Samuel Wendell Williston.

Water reptiles of the past and present online

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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 com-
pleting 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



SAUROPTERYGIA 77

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 differ-
ent 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 ani-
mals 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



78 WATER REPTILES OF THE PAST AND PRESENT

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 com-
posing 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, Brachau-




FiG. 32. — Skeleton of Trinacromerum osbomi, a Cretaceous plesiosaur, as mounted
in the University of Kansas Museum.

chenius 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.

Not only was the number of vertebrae so extraordinarily
increased in many plesiosaurs, but in the longest necks the verte-
brae 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.



SA UROPTERYGIA




8o



WATER REPTILES OF THE PAST AND PRESENT



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.

The ribs in the cervical region are short, but so locked together
posteriorly as not to permit much lateral motion. They are




Fig. 34. — Cervical vertebrae, from the side and behind, and dorsal vertebra from
in front of Polycotylus, a Cretaceous plesiosaur: az, anterior zygapophysis; pz, pos-
terior zygapophysis, r, r, r, cervical ribs; d, articulation of dorsal rib.



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 trans-
verse 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 flat-
tened from above downward, and broad; indeed, this shape is



SAUROPTERYGIA 81

quite certain because of the very broad expanse of the coracoids,
between the articulations of the front legs.

The shoulder girdle or pectoral arch is strangely unlike that of
any other reptiles. There is no breast bone, since the breast bone




Fig. 35. — Pectoral girdle of Trinacromerum from above: ic, interclavicle; cl,
clavicle; sc, scapula; c, coracoid.

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 coracoids



82 WATER REPTILES OF THE PAST AND PRESENT

join 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 inter-
clavicle 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 under-
stood 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
fiat, 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,



SAUROPTERYGIA 83

however, was not flat like the under shell of a turtle, but rounded
from side to side.




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



84



WATER REPTILES OF THE PAST AND PRESENT



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




Fig. 37. — Pelvic girdle of Elasmosaurus: p, pubis; is, ischium; il, ilium



so long and strong; they are very short in the cetaceans, the sire-
nians, 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.

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



SAUROPTERYGIA



85



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




4
WW?

iffl



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 indi-
vidual; /, femur; fb, fibula; t, tibia; h, humerus; r, radius; u, ulna.

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



86



WATER REPTILES OF THE PAST AND PRESENT



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.




Fig. 39. — Pectoral girdle (in part) and
front paddles of Elasmosauriis (after Riggs) :
sc, scapula; h, humerus; cor, coracoid; r,
radius; u, ulna.



m



B
w




In Fig. 38 on p. 85 are shown two paddles, the front and
hind paddles of a single individual of a very specialized ple-
siosaur from the Upper Cretaceous of Kansas (Trinacromerum) .
The long arm and thigh bones are followed by remarkably short and



SAUROPTERYGIA 87

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, Thauma-

tosaurus, 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 Elasmo-

saurus, which, though one of

the latest of all plesiosaurs in _ ■ 01 „ , _, , .

L .biG. 40. — bkull or Klasmosatiriis irom the

geological history, has the s ide: p,n, premaxilla; m, maxilla; po, post-
structure of its paddles some- orbital; j, jugal.
what intermediate between

that of the earlier Plesiosaurus and the later Trinacromerum.
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, mPlio-
saurus, 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 pecu-
liar to the reptiles and birds — to which the lower jaws are articu-
lated, are, as in the ichthyosaurs and crocodiles, rigidly fixed and




88 WATER REPTILES OF THE PAST AND PRESENT

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 attach-
ment 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




Fig. 41. — Skull of Trinacromer'um from the side: ang, angular; d, dentary; pm,
premaxilla; po, postorbital; j, jugal; sur, surangular.

and the mosasaurs, whereas the crocodiles, thalattosaurs, phyto-
saurs, 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; possi-
bly we may infer from their large size in the plesiosaurs that they
were not at all clumsy in their movements. There is a large open-
ing for the pineal body, the so-called eye in the roof of the brain
cavity, though its possession does not necessarily imply the pos-
session of a functional organ.

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



SAUROPTERYGIA



89



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.

As is the case with both the ichthyosaurs and mosasaurs,
skeletons of plesiosaurs have been discovered with nearly all their




Fig. 42. — Restoration of Trinacromerum, a Cretaceous plesiosaur; length about
ten feet.



bones in their relative positions, and with impressions of skin and
outlines of body made before decomposition. Though our knowl-
edge 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



QO WATER REPTILES OF THE PAST AND PRESENT

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 ga vials. 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 consid-
erable 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 move-
ments 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.



SA UROPTER YGIA 9 1

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 con-
sider 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, how-
ever 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.



9 2



WATER REPTILES OF THE PAST AND PRESENT



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.




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 plesio-
saur nearly a peck of smoothly polished, rounded, and siliceous



SAUROPTERYGIA 93

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


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Online LibrarySamuel Wendell WillistonWater reptiles of the past and present → online text (page 7 of 19)