SAUROPTERYGIA
Very scanty are the early human records of those strange reptiles
known as the plesiosaurs. Were one to search through the many works
published during the latter half of the seventeenth century and all
of the eighteenth, devoted to “lapides petrifacti,” “figured stones,”
“reliquia diluvii,” or by whatever other fanciful names fossils were
known, here and there he would probably find descriptions and figures
of bones of these reptiles. It would hardly seem that plesiosaurian
bones could have been overlooked by the curious, so abundant are they
in many places. But there is no such history of the early discovery
of the plesiosaurs as there is of the ichthyosaurs and mosasaurs.
Their birth into human history was very formal and proper, under
the ministrations of a learned doctor of science, the renowned
Conybeare, of whom we shall speak again. It was he, who with De la
Bêche, late Director of the British Geological Survey, described for
the first time, in 1823, one of these reptiles, to which he gave the
name _Plesiosaurus_, meaning “like a lizard.” He distinguished the
plesiosaurs from ichthyosaurs, with which it is possible that they had
previously been confounded, and gave a good description of considerable
material. Cuvier, a little later, gave a more complete description
of the same remains which had served Conybeare and De la Bêche for
their original description, and for the first time made it evident
that fossil plesiosaurs were widely and abundantly distributed over
the earth. The closing sentence of Cuvier’s chapter devoted to the
discussion of these creatures in his _Ossemens Fossiles_ was really
prophetic, not only of the many discoveries of the plesiosaurs yet to
be made, but of all other extinct animals as well: “I doubt not that,
in a few years it may be, I shall be compelled to say that the work
which I have today finished, and to which I have given so much labor is
but the first glimpse of the immense creations of ancient times.”
[Illustration: FIG. 31.—Restoration of _Plesiosaurus guilelmi
imperatoris_ (left figure) and _Thaumatosaurus victor_ (right figure),
Liassic plesiosaurs. (From E. Fraas.)]
In quick succession there followed many other discoveries of
plesiosaurs, not only in England but elsewhere in Europe. The famous
English anatomist and paleontologist, Sir Richard Owen, to whom we
owe, perhaps, more than to anyone else our present knowledge of these
animals, the eccentric Hawkins of England, the learned von Meyer of
Germany, and, in later times, more especially Seeley and Andrews of
England, Fraas of Germany, Bogalobou and Riabanin of Russia, as well
as many others, have brought to light during the past century many and
varied forms of those sea-reptiles. Blaineville in 1835 gave to the
plesiosaurs an ordinal rank under the class Ichthyosauria, and even the
astute Owen in 1839 united them with the ichthyosaurs as a suborder of
his Enaliosauria, or “sea-saurians.” He called them Sauropterygia, or
“reptile-finned,” and these terms, Enaliosauria, Ichthyopterygia, and
Sauropterygia, have long persisted in works on natural history because
of the prestige of Owen’s name. As we shall see later, the plesiosaurs
are really of remote kinship to the ichthyosaurs, and there is no such
natural group as the Enaliosauria. It often takes years to distinguish
between apparent and real relationships among living organisms, and
both of these groups of sea-saurians have had a sorry experience in the
treatment they have received from nomenclators.
Perhaps because of the writings of Dean Buckland in his famous
_Bridgewater Treatise_, in large part a theological disquisition,
though of real scientific merit, the ichthyosaurs and plesiosaurs
early became widely and popularly known, and, even to this day, these
reptiles, together with the dinosaurs, first made known by Rev. Dr.
Mantell, are often supposed to be the most typical and horrid of
monsters. Many and fabulous are the tales that have been told of
them in literature both grave and gay. The preacher adduced them as
evidences of the great world-catastrophe told in biblical history, and
the German student sings of them to the tune of the “Lorelei”:
Es rauscht in Schachtelhalmen, verdächtig leuchtet das Meer;
Da schwimmt mit Thränen in Auge ein Ichthyosaurus einher.
Ihn jammert der Zeiten Verderbniss, denn ein sehr bedenklicher Ton
War neuerlich eingerissen in der Liasformation.
Der Plesiosaurus, der alte, der jubelt in Saus und Braus;
Der Pterodactylus selber flog jungst betrunken nach Haus.
Der Iguanodon, der Lümmel, wird frecher zu jeglicher Frist;
Schon hat er am hellen Tage die Ichthosaura geküsst.
We now know that they were not the monsters of horrid mien that they
were once supposed to be: the largest plesiosaurs, were they living
today, would find unopposable foes in the vicious and cruel crocodiles.
They were relatively stupid and slow, cruel enough to the smaller
creatures, but of limited prowess. But in structure and habits they are
among the most remarkable of all the animals of the past or present.
Although their remains are among the most abundant and widely
distributed of all fossil reptiles, the plesiosaurs as a whole are less
perfectly known than either the ichthyosaurs or the mosasaurs, and it
has been within a comparatively few years only that an approximately
complete knowledge of any form has been obtained. This is partly due
to the fact that the order comprises vastly more kinds, more species,
genera, and families than does any other order of marine reptiles;
partly because their remains, though widely distributed over the earth,
and in rocks of many geological epochs, are seldom found completely
preserved; usually specimens comprise only a few bones or single bones,
and complete skeletons are rare. Were there but few kinds, the many
specimens discovered would mutually supplement each other, finally
completing our knowledge; but the fragments of many kinds only add to
our confusion. Nevertheless, because the plesiosaurs lived so long in
geological history, their remains are found in rocks of many different
kinds, and since it is improbable that any of them had great specific
longevity, it is very probable that all these described species, or
most of them, often made known from single bones, will eventually be
found to be distinct, and that many more will be added to them. It does
not seem improbable that within the next forty or fifty years not less
than a hundred species of plesiosaurs will have been discovered in
North America alone. At the present time perhaps that many have been
described from the whole world.
When Blaineville gave the name Plesiosauria to the aquatic reptiles
described by Conybeare, Cuvier, and others, he had no knowledge of
others of an intermediate kind between them and land reptiles. His
group-term then can be properly applied only to the truly aquatic
forms, and Owen’s name Sauropterygia becomes available in a wider
sense to include all the known types belonging to the order of which
the plesiosaurs form a part. Of this order then there are two clearly
marked divisions or suborders, the Plesiosauria and the Nothosauria,
the former having a complete aquatic adaptation, the latter only
a partial one. While the two suborders are evidently allied, some
authors have suggested that their differences are only familial; others
have thought that they are really orders. We shall see how close the
relationships are.
PLESIOSAURIA
It was Dean Buckland who facetiously likened the plesiosaurs to a snake
threaded through the shell of a turtle, and the simile was not an
inapt one in his day. The vernacular designation of them—long-necked
lizards—conveys the same impression of their chief peculiarity, but the
name is less applicable than it once was, since recent discoveries have
brought to light forms with a relatively short neck.
Though the plesiosaurs are nearly perfectly adapted to an aquatic
life, the adaptation was, in many respects, of a very different kind
from that of the ichthyosaurs—so very different that we have not yet
quite finished conjecturing as to the habits of the living animals. As
already suggested in the popular name, the most striking characteristic
of the typical plesiosaurs, the one which suggested to Buckland his
frequently quoted simile, is the ofttimes enormously long neck,
proportionately longer than that of any other known creatures of the
past or present. In other truly aquatic animals the neck is actually
shortened in the acquirement of a fish-like shape, and the number of
bones composing it reduced. In the Sauropterygia the neck is usually
longer than any truly land animals ever possessed, the longest-necked
forms having as many as seventy-six vertebrae in the cervical region.
The elongation of the neck among mammals is always due to an increase
in the length of the individual bones, never to an increase in the
number from seven, with but a single exception—a South American sloth
which has nine cervical vertebrae. The long neck of birds is due both
to an increase in the length of the individual vertebrae and to an
increase in their number, to as many as twenty-one. But the elongation
of the neck among plesiosaurs was very variable indeed; sometimes it
was ten or twelve times the length of the head, at other times it was
even shorter than the head. And the number of bones composing it was
also extremely variable, scarcely any two species having the same,
the known extremes being seventy-six and thirteen. In _Elasmosaurus
platyurus_, for instance, the longest-necked plesiosaur known, the head
was two feet in length, the neck twenty-three, the body nine, and the
tail about seven; on the other hand, in the shortest-necked plesiosaur
known, _Brachauchenius Lucasi_, the head was two and one-half feet
in length, the neck less than two feet, and the body about five; the
length of the tail is unknown.
[Illustration: FIG. 32.—Skeleton of _Trinacromerum osborni_, a
Cretaceous plesiosaur, as mounted in the University of Kansas Museum.]
Not only was the number of vertebrae so extraordinarily increased in
many plesiosaurs, but in the longest necks the vertebrae themselves, as
in birds, were more or less elongated, especially the posterior ones,
which may be six or seven times the length of the anterior ones. Not
only was the neck of such great length in many plesiosaurs, but it also
tapered very much toward the head.
The vertebrae are always biconcave, but the cavities are shallow,
saucer-like, sometimes almost flat at each end, and very different from
the conical fish-like cavities of ichthyosaurian vertebrae.
[Illustration: FIG. 33.—Restoration of _Elasmosaurus platyurus_, an
Upper Cretaceous plesiosaur.]
Often the vertebrae are short throughout the vertebral column;
sometimes the posterior cervicals and the dorsals are elongated and
very robust. The trunk or body proper was never much elongated in the
plesiosaurs, having only from twenty-five to thirty vertebrae. The
tail was always shorter than the trunk, and it tapered rapidly to the
extremity; in some specimens it has been observed to turn up slightly
near the extremity, as though for the support of a small terminal fin.
[Illustration: FIG. 34.—Cervical vertebrae, from the side and behind,
and dorsal vertebra from in front of _Polycotylus_, a Cretaceous
plesiosaur: _az_, anterior zygapophysis; _pz_, posterior zygapophysis,
_r_, _r_, _r_, cervical ribs; _d_, articulation of dorsal rib.]
The ribs in the cervical region are short, but so locked together
posteriorly as not to permit much lateral motion. They are sometimes
double-headed in the neck, sometimes single-headed, but both heads
when present articulate or are attached to the body of the vertebrae,
distinguishing them at once from those of other animals, except the
ichthyosaurs. In the dorsal region the ribs are attached high on the
arch to the extremity of the stout transverse processes by a single
head, very much as they are in some cetaceans, and quite unlike the
condition in any other known reptile. They end freely below, having
no attachment to a breast bone or other bony parts. Because of their
shape and position as frequently found, the body in life must have been
flattened from above downward, and broad; indeed, this shape is quite
certain because of the very broad expanse of the coracoids, between the
articulations of the front legs.
[Illustration: FIG. 35.—Pectoral girdle of _Trinacromerum_ from above:
_ic_, interclavicle; _cl_, clavicle; _sc_, scapula; _c_, coracoid.]
The shoulder-girdle or pectoral arch is strangely unlike that of any
other reptiles. There is no breast bone, since the breast bone is a
comparatively late development in reptiles, not appearing, probably,
until after the plesiosaurs had begun their existence. Taking the
place of the sternum, the very large and broad coracoidsjoin each
other in the middle, forming a sort of subdermal armor on the under
side of the body in front. In some of the largest plesiosaurs these
two bones measured together about six feet in length by four in
width. Though so very large they are thick only in front between the
articulations of the forelegs. The shoulder-blades are much reduced
in size and are extraordinarily modified. The blade proper, that is,
that part extending backward and upward, is narrow and small, affording
but little surface for the attachment of muscles. On the inner side,
extending toward the middle in front of the coracoids, there is another
projection, often broad and large, to which was attached the clavicles
when present, and often this projection met its mate of the opposite
scapula in the middle in front of the coracoids in a broad union. The
clavicles or collar-bones are small and thin, and sometimes absent;
they also are united in the middle posteriorly with the coracoids when
the scapula did not intervene. And the interclavicle also is sometimes
wanting. Altogether the pectoral bones form a very large, broad, and
concave trough inclosing the whole of the under side of the anterior
part of the body. This extensive surface must have furnished attachment
to stout and strong muscles controlling the downward and inward motion
of the paddles.
There is a well-developed sacrum of three vertebrae for the support
of the pelvis or hip bones. The reason for its persistence in animals
so thoroughly adapted for life in the water will be understood later.
The ilium is slender; it was attached to the sides of the sacrum by
ligaments, only, not forming a firm union, but strong nevertheless.
The pubes and ischia, the other bones of the pelvis on the under side
of the body, like the corresponding bones of the pectoral girdle, were
enormously enlarged, forming great flat, bony plates.
Besides these large bony plates of the shoulder and pelvic girdles,
the short abdominal region was inclosed by numerous series of strong
ventral ribs, that is, overlapping rod-like bones on each side,
connected with a central piece. It will be seen that the whole under
side of the body, from the base of the neck to the base of the tail,
was well protected by bones, rigid and unyielding in front and behind,
flexible for a short space below the abdomen; this surface, however,
was not flat like the under shell of a turtle, but rounded from side to
side.
[Illustration: FIG. 36.—Pelvic girdle from above of _Trinacromerum
osborni_, an Upper Cretaceous plesiosaur: _p_, pubis; _is_, ischium;
_il_, ilium.]
Many of the characteristics of the limbs of the plesiosaurs are
peculiar to themselves; others they had in common with other aquatic
reptiles and mammals. The paddles resemble those of the ichthyosaurs
more nearly than those of any other reptile, and it was doubtless this
superficial resemblance which so long deceived the early anatomists
as to the affinities of the two orders. Unlike all other aquatic
animals, however, the plesiosaurs have the hind limbs nearly or quite
as large as the front ones, and they doubtless were equally effective
in function. The humerus and femur are always elongate, though broad
and massive. In no other aquatic animals, save the marine turtles,
do we find these bones relatively so long and strong; they are very
short in the cetaceans, the sirenians, the ichthyosaurs, mosasaurs,
thalattosaurs, and the marine crocodiles, in front at least. The strong
muscular rugosities of the plesiosaurian bones are very suggestive of
powerful swimming muscles.
[Illustration: FIG. 37.—Pelvic girdle of _Elasmosaurus_: _p_, pubis;
_is_, ischium; _il_, ilium.]
[Illustration: FIG. 38.—Paddles of Plesiosaurs: _A_, right hind
paddle of _Thaumatosaurus_, after Fraas; _B_, right hind paddle of
_Trinacromerum_; _C_, right front paddle of same individual; _f_,
femur; _fb_, fibula; _t_, tibia; _h_, humerus; _r_, radius; _u_, ulna.]
The bones of the forearms and legs, the wrists and ankles are all
polygonal platelets of bones, closely articulating with each other.
The finger and toe bones have a more elongated, hour-glass shape
than those of the ichthyosaurs, resembling more nearly those of the
mosasaurs, indicating a greater flexibility than the ichthyosaurs
possessed. The ichthyosaur paddles must have been quite like the fins
of fishes in function, while doubtless those of the plesiosaurs were
capable of a more varied use, as indeed was required of them. Their
articulation with the trunk was more of a ball-and-socket joint than in
the other reptiles, showing possibility of considerable rotation on the
long axis, and an antero-posterior propelling action. The paddles were
certainly more powerful than those of any other aquatic air-breathing
animals. There were no additional digits, all plesiosaurs having
neither more nor less than five in each hand and foot. Hyperphalangy
was sometimes carried to an excessive degree, some digits of some
species having as many as twenty-four bones, a larger number than has
been observed in any other air-breathing vertebrate.
[Illustration: FIG. 39.—Pectoral girdle (in part) and front paddles
of _Elasmosaurus_ (after Riggs): _sc_, scapula; _h_, humerus; _cor_,
coracoid; _r_, radius; _u_, ulna.]
In Fig. 38 on p. 85 are shown two paddles, the front and hind paddles
of a single individual of a very specialized plesiosaur from the
Upper Cretaceous of Kansas (_Trinacromerum_). The long arm and thigh
bones are followed by remarkably short and broad bones in place of
the elongated forearm and leg bones of the land reptiles. Not only
are these bones much broader than they are long, but there have been
developed additional bones back of them in the same row—new bones
which have no counterpart in any terrestrial reptiles. In the first of
the three figures is shown a hind paddle of one of the earliest known
plesiosaurs, _Thaumatosaurus_, from the lower part of the Jurassic of
Germany. It will be seen here that the tibia and fibula are much more
elongated than in _Trinacromerum_, and much more like the leg bones
of land reptiles. A still more primitive stage in the evolution of
the swimming paddle of the plesiosaurs will be seen in Fig. 48 on p.
99, the possibly ancestral, amphibious nothosaur. Here the tibia and
fibula, while relatively very much shorter than in any land reptile,
still have, together with all the other bones of the leg, a terrestrial
or amphibious type. In Fig. 39 is seen the front paddles of the
long-necked _Elasmosaurus_, which, though one of the latest of all
plesiosaurs in geological history, has the structure of its paddles
somewhat intermediate between that of the earlier _Plesiosaurus_ and
the later _Trinacromerum_.
[Illustration: FIG. 40.—Skull of _Elasmosaurus_ from the side: _pm_,
premaxilla; _m_, maxilla; _po_, postorbital; _j_, jugal.]
The skull of the long-necked plesiosaurs is surprisingly small in
comparison with the remainder of the skeleton, often very snake-like
in shape, though very un-snake-like in structure. The short-necked
plesiosaurs had often a relatively larger skull, in _Pliosaurus_, for
instance, more than five feet long, sometimes rather broad and short,
sometimes remarkably long and slender. The external nostrils were
situated far back, very near the eyes, and were very small. The eyes,
of considerable size, though by no means so large as those of the
ichthyosaurs, were directed laterally, and were provided with a ring of
bony sclerotic plates—rather small and weak ones, however. The quadrate
bones—bones peculiar to the reptiles and birds—to which the lower jaws
are articulated, are, as in the ichthyosaurs and crocodiles, rigidly
fixed and immovable. The lower jaws, always rather slender, are firmly
united in front, sometimes for a long distance, as in the modern
gavials. The teeth of the broad-headed plesiosaurs are long, slender,
pointed, and recurved, of a murderously cruel shape; they are deeply
implanted in sockets, and number from twenty to thirty on each jaw
above and below. There are no teeth on the bones of the palate, such
as the mosasaurs possessed. The slender-jawed, gavial-like plesiosaurs
have more numerous, but smaller teeth. The surface of the skull on each
side behind, for the attachment of the muscles closing the mandibles,
is of great extent; in some this surface is increased by a high, thin
crest in the middle, as in strongly carnivorous animals, all of which
give conclusive evidence of the powerful muscles used in biting and
seizing. There is but one temporal opening on each side, as in the
ichthyosaurs and the mosasaurs, whereas the crocodiles, thalattosaurs,
phytosaurs, etc., have two. The brain cavity of all plesiosaurs is
small, though the cavities of the internal ears, the semicircular
canals at least, are large. The semicircular canals in vertebrates have
little or nothing to do with the function of hearing; they serve rather
for equilibration, for the co-ordination of muscular movement; possibly
we may infer from their large size in the plesiosaurs that they were
not at all clumsy in their movements. There is a large opening for
the pineal body, the so-called eye in the roof of the brain cavity,
though its possession does not necessarily imply the possession of a
functional organ.
[Illustration: FIG. 41.—Skull of _Trinacromerum_ from the side: _ang_,
angular; _d_, dentary; _pm_, premaxilla; _po_, postorbital; _j_, jugal;
_sur_, surangular.]
The Plesiosauria included some of the largest aquatic reptiles that
have ever existed, equaled, perhaps, though not exceeded, by some of
the extinct crocodiles. The largest known are probably those of the
Kansas chalk, or the Jurassic of Wyoming, which probably reached a
length of nearly or quite fifty feet, of which the neck formed about
one-half. Some of them had paddles more than six feet in length. The
head of the largest was about five feet in length, or about the size
of that of the largest known ichthyosaurs and mosasaurs. The smallest
known adult plesiosaurs were nearly ten feet in length. The teeth of
the largest and most carnivorous plesiosaurs sometimes measure four
inches in length.
[Illustration: FIG. 42.—Restoration of _Trinacromerum_, a Cretaceous
plesiosaur; length about ten feet.]
As is the case with both the ichthyosaurs and mosasaurs, skeletons
of plesiosaurs have been discovered with nearly all their bones in
their relative positions, and with impressions of skin and outlines of
body made before decomposition. Though our knowledge of the external
appearance of the plesiosaurs when alive is perhaps not as full as we
could wish, it is sufficient to give us a fairly good conception of
what the animals really were. The skin was smooth and bare, without
scales or plates of any kind, and Dames has described a terminal or
nearly terminal fleshy dilatation of the tail, forming a sort of caudal
fin, which may have aided as a steering apparatus. Mounted skeletons
are preserved in a few museums, notably the British Museum, the
American Museum of New York City, and the museum of the University of
Kansas. Many nearly complete skeletons, however, preserved as they were
found in the matrix, are shown in various museums.
With these, principal facts regarding the structure, size, and external
form of these animals we may venture to draw certain conclusions, or at
least to offer certain conjectures as to their habits in life.
Because of the rigid structure of the jaws, united in front and
incapable of any lateral movement posteriorly, quite as are the jaws
of crocodiles, we are sure that prey of any considerable size could
not have been swallowed whole. The crocodiles tear away portions of
the flesh of their victims by quick, powerful jerks, and it is very
probable that the flat-headed plesiosaurs tore their food apart in
the same manner. In these kinds the teeth are much larger and more
irregular in size than are those of the long-snouted plesiosaurs, and
their use was certainly as much for tearing as for seizing. There are
the same differences between the size of the head and the size of the
teeth among the various plesiosaurs that there are among the modern
crocodiles and gavials. While the crocodiles seize and destroy even
larger prey, drowning and tearing their victims to pieces, the gavials
are more exclusively fish-eating, for which their small, sharp, and
more numerous teeth especially fit them. Their food, of small size,
is swallowed entire, and they are comparatively harmless, so far as
animals of considerable size are concerned.
The long neck, the thickset body, and short, stout tail are not at
all what we should expect to find in quick-swimming animals. We may
therefore assume that the motions of the plesiosaurs through the
water were more turtle-like than fish-like. The tail, even though
provided with a terminal, fin-like dilatation, was of little use in
the propulsion of the body, since the range of its movements was
restricted; it possibly served in a measure as a steering organ, a
rudder. The large, freely movable paddles must have been effective
organs of locomotion, and this function accounts for the relatively
large size of the posterior pair, and the firm union of the pelvis with
the vertebral column through the sacrum. With the hind limbs used as
oar-like organs, a firmer union with the skeleton was required than
the soft yielding flesh would permit. At the same time this union
was ligamentous only, not bony and unyielding, since the limbs were
never used to support the body upon the ground; and it is of interest
to observe that the ilia are directed, not upward and forward, but
upward and backward to the sternum, precisely the position that would
be expected with the force or thrust coming from behind, and not
below the yielding ligaments. Were the tail longer and more powerful,
the hind limbs would have been smaller and weaker, of use chiefly in
equilibration, involving the loss of any connection with the vertebral
column and the disappearance of the sacrum. It is of interest, finally,
to observe that many of the slender-jawed plesiosaurs had a relatively
short neck; they were doubtless more distinctively fish-eating in
habit, and possessed greater speed. That the limbs of plesiosaurs
were powerful propelling organs is also conclusively proved by their
structure. Quite unlike all those animals whose locomotion in the water
is chiefly effected by the tail, the humeri and femora, the upper
arm and thigh bones were elongated, and not shortened. They form the
rigid and stout handles of oars whose blades are the thinner, flexible
forearm, wrist, and fingers, or the corresponding foreleg, ankle,
and toes. No other purely aquatic reptiles, save the turtles, which
likewise are of the oar-propelled type, have elongated arm and thigh
bones.
Textbook illustrations of the plesiosaurs usually depict the necks,
like those of the swans, freely curved, and a popular scientific
article in one of our chief magazines a few years ago depicted one
of them with the neck coiled like the body of a snake. One noted
paleontologist, indeed, not many years ago described the plesiosaurs as
resting on the bottom in shallow waters with the neck uplifted above
the surface viewing the waterscape! And when we consider the fact that
some species of the elasmosaurs had a neck not less than twenty feet in
length, such a flexible use of it would not seem improbable. But the
plesiosaurs did not and could not use the neck in such ways. They swam
with the neck and head, however long, directed in front, and freedom
of movement was restricted almost wholly to the anterior part. The
posterior part of the neck was thick and heavy, and could not have been
moved upward or downward to any considerable extent and not very much
laterally. From all of which it seems evident that the plesiosaurs
caught their prey by downward and lateral motions of their neck, rather
than by quick swimming.
[Illustration: FIG. 43.—Gastroliths and bones of an undetermined
plesiosaur from the Lower Cretaceous of Kansas.]
About thirty years ago, the late Professor Seeley, a well-known English
paleontologist who devoted much attention to the study of these
reptiles, found with the remains of a medium-sized plesiosaur nearly a
peck of smoothly polished, rounded, and siliceous pebbles. He believed
that their occurrence with the skeleton was not accidental, but that
they had been intentionally swallowed by the animal when alive, and
formed at its death a part of its stomach contents. Even earlier than
this the same habit had been noticed. Nearly at the same time that
Seeley mentioned the peculiar discovery he had made the present writer
found several specimens of plesiosaurs in the chalk of western Kansas
with which similar pebbles were associated, an account of which was
given soon afterward by the late Professor Mudge. Since then numerous
like discoveries have made it certain that the plesiosaurs usually,
if not always, swallowed such pebbles in considerable quantities, for
what purpose we do not yet feel sure; one can only hazard a guess. The
small size of the pebbles, or gastroliths, as they have been called,
a half-inch or less in diameter, found with skeletons of large size,
indicate much more complete digestion of the hard parts of their food
than is the case with many other reptiles; no solid substance of size
could have passed out of the plesiosaur stomach, and such is the case
with the modern crocodiles, which have a like habit of swallowing
pebbles. That the plesiosaurs picked up these siliceous pebbles,
sometimes weighing a half-pound, accidentally with their food is highly
improbable; they surely had something to do with their food habits. It
is not at all unreasonable to suppose that the plesiosaurs, because of
their comparative sluggishness, fed upon anything of an animal nature,
whether living or dead, which came in their way; that carrion, squids,
crustaceans, and fishes were all equally acceptable; they were probably
largely scavengers of the old oceans. Barnum Brown found among the
stomach contents of a plesiosaur fragments of fish and pterodactyl
bones, and cephalopod shells. Gallinaceous birds, most of which have
the same pebble-swallowing habit, have a thick-walled muscular stomach
or gizzard, in which the pebbles serve as an aid in the trituration of
food. Modern crocodiles, with the same pebble-swallowing habit, have a
thick-walled muscular stomach, gizzard-like, though of course not as
large as in birds; and the same habit has been noted by Des Longchamps
in the ancient teleosaur crocodiles.
It is hardly possible yet to decide whether or not the plesiosaurs
were denizens of the open oceans for the most part, far from land.
That many of them were rovers is quite certain. With the skeleton of
a large plesiosaur found some years ago in western Kansas, there were
many siliceous pebbles which could have come only from the shores
of the old Cretaceous seas about the Black Hills, hundreds of miles
distant. Some of the pebbles are red quartzite, quite identical with
that of the bowlders brought to Kansas millions of years later by the
glacial drift from outcroppings near the northern line of Iowa. The
bones of plesiosaurs are often found in deposits believed to be of
deep-water origin. But they are also found in Kansas associated with
the remains of small turtles, flying reptiles, and birds which could
only have lived near the shores. Indeed, their remains have often
been found with those of strictly fresh-water animals which had been
brought down by the floods to the seas. Their wide but rather sparse
distribution in all kinds of marine sediments would rather indicate
that they were at home far out in the tempestuous ocean or near the
shores in protected bays, though probably they preferred the shallow
water littoral regions. One conclusion is quite justified: they were
not gregarious, as were the ichthyosaurs.
It is not certain that the plesiosaurs were viviparous, though there
are good reasons for the belief that they were. Remains of two embryos
were found years ago in England associated in such a way that it is
reasonable to suppose they were unhatched young, though embryos have
never yet been found associated with skeletons of adults, as have those
of ichthyosaurs in numerous instances. Bones of young, often quite
young, plesiosaurs, are frequently found in shallow water deposits, and
if the young were actually born alive they must have swum freely in the
open waters while yet of very tender age. Rather singularly, however,
the remains of these young plesiosaurs always occur as isolated bones.
In geological range the plesiosaurs were very persistent, extending
through nearly all the Mesozoic. They began their career as fully
evolved plesiosaurs, so far as we now know, near the close of the
Triassic period, and reached their culmination in the Upper Cretaceous,
but survived to the close of that period. In the beginning of their
career they were associated with the marine crocodiles and the
ichthyosaurs, but outlived them to find companions and probably
enemies in the huge and voracious mosasaurs of the later Cretaceous
times. At no time do they appear to have been especially numerous, nor
does it seem probable that they were ever a dominant type of marine
vertebrate life, though their remains occur everywhere that marine
deposits of the Jura and Cretaceous are known. Indeed, it may be said
with almost certainty that rocks of these ages and of that character
everywhere in the world contain fossil plesiosaurs. Their bones have
been made known from Europe, Asia, Africa, Australia, and North and
South America. From North America thirty or more species have been
described from New Jersey, Alabama, Mississippi, Texas, Arkansas,
Kansas, Nebraska, Colorado, New Mexico, Wyoming, North and South
Dakota, California, etc.
The cause of their final extinction no one knows, nor can we
conjecture much about it with assurance. That climatic conditions
became unfavorable for them is highly improbable, considering
their cosmopolitan habits; they were not discriminating in their
environments. After successfully withstanding their fiercest foes, the
ichthyosaurs, crocodiles, and mosasaurs, and large carnivorous fishes,
it does not seem probable that they would succumb to lesser enemies,
though it may be that they were finally attacked successfully, not
in the fulness of their strength as adults, but while young, by more
insidious enemies. More probably after their long life of millions of
years they had grown old, as everything grows old, and had become so
fixed and unplastic in their structure and habits that even slight
causes were at last their undoing. When we shall have bridged over
that still imperfectly known transition period between the great Age
of Reptiles and the greater Age of Mammals we shall have learned
more definitely some of the causes of the extraordinary revolution
in vertebrate life that then occurred. The plesiosaurs went out
with nearly all of their kind, the mosasaurs, the pterodactyls, the
dinosaurs; and, so far as we now know, their places in the sea, land,
and air were not immediately taken by any other creatures.
[Illustration: FIG. 44.—_Nothosaurus_; restoration after E. Fraas;
landscape by Dorothy Williston.]
NOTHOSAURIA
A few years after the discovery of the plesiosaurs by Conybeare, the
remains of animals of allied kinds were found in the Triassic rocks of
Bavaria. At first they were supposed to be those of true plesiosaurs,
and even the astute Cuvier was not very clear about them. Cuvier was
the first to call attention to them, expressing the opinion that
some of the fossils were of previously unknown animals allied to the
crocodiles, lizards, and plesiosaurs. It was von Meyer, however, who
first introduced a nothosaur to the scientific world under the name
_Conchiosaurus_. A year later Count George of Münster described other
forms under the name _Nothosaurus_, meaning “false lizard.” Count von
Münster was a most zealous collector of the fossils of the Triassic
deposits of Bavaria, amassing, after thirty years of active and
enthusiastic labor, a very large amount of material, which, at his
death, was purchased by the King of Bavaria and placed in the hands of
von Meyer for study. Von Meyer was to Germany what Owen was to England,
a man of deep learning, having an extensive knowledge of comparative
anatomy, and being thorough and critical in his work. His descriptions
and illustrations of these rich collections made by von Münster are
masterpieces of scientific thoroughness. He recognized in _Nothosaurus_
and other allied forms from the Bavarian Triassic a distinct group of
semiaquatic reptiles allied to the plesiosaurs, and his conclusions
have never been gainsaid. In more recent years additional remains
of these animals from Bavaria and other places in Europe have been
described, but none are known from other parts of the earth, or from
other than Triassic rocks. Altogether about ten genera and about twice
as many species have been described, probably all belonging in one
family, and all by common consent now classified with the Sauropterygia.
[Illustration: FIG. 45.—Head and neck of _Nothosaurus_; photograph of
specimen in the Senckenberg Museum, from Dr. Dreverman.]
[Illustration: FIG. 46.—Pectoral girdle of _Nothosaurus_, from
photograph by E. Fraas: _icl_, interclavicle; _cl_, clavicle; _sc_,
scapula; _cor_, coracoid.]
[Illustration: FIG. 47.—Pelvic bones of _Nothosaurus_: _il_, ilium;
_ac_, acetabulum; _p_, pubis; _is_, ischium. (After Andrews.)]
The Nothosauria were much smaller reptiles than the plesiosaurs, none
of them perhaps exceeding the size of the smallest known plesiosaurs.
They were semiaquatic in habit, with many curious resemblances to
other semiaquatic reptiles of a later time known, as the dolichosaurs.
The neck is more or less elongated, having about twenty vertebrae in
the longest-necked forms; the body is moderately long, and broad,
and the tail is relatively short. The vertebrae and ribs are quite
like those of the plesiosaurs, that is, the vertebrae are gently
concave at each end, and the dorsal ribs are attached by a single
head to the transverse process high up on the arch; the cervical ribs
are double-headed, precisely like those of the older plesiosaurs,
one of the characters which insistently proves the relationships of
the two groups. The bones of the shoulders (Fig. 46) also have many
resemblances to the extraordinary ones of the plesiosaurs, though
they are much less specialized. There was no sternum; the coracoids
are large, though very much smaller than those of the plesiosaurs.
The collar-bones are large and strong, joining each other in front
of the coracoids and firmly united with the shoulder-blades at the
outer extremity. Four vertebrae are united to form a sacrum, and their
union with the hip bones (Fig. 47) was much firmer than was the
case with the plesiosaurs. The limbs are elongated, but it will be
observed in the figures (Fig. 48) that the radius and ulna, tibia and
fibula, that is, the bones of the forearm and of the leg proper, are
relatively very short as compared with the humerus and femur, a sure
indication of the beginning of aquatic habits. The toes and fingers
were doubtless webbed, and there was no increase in the numbers of
bones in the digits, so conspicuous in the plesiosaurs. The external
nostrils are large, but are not situated so far back near the eyes as
in the plesiosaurs. There is a large pineal opening in the top of the
skull, as in the plesiosaurs, but no sclerotic or bony plates have
been observed in the eyes. They had ventral ribs like those of the
plesiosaurs.
[Illustration: FIG. 48.—Legs of _Lariosaurus balsami_, an Upper
Triassic nothosaur: _h_, humerus; _r_, radius; _u_, ulna; _i_,
intermedium; _ue_, ulnare; _f_, femur; _fi_, fibula; _t_, tibia; _a_,
astragalus; _c_, calcaneum. (After Abel.)]
No impressions of scales or bony plates have ever been found with the
remains of the nothosaurs, and it is the belief that the skin was
bare. A good idea of their general appearance will be gained from the
accompanying restoration adapted from that of Professor Fraas (Fig. 44)
and the restoration of the less highly specialized _Lariosaurus_, made
from a very complete skeleton in the Frankfort museum (Fig. 49).
It has been thought that these nothosaurs, so intermediate in structure
between the true plesiosaurs and land reptiles, were the actual
ancestors, but this is rather doubtful. It is probable that they were
only very closely akin to the real ancestors, since in some ways they
had become specialized too much, and, as we have already explained,
highly specialized characters or organs can never go back to their
earlier condition. The nothosaurs do prove beyond all possibility of
doubt that the plesiosaurs were at least the descendants of animals
closely allied to them, so closely, indeed, that it is doubtful whether
we could distinguish external differences were all of them actually
living at the present time.
[Illustration: FIG. 49.—_Lariosaurus balsami_.]
We have repeatedly seen that all aquatic animals have some or all the
bones of the limbs shortened, and it is of interest to observe that the
early plesiosaurs had longer forearm and foreleg bones than the later
ones, just as we have seen was the case with the early ichthyosaurs.
It would seem probable that all the early plesiosaurs had long necks,
though some of the late ones in Cretaceous times had relatively short
necks, shorter even than the known nothosaurs possessed.
The nothosaurs doubtless lived about the shores of the ancient seas,
spending much of their time in the water, leaving it perhaps when hard
pressed by their enemies, as do some modern reptiles, or to rear their
young. The teeth of the nothosaurs are long and slender in front,
shorter behind. The animals must therefore have been carnivorous in
habit, feeding probably upon such fishes as they could catch, and the
various invertebrates which live in shallow water. The structure of the
jaws and their attachments are quite as in the plesiosaurs, proving
that they could not have swallowed large objects; but the skull is
broader and flatter than that of most plesiosaurs, indicating habits
not unlike those of the modern alligators and crocodiles.
Some time we shall doubtless find remains of nothosaurs or nearly
allied animals elsewhere than in Europe, but probably not from later
deposits than the Triassic. So far as we now know, their geological
range and geographical distribution were much restricted; they
evidently wholly died out shortly after the plesiosaurs appeared.