the
recent discoveries of the earlier ichthyosaurs have demonstrated a
positive increase in the aquatic adaptations of the later forms, but
also because a double origin of any type of animal life is quite out of
accord with all known facts and principles of paleontology. It is quite
possible for animals, in becoming adapted to peculiar environmental
and food conditions, to acquire certain resemblances to other animals,
but quite impossible for them to acquire their actual structure. The
ichthyosaurs are true reptiles, and all reptiles must have had a common
origin.
[Illustration: FIG. 53.—_Baptanodon (Ophthalmosaurus)._ Skull from
the side, from above, and from below (after Gilmore): _ang_, angular;
_bs_, basisphenoid; _d_, dentary; _fr_, frontal; _j_, jugal; _la_,
lacrimal; _mx_, maxilla; _na_, nasal; _oc_, occipital condyle; _p_,
palatine; _pa_, parietal; _pm_, premaxilla; _po_, postorbital; _ps_,
parasphenoid; _pt_, pterygoid; _pf_, prefrontal; _sa_, surangular;
_sp_, splenial; _sq_, squamosal; _st_, supratemporal; _q_, quadrate;
_qj_, quadratojugal.]
[Illustration: FIG. 54.—Occiput of _Baptanodon (Ophthalmosaurus)_:
_pa_, parietal; _soc_, supraoccipital; _sq_, squamosal; _exoc_,
exoccipital; _op.o_, paroccipital; _sta_, stapes; _st_, supratemporal;
_qu_, quadrate; _qj_, quadratojugal; _pt_, pterygoid; _bs_,
basisphenoid; _sag_, surangular; _ag_, angular; _art_, articular;
_pra_, prearticular. (After Gilmore.)]
We are sometimes in doubt, however, as to whether characters resembling
those of other animals are really acquired as adaptations to peculiar
environments, that is, parallel, convergent, or homo-plastic
characters, or whether they are due to heredity from remote ancestors.
The reptilian characters of the ichthyosaurs, however, are so emphatic
that they can only be ascribed to heredity. Ichthyosaurs are as truly
reptilian as crocodiles or snakes, notwithstanding their fish-like form
and habits. The ichthyosaur ancestors were once truly land reptiles—of
that we are as sure as we well can be. Some have thought that those
ancestors were the primitive Rhynchocephalia, but most are now
convinced that they were among the most primitive of reptiles, a branch
probably from the cotylosaurs or cotylosaurian ancestors. Probably of
all the extinct forms that we know the Proganosauria come the nearest;
indeed it is not impossible that they may have been the actual forbears
of the ichthyosaurs.
The ichthyosaurs varied in length from two to thirty feet, but the
different species, especially all the later ones, resembled each other
pretty closely in shape; the beak was more slender in some than in
others, and the shapes of the fins and paddles varied not a little, as
we shall see. The jaws were long and slender, provided with numerous
rather small but sharp and recurved teeth, especially well fitted for
the seizure and retention of slippery prey. The teeth were inserted,
not in separate sockets, as are those of the crocodiles and many other
reptiles, but in long, deep grooves, and were easily lost, indeed so
easily lost that one late American form was originally described as
edentulous, and it was not till a number of years had elapsed that
the teeth were found. The nostrils were small, and situated far back
on the sides of the face, near the eyes. The eyes were very large,
not only in proportion to the size of the skull, but, in the largest
species, actually attaining in some, perhaps, the size of a human head.
The eyeball was surrounded in front by an extraordinarily large and
strong ring of ossifications in the sclerotic membrane, giving not only
protection to the eye under the varying pressure of the water, but
also greater control over vision. The neck was very short, so short,
in fact, that no construction was visible in the living animal between
the head and body; it was capable of only slight movement. The trunk
was elongated and relatively slender, sometimes with more than fifty
vertebrae in it. The tail also was long and flattened, ending in all
the later species in a large fleshy fin, resembling the caudal fin of
many fishes in shape and doubtless also in function. There was also a
large dorsal fin, supported by hardened or calcified sinews, in shape
like the dorsal fin of most fishes and many cetaceans. This character
is absolutely unique among reptiles, so far as is known, and was one of
the extreme specializations of water life. The hind limbs were smaller,
often much smaller than the fore ones, and both were quite fin-like in
life, or rather flipper-like, though not at all fin-like in structure.
The skin was smooth and bare. In brief, to quote Fraas’s words:.
The general aspect of the ichthyosaurs was very
dolphin-like. The body was everywhere naked and probably
dark in color. The head was produced in front into a
long, slender snout, and was closely joined to the body
posteriorly without indications of a neck. The body itself
was cylindrical, expanded in front by the large thorax and
abdomen, but rapidly diminishing into the long, slender, and
strong tail. Close behind the head were the front paddles,
which in some species were broad and shovel-like, in others
elongated and pointed. The hind paddles were smaller than
the front ones, sometimes greatly reduced in size, their
function replaced by that of the very broad tail.
From the foregoing descriptions and the restoration shown in Fig. 51,
we see how very fish-like, or rather dolphin-like, these animals were
in the external form—so fish-like that the name _Ichthyosaurus_ is not
misleading, though Koenig gave it in the mistaken belief that they were
really allied to the fishes. When to these external features certain
other fish-like details of the skeleton are added, we do not wonder
that the early observers were so long in doubt about them. A more
careful examination of the skeleton will, however, disclose so many
truly reptilian characters that their external appearance and habits
lose all significance.
The vertebrae are deeply biconcave and fish-like, it is true, but a
consideration of the reasons therefor will convince us that any other
kind of vertebrae would be more remarkable. At the time when the
ichthyosaurs must have originated, at the time when the first known
ichthyosaurs appeared in geological history indeed, all reptiles had
biconcave vertebrae, and for the most part at least deeply biconcave
ones. The vertebrae remained fish-like throughout all their history,
perpetuating their type until most other reptiles had developed a
firmer one, because such vertebrae were best adapted for the quick,
pliant movements of the spinal column so necessary for the well-being
of the animals in the water. In the modern dolphins, animals in shape,
size, and habits most wonderfully allied to what these old reptiles
must have been, the small, flat-ended vertebrae are widely separated by
disks of flexible cartilage.
Not only were their vertebrae fish-like in form, but there are other
characters in the spinal column of a primitive or generalized nature.
As in all aquatic animals, the articulating processes between the
vertebrae are either weak or wanting in the posterior part of the
column. And they were not only small, but were situated, in many,
high up, very remarkably resembling the peculiar arrangement of the
articulations in the dolphins.
There is no sacrum, that is, there were no united vertebrae posteriorly
for the attachment and support of the pelvis, as no such support was
needed. In only one other group of aquatic reptiles was the sacrum
lost, though it has wholly disappeared in the cetaceans and sirenians
among mammals. The chevron bones of the tail, usually bony arches on
the under side of the tail for the protection of the blood-vessels, in
crawling reptiles, were very imperfectly developed in the later forms,
though normal in shape in the early ones. The ribs are numerous, long,
and slender, very much resembling those of the fish-eating dolphins.
They usually had, however, two attachments to the body of the vertebra
and none to the arch, differing in this respect from all other animals.
Of the shoulder bones, the scapula or shoulder-blade, as usual among
water animals, is short and broad. In the place of a sternum the
coracoids joined each other broadly in the middle, just as they did
in the oldest known land reptiles. And there were clavicles and an
interclavicle. Below the abdomen behind were numerous slender bones
called ventral ribs. The pelvis is very weak, and was suspended
below the spinal column in the fleshy walls of the abdomen. The hind
legs were so small that little support was necessary for them, and,
because they were not used either for the support of the body or for
propulsion, they did not require a firm union with the skeleton.
Doubtless had the ichthyosaurs continued to the present time, they
would have lost entirely the hind legs, as have the cetaceans.
[Illustration: FIG. 55.—Pectoral girdle of _Baptanodon
(Ophthalmosaurus)_, an American Upper Jurassic ichthyosaur. (After
Gilmore.)]
It is in the limbs that most extraordinary differences from all other
animals are seen. So great are these differences that it has been a
puzzle to naturalists to understand how they could have arisen. In no
other animals above the fishes, that is, in no other reptiles, in no
amphibians, birds, or mammals, are there ever more than five fingers or
five toes, the number with which air-breathing animals began. Fingers
and toes may be lost and often are lost in all groups of life, until
a single one in each limb may remain, as in the domestic horse. An
increase of fingers and toes, however, seems to be an impossibility
in evolution, and doubtless of real fingers and toes it is an equal
impossibility. All naturalists are now agreed that a specialized
character can never revert to a generalized condition, or rather to a
generalized structure, that an organ once functionally lost can never
be regained by descendants. A character once lost is lost forever;
horses of the future can never have more than one finger or one toe in
each limb.
[Illustration: FIG. 56.—Front paddle of _Ophthalmosaurus_ (after
Andrews): _h_, humerus; _r_, radius; _u_, ulna; _p_, pisiform; _re_,
radiale; _int_, intermedium; _ue_, ulnare.]
[Illustration: FIG. 57.—Front paddle of _Merriamia_, a Triassic
ichthyosaur. (After Merriam.) Explanations as in Fig. 56.]
And there was an increase in the ichthyosaurs, in some not only of
the number of digits in each limb, but in all of the number of bones
in each digit, a character found also in the unrelated mosasaurs and
plesiosaurs. This increase in finger and toe bones, or hyperphalangy
as it is called, is one of the most peculiar of all the adaptations to
water life, changing the feet and hands from the ordinary walking type
to the fish-like swimming type. The bones beyond the humerus and femur
in the ichthyosaurs were so increased in number and so changed in form
and relations that they bear little resemblance to the corresponding
bones of other reptiles. They are merely polygonal platelets of bone,
articulating on all sides and fitting closely together, permitting
flexibility, but not much else.
It is now believed that the increase, not only of additional
digits, sometimes to as many as ten in each hand and foot, but
of the finger and toe bones as well, was the result of a sort of
vegetative reproduction. The margins and ends of the flippers were
doubtless hardened by cartilage or fibrous material, and because of
the action of the limbs this cartilagenous material broke up into
nodules each of which took on ossification finally. Among the whales,
where hyperphalangy also occurs, though to a less extent, it has
been thought that the increase in number has been due simply to the
ossification of the parts of each bone normally present, that is, to
the epiphyses, which became separated from the shaft of each bone. But
this explanation will hardly suffice for the fingers and toes of the
plesiosaurs and ichthyosaurs, for there are altogether too many such
ossifications; and besides, the bones in these animals, as in most
reptiles, did not have epiphyses, or terminal separate ossifications of
the bones of the skeleton.
It will be observed from the figures that the arm and thigh bones
of _Ichthyosaurus_ are very much shortened—a striking adaptation to
water life, so conspicuously seen in the modern whales and dolphins
as well as in the mosasaurs, thalattosaurs, etc. So characteristic
indeed is this shortening that, were every other bone of the skeleton
of an ichthyosaur unknown save the humerus or femur, it would be quite
certain from these alone that the animal was thoroughly aquatic in
habit.
About sixty years ago a rather aberrant form of ichthyosaur, now
known as _Mixosaurus_, was discovered in rocks of Triassic age, that
is, of much greater age than any ichthyosaurs previously found, in
which not only the forearm but also the lower leg bones were longer,
resembling more the corresponding bones of land animals. It was from
the examination of specimens in 1887 of these mixosaurs that the
late Professor Baur became convinced that the ichthyosaurs were the
descendants of land reptiles, and not directly of the fishes as they
were universally thought to have been at that time. As Professor Baur
very pertinently said, if the ichthyosaurs were descended from the
fishes directly, the earliest forms should be more nearly like the
fishes than the later ones, whereas just the opposite was the real
fact. The arguments which he gave in support of his contention were so
convincing that they found immediate acceptance among all naturalists.
Fortunately within the past fifteen years many other remains of early
ichthyosaurs from the Triassic rocks of California have been brought
to light by Professor Merriam, remains which throw a flood of light
upon the early, though not the earliest, history of these strange
reptiles. He has recognized among the forms he has discovered, not
only new species, but several new genera, and perhaps new families
of ichthyosaurs. His studies have demonstrated so well the stages of
evolution between the early ichthyosaurs and the later ones in their
progressive adaptation to water life that it will be of interest to
summarize them here.
In the early ichthyosaurs locomotion was largely by the aid of the
limbs; in the later ones almost exclusively by the aid of the caudal
fin. In the former the paddles were larger and the bones longer, more
like those of land animals; in the latter they were relatively smaller
and shorter, and more fin-like. In the digits of the early forms the
finger and toe bones were more elongated and fewer in number. The hind
limbs were nearly as large as the front ones in the Triassic, often
very much smaller in the later ichthyosaurs; and the increased number
of digits occurs only in the later forms.
In the Triassic ichthyosaurs, all classed in the family Mixosauridae,
the pelvis was larger and more firmly connected with the body than in
the later forms.
The skull of the early forms was relatively shorter, as compared with
the trunk, the jaws shorter as compared with the head, the eyes were
relatively small, the teeth in some less numerous, and set in distinct
sockets like those of land reptiles; the vertebrae were relatively
longer and less fish-like, and their articulations more like those of
land reptiles.
The distal part of the tail was not bent downward so sharply, that
is, the terminal fin was smaller, or the tail may have been simply
flattened near its end and not really fin-like. The scapula was longer
and less fan-like in shape.
And all these are remarkable evidences of an increased adaptation
to water life in the more recent ichthyosaurs over the older ones.
Were someone now so fortunate as to find ichthyosaurs in late Permian
rocks, we should doubtless have the nearly complete chain between the
most highly specialized type of water reptiles and their terrestrial
ancestors.
[Illustration: FIG. 58.—Caudal fin of _Ichthyosaurus_, after Baur (left
figure); caudal fin of _Mixosaurus_, after Wiman (right figure).]
From the structure of the skeleton alone the early observers were
justified in inferring much concerning the shape and habits of the
living ichthyosaurs. Later discoveries have added so many definite
facts that, at the present time, we know more about their habits
than we do of any other extinct reptiles. In various places in
England and Germany, especially in Würtemberg, the remains of
ichthyosaurs are found in extraordinary abundance and perfection, not
only whole skeletons lying in the positions which they had assumed
after the decomposition of their bodies, but also often the actual
remains, carbonized, of the skin, muscles, and ligaments, as well as
delicate impressions of external parts. Many of these skeletons are
obtained from the numerous stone quarries, where they are a sort of
“by-product,” the sums received for them adding not a little to the
income of the quarrymen. So many are obtained in this and other ways
that specimens of ichthyosaurs are perhaps more frequently seen in the
museums of the world than those of any other extinct backboned animal.
Fairly complete skeletons may now be purchased of dealers in such
things for from fifty to seventy-five dollars. As may be supposed, the
best and most complete collections of these fossil remains are those of
the British Museum in London and the museum in Stuttgart. From a study
of those of the last-mentioned museum Professor Fraas has learned many
interesting facts and reached many interesting conclusions regarding
the life-habits of the ichthyosaurs. In the accompanying figure (Fig.
59) is shown a photographic reproduction of a very complete specimen,
in which not only is the outline of the whole body shown, but also much
of the carbonized remains of the muscles and skin has been detected.
[Illustration: FIG. 59.—_Ichthyosaurus quadricissus._ (From a
photograph from B. Hauff, Paleontologisches Atelier, Holzmaden.)]
The attachment of the paddles to the body was broad antero-posteriorly,
proving conclusively that they could not have been much used in
propulsion, either in the water or upon land, since such use would
require a fore-and-aft movement, and a consequent twisting or rotation
of the whole arm or leg, which, because of the broad attachment, must
have been very difficult, if not impossible.
Microscopic examination of the remains of skin preserved disclosed
an abundance of dark pigment, indicating, Professor Fraas believes,
that the skin was dark colored above. Doubtless, also, the under
side, as in nearly all swimming animals of the present time, was of
a lighter color, because such coloration rendered the animals much
less conspicuous in the water when seen either from above or below.
That the skin was bare is proved by many impressions or molds of it
that have been discovered in the rocks, in which many fine creases are
seen, but nothing suggesting scales or bony plates, save on the front
edge of the paddles, where impressions of overlapping scales have been
observed. This is an interesting fact, bearing witness that their land
ancestors had been covered everywhere with scales, much like those
of existing lizards and other reptiles. Scales or bony plates were
not only useless to the ichthyosaurs in the water, since they could
afford no protection, but would have been detrimental in increasing the
resistance in swimming.
That the ichthyosaurs were predaceous animals is of course evident
from their teeth, adapted for the seizure and retention of slippery
prey, but not for tearing or comminuting. The fossilized remains of
food found between the ribs of some specimens, in the place where the
stomach was, together with fossil excrement, called coprolites, usually
attributed to these animals, prove that they fed largely upon fishes,
squids, belemnites, and probably other invertebrates. One ichthyosaur
specimen preserved in the Stuttgart Museum has preserved in its stomach
contents a mass composed of the remains of more than two hundred
belemnites.
Most interesting of all is the fact that, not very rarely, embryonic
skeletons of ichthyosaurs have been found associated with the remains
of adult animals, in such positions that they must have been inclosed
within the body cavity at the death of the animals. As many as seven
such embryonic skeletons have been observed with a single specimen. At
first it was supposed that these skeletons were of small ichthyosaurs
which had been swallowed whole as food, since it is not at all likely
that these predaceous reptiles were discriminative in their choice
of food when hungry. It is not improbable that in some cases this is
the true explanation of the smaller skeletons within the larger ones,
but it cannot be true of all, since wherever the small skeletons are
identifiable they have been found to belong to the same species as the
adult, and it would be absurd to suppose an ichthyosaur bent upon its
prey would be at all likely to select as many as seven young animals,
all of the same size and all of its own species. Furthermore, some
of these young skeletons have been found in such positions as would
indicate that they were inclosed within their egg-covering at the time
of their death. Some of these embryos measure as much as twenty inches
in length.
Because the ichthyosaurs were born alive, and because so many of their
skeletons are found with their various parts in orderly relation
to each other, it is inferred with much probability that they were
inhabitants, in large part at least, if not exclusively, of the open
and deeper oceans. Had they been oviparous they must necessarily have
laid their eggs upon the beaches, since no reptiles of the present
time lay eggs in the water, and we have no other indications that
the reptiles of the past have ever done so. And such habits would
necessitate the periodical return to land. Had they been denizens of
shallow waters, like the mosasaurs and plesiosaurs for the most part,
their skeletons must surely have been disturbed by the currents and
tides, as also by predaceous fishes, breaking up or displacing them or
carrying away their bones. In shallow waters, also, the decomposing
bodies would have been more liable to despoliation by the many
scavengers of the seas.
The ichthyosaurs must have been quite helpless upon land, their limbs
being of little more use for locomotion than are the fins of fishes.
Breathing air as they did, they were of course not suffocated when
exposed, unless, as is the case with the whales, the feeble attachment
of the ribs prevented the action of the respiratory muscles. If
accidentally thrown upon the beaches, they doubtless were able to
return to their home element more easily than the fishes can, by
flopping, wriggling, and turning. As we have seen, the food consisted
in part, perhaps the larger part, of small invertebrates, and because
the bones of the lower jaws were closely united, permitting little or
none of that expansion so characteristic of the snakes, all their prey
must have been of relatively small size. In habit the ichthyosaurs were
doubtless, like the dolphins and gavials, inoffensive and harmless,
so far as animals of larger size were concerned. The abundance of
their remains often found in restricted localities, while deposits of
like age and character not far distant may be almost free from them,
suggests that in all probability the ichthyosaurs, or the later ones at
least, were more or less gregarious in habit as are the sea-mammals.
They probably lived in schools, as do the porpoises, each species
keeping to its restricted locality and not wandering far.
The ichthyosaurs began their existence, so far as we now know, about
the middle of Triassic times and continued to near the middle of Upper
Cretaceous, when they disappeared forever from geological history.
As we have seen, however, the earliest forms that we know were true
ichthyosaurs in all respects, though more primitive than the later
ones, indicating a long previous existence of which we yet have no
knowledge. Their remains have been found widely distributed in Triassic
rocks of Europe, Spitsbergen, Australia, and North America. During the
Jurassic period they lived in great numbers and variety throughout
the region that is now Europe. In North America the only marine rocks
of this period that we know of have yielded numerous remains. These
American ichthyosaurs were, however, among the most specialized of all
ichthyosaurs—the culmination of their development. They were originally
named _Sauranodon_ in the belief that they were toothless, but in
recent years their teeth, small and numerous, have been discovered. And
the genus seems also to be identical with one previously named from the
Jurassic of Europe called _Ophthalmosaurus_. The last known remains
of ichthyosaurs have recently been found in the Benton Cretaceous of
Wyoming. Scanty remains of ichthyosaurs are also known from Australia
and New Zealand. Why the ichthyosaurs should have gone out of existence
before the plesiosaurs and mosasaurs did, one cannot say; possibly
their stock had grown old and feeble.