CROCODILIA
The order of reptiles to which the name Crocodilia is technically
applied comprises less than twenty-five living species, popularly
known as crocodiles, alligators, caimans, and gavials. They are often
of great size, ugly and repulsive in appearance, cruel and vicious
in habit, wholly carnivorous, and denizens, almost exclusively, of
fresh-water lakes or rivers in tropical and subtropical regions; a few
only venture into the sea near the shores. They are all excellent and
powerful swimmers, but are by no means exclusively aquatic in habit,
many of them spending a large part of the time on the shores; and they
invariably seek the land for the deposition and hatching of their eggs.
In size they are the largest of living reptiles, some of the existing
species reaching a length of twenty-five feet, while some extinct
species were probably fully twice that length.
The geological history of the crocodiles is a very ancient one,
reaching back at least as far as the early Jurassic and probably to
the Triassic. The culmination of the order, at least so far as size,
variety, and numbers are concerned, was doubtless before the close of
the Mesozoic. The early crocodiles, however, were of a more generalized
structure in some respects, though specialized in others, because of
which naturalists in the past have usually divided the order into three
or four chief subdivisions, or suborders, two of which, the Mesosuchia
and the Thalattosuchia, became extinct before or during Cretaceous
time. The third suborder, the Parasuchia of many textbooks, has now
been unanimously separated by paleontologists from the Crocodilia as an
independent order. The fourth suborder of the textbooks, the Eusuchia,
a word meaning _true_ crocodiles, appeared in geological history, so
far as we yet know, shortly before the extinction of the Mesosuchia,
and is best known from the forms now living, all of which belong to
it. Although the modern crocodiles can hardly be called, as a group,
purely aquatic reptiles, we shall find it of interest, because of their
intimate relation to the older and more strictly marine forms, to speak
of them somewhat in detail.
MODERN CROCODILES, EUSUCHIA
The crocodiles of the present—and we use the word in the technical
sense of Crocodilia—because of their general resemblance to the
lizards, or true “saurians,” were classed with them by the older
naturalists, whence comes the popular name alligator, a corruption
of the Spanish _el lagarto_, or “the lizard,” given to some of
the South American forms by early explorers. But this resemblance
is a superficial one only, as was early recognized by comparative
anatomists. The crocodiles, indeed, are only remotely related to the
lizards.
[Illustration: FIG. 98.—Senegal crocodile. (By permission of the New
York Zoölogical Society.)]
The head or cranium is flattened and broad, the facial part or snout
sometimes greatly elongated and slender, and the external nostrils
are always situated at the front end. The bones of the upper surface
of the cranium and face have many pit-like depressions. The neck is
short and stout, and but little movable. The body is somewhat depressed
and flattened, not cylindrical and slender, as in the more typical
water reptiles. The tail is much elongated and compressed, forming a
powerful means of propulsion in swimming, its breadth being increased
by a vertical row of horny plates above. The limbs are of the ordinary
elongated type—ambulatory rather than swimming legs—and are not of much
use for propelling the body in the water; the front legs indeed are
usually held close to the body while the animal is swimming. The toes,
however, are partially connected by webs, to a slight extent only in
the alligators and crocodiles, but much more so in the long-snouted
gavials. The feet have five toes in front and four behind; and the loss
of the fifth toe can only be ascribed to terrestrial habits. The body
is covered more or less with horny scutes or scales, beneath which are
several rows of thickened, pitted, bony plates on the dorsal side, and
sometimes also on the under side, forming a more or less extensive
bony armor. The eyes have movable lids, as in most lizards, and the
ear-opening is small.
But the external appearance of these reptiles is not sufficient to
distinguish them widely from other groups, and we must resort to
the internal structure, especially that of the skeleton, for the
more essential differential characters. The most crucial of these,
the one which more than any other determines their relationships,
consists in the position and shape of the bone with which the lower
jaw articulates, the quadrate bone, so characteristic of reptiles. As
in the plesiosaurian and ichthyosaurian skulls, it is firmly united
with the adjoining bones, not articulating freely with them, as in the
lizards and snakes. But this fixed relation of the bones is very unlike
that of the plesiosaurs, ichthyosaurs, and turtles, in that it is held
in place by _two_ bony bars or arches, the upper extending forward to
unite with the bones at the back part of the orbit, the lower, with
the hind extremity of the upper jaw. The lower jaws are rigidly united
in front, sometimes for a long distance; they have, almost always,
a hole or opening through the hinder part, known in but few other
reptiles. The bones of the palate are all firmly united, forming a
nearly complete roof, very unlike the condition in the mosasaurs and
lizards. The palate also is very peculiar in the development of a
plate of bone below the nasal chambers, forming a complete bony canal
on each side through which the respiratory air passes far back to the
internal opening of the nostrils above the pharynx, and not, as in
other reptiles—save the immediate ancestors of the mammals—entering the
mouth at the front end. This peculiar arrangement of the air-passages,
so like that of ourselves, has much to do with the water habits of the
crocodiles, as we shall see.
The teeth are conical in shape, and are always restricted to the edges
of the jaws, above and below. They are inserted deeply and firmly in
sockets, and are replaced frequently by new ones growing beneath them,
pushing the older ones out as their usefulness becomes impaired by
injury or by use. In some species there are as many as thirty teeth in
each side of the jaws, above and below, although the broad-headed kinds
have a smaller number.
[Illustration: FIG. 99.]
[Illustration: FIG. 100.
FIG. 99.—Skull of _Alligator mississippiensis_, from below.
FIG. 100.—The same, from above: _bo_, basioccipital; _bs_,
basisphenoid; _f_, frontal; _j_, jugal; _l_, lacrimal; _m_, maxilla;
_n_, nasal; _p_, parietal; _pa_, palatine; _pm_, premaxilla; _pf_,
prefrontal; _pr_, postfrontal; _pt_, pterygoid; _q_, quadrate; _qj_,
quadratojugal; _tr_, transverse.]
The neck is short, as has been stated, but it always includes in living
forms nine vertebrae, a number probably slightly in excess of that
of their terrestrial forbears. By the peculiar mode of attachment of
the short “hatchet-shaped” ribs, much lateral movement of the neck
is prohibited, nor is any very great vertical movement possible. The
vertebrae of the whole column, save the atlas, the second sacral, and
the first caudal—which is a very remarkable anomaly—are concave in
front and convex behind, agreeing in this respect with those of all
other living reptiles, save the turtles, the tuatera, and some lizards.
The ribs of the neck have their two heads attached, one to the body of
the vertebra, the other to the arch, but those of the dorsal region,
though double-headed, have both become attached to the transverse
projection of the arch, a seemingly trivial character, but one which
immediately distinguishes all crocodiles from all other water reptiles,
and from all terrestrial reptiles, indeed, save the Parasuchia,
Pterosauria, and Dinosauria. The pelvis is firmly attached to the
spinal column by two sacral vertebrae.
[Illustration: FIG. 101.—Vertebrae of gavial from the side (cervical),
and from in front (dorsal): _az_, anterior zygapophysis; _pz_,
posterior zygapophysis; _d_, diapophysis; _r_, cervical rib; _c_,
articulation for head; _t_, for tubercle of dorsal rib.]
The collar-bones, or clavicles, are wanting in crocodiles; there is a
slender interclavicle; and the shoulder-blade and coracoid are well
developed (Fig. 102). The bones of the pelvis are loosely united with
each other as they are in most reptiles (Fig. 104). The pubes, the
anterior bones below, unlike those of all other reptiles, do not help
to form the acetabulum or socket for the articulation of the thigh
bone, nor do they articulate with each other. This single character
sharply distinguishes a crocodile from all other reptiles, living or
extinct, and is almost the only character that separates the order from
the dinosaurs, aside from the peculiar structure of the nasal passages
in the skull. On the under side of the body, connected with the front
end of the pubes, there are seven or eight pairs of abdominal ribs,
corresponding to the plastron of the turtles and similar to those of
the ichthyosaurs and plesiosaurs. The mosasaurs have no such ribs.
[Illustration: FIG. 102.]
[Illustration: FIG. 103.
FIG. 102.—Scapula (_sc_) and coracoid (_cor_) of gavial.
FIG. 103.—Front foot of crocodile: _u_, ulna; _r_, radius; _re_,
radiale; _ue_, ulnare; _p_, pisiform.]
Furthermore, the crocodiles differ from all other living reptiles in
having a four-chambered heart, like that of birds and mammals, that
is, a heart with two auricles and two ventricles. This more perfect
structure of the circulatory organs does not, however, insure at all
times a complete separation of the pure or arterial blood from the
impure or venous blood, since the blood may be more or less intermixed
outside of the heart by a connection between the venous and the
arterial systems. Whether these imperfectly developed organs, so
suggestive of a higher and more perfect mode of respiration, are the
vestiges of what were once among some reptiles functional structures,
or whether they are rudiments of a higher organization, developing
independently in these creatures, cannot be positively determined,
but it seems very probable that, far back in geological times, some
reptiles, especially the pterodactyls and dinosaurs, had their
respiratory and circulatory systems more like those of the birds and
mammals of today. Unfortunately, however, if such was the case, we may
never be able to prove it, although proof would not be impossible;
stranger things than fossil hearts have been found by paleontologists!
The stomach, moreover, in the crocodiles is fashioned somewhat after
that of the birds, with an imperfect division into crop and gizzard.
Some crocodiles of today have the habit of swallowing hard pebbles,
as do many birds. There is an old myth that the crocodile of the Nile
swallows a pebble on each of its birthdays, thus giving reliable
information as to its age by the number found in its gizzard at
its death! And this habit has been suggested for some of the most
ancient crocodiles, the teleosaurs, by the recurring presence of
siliceous pebbles found with the remains of their skeletons. And we
have seen this pebble-swallowing habit was also characteristic of the
plesiosaurs, with whose remains “stomach-stones,” or gastroliths, as
they have been called, are often found.
[Illustration: FIG. 104.—Pelvis of crocodile: _il_, ilium; _is_,
ischium; _pu_, pubis.]
All of these various characters of the skeleton and fleshy parts are
pretty conclusive evidence that the crocodiles, ugly creatures that
they are, today enjoy the highest rank among cold-blooded animals. They
are perhaps in some respects of not so high a type of reptiles as were
some of the extinct reptiles, but that they have survived so long, so
many millions of years, is pretty good evidence of endurance, to say
the least.
Living crocodiles belong to three distinct groups or families: the
true crocodiles and alligators; the long-snouted crocodiles or Borneo
gavials; and the true gavials of India. Members of the first of these
families are really only subaquatic, or amphibious in habit; they move
about on land with entire freedom, and often seek their food there.
Certain marked aquatic characters they do possess, in the skull and
tail, as we shall see. They are indigenous to southern China, India,
Africa, Madagascar, the southern part of the United States, Central
America, and the northern part of South America. The members of this
family are distinguished by the more or less broad and flat head, the
possession of comparatively few teeth of large size, and by having the
toes less completely webbed. The crocodiles proper differ from the
caimans and alligators especially in the arrangement of the teeth.
During later geological times, that is, during early Tertiary times,
the geographical range of the Crocodilidae was much more extended
than it is at present, the remains of many often very large species,
being found in the lake deposits of the northwestern parts of the
United States, Great Britain, Germany, France, etc. Yet earlier, in
the late Cretaceous rocks of the United States, in Texas, and Wyoming
especially, there have been found rather scanty remains of a gigantic
crocodile which must have been nearly fifty feet in length when living.
The second family, the Tomistomidae, or long-snouted crocodiles,
comprises but two living species, both restricted at the present time
to Borneo. These crocodiles have a moderately slender snout, because
of which they are sometimes called gavials, though it is not nearly so
slender as that of the true Gangetic gavial. This family is probably
older than either of the other living ones, and is the only one known
with certainty to have lived during much of the Cretaceous times,
several species of considerable size having been found in New Jersey
and Europe. Their feet are better webbed than are those of the true
crocodiles and alligators, the front feet partly, the hind feet wholly
so. In general structure they seem to be the most primitive of the
living Crocodilia, and may have been the ancestors of all modern forms.
[Illustration: FIG. 105.—Gavial. (By permission of the New York
Zoological Society)]
The third family, the Gavialidae, also comprises but two living
species, both restricted in habitat to the rivers of India. Of these
the famed gavial of the Ganges is the better known and the more highly
specialized. The skull of this species has an exceedingly long and
slender snout; the teeth are numerous, small, and slender; and the feet
are more fully webbed than are those of other members of the order. In
habits the gavials are more distinctly aquatic than are the crocodiles
and alligators. They feed almost exclusively upon small fishes, for
the seizure and retention of which their small and sharply pointed
teeth are admirably adapted. The hind feet are relatively long, a
character that will be better understood when comparison is made with
those of the Thalattosuchia. Although attaining a large size, fully
twenty-five feet in length, they are comparatively harmless animals,
never attacking human beings or other large animals, as do some of the
crocodiles proper. The gavials have lived a long time in the Indian
regions, the Gangetic gavial itself having been found in deposits
of Pleiocene age, perhaps the oldest known of all living species of
air-breathing vertebrates. Some of the extinct gavials of the same
region attained a length of nearly or quite fifty feet, possibly the
longest, if not the largest, of all swimming reptiles of ancient or
modern times. Extinct gavials have been reported from South America,
but are not yet fully known.
While the fish-eating gavials swallow their prey whole, the crocodiles,
caimans, and alligators prey upon all living animals that come within
their reach, whether large or small, and they will often leave the
water to seize their intended victims, such as pigs, sheep, birds, or
even human beings. Their teeth, as has been already stated, are much
larger, longer, and more irregular in size than those of the gavials.
Their victims are often drawn under the water and drowned, the peculiar
posterior position of the internal nostrils permitting the animals to
breathe with the mouth and to firmly hold their prey under water, while
the extremity of the snout and the external nostrils are above the
surface.
As the firm, unyielding bony palate, the fixed position and
articulation of the lower jaws, and their rigid attachment to each
other in front do not permit creatures of large size to enter the
gullet whole, the crocodiles and alligators must tear their food to
pieces, which they do by quick, strong jerks from side to side, aided
by the powerful tail; or they may twist off a limb or some other part
of their victims by a rapid rotation of the whole body, two assisting
in this operation, rotating in opposite directions.
Living crocodiles lay from twenty to sixty eggs, according to the
species; these eggs are sometimes the size of a goose egg, and are
covered with a hard shell. They are laid either in a deep excavation
in the sand and covered over by the parent; or under leaves and straw.
The female remains on guard until the eggs are hatched, of which she
is apprized, it is said, by a peculiar noise uttered by the partly
imprisoned young. She thereupon reopens the nest, and guides her
liberated infants to the water, where she leaves them to their fate.
Whether this remarkable habit is one that has been acquired in recent
times or not is uncertain, but because it has been observed in a
number of unrelated forms, it is probable that the instinct is of long
inheritance, and may account for certain peculiarities of structure in
some of the ancient members of the order. Doubtless the habit arose
because of the unprotected places in which the eggs are necessarily
laid on the shores and beaches, and because the eggs are comparatively
so few in number. The sea-turtles likewise lay their eggs in hollows
scooped out of the sand of the beaches, but the parents give no further
care to their eggs, nor to their newly hatched offspring, a neglect
which is compensated for by the much larger number of eggs they lay,
because of which the chances are much greater that a few will survive
the more numerous vicissitudes to which the eggs and young turtles are
exposed.
ANCIENT CROCODILES, MESOSUCHIA
The name Mesosuchia, meaning “middle crocodiles,” by which the ancient
members of the Crocodilia have generally been known, was given by
Huxley in the belief that they were intermediate between the “true”
or modern crocodiles and an ancient group which he united with the
order under the name “Parasuchia.” A fuller and better knowledge of
the members of this last group has proved very conclusively that they
are really less allied to the crocodiles than are some other orders of
reptiles, the dinosaurs for instance, and should be properly classed
by themselves as a distinct order. And, more recently, it has also
become quite apparent that the old crocodiles should not be separated
so widely from the modern ones as Huxley proposed; that the differences
distinguishing them from the recent members of the order are really
not of more than family importance. We thus have left but two chief
divisions of the Crocodilia, the Eusuchia and Thalattosuchia; and the
latter group even, by some authors, perhaps rightly, are included under
the true crocodiles as a family only.
[Illustration: FIG. 106.—_Teleosaurus_; skull, from above.]
These older crocodiles, the Mesosuchia of Huxley, comprise a
considerable number of extinct forms which lived as far back as the
early part of the Jurassic, and continued nearly to, if not actually
into, Cenozoic time, that is, to the Eocene. They differ from all
living forms, chiefly in having, not concavo-convex but biconcave
backbones, that is, the more primitive vertebrae with which all
reptiles began. Nor was the internal opening of the nasal passages so
far back in the mouth as in the later forms. In other respects they
did not differ very greatly from some of those now living. All the
earliest kinds that we know of—the teleosaurs—had a long, slender
snout, resembling very much that of the modern gavials. And they were,
for the most part, incased in a more complete bony armor, on both the
dorsal and the ventral sides; and the front legs were smaller than
those of the gavials even. The resemblance of the living teleosaurs to
the modern gavials must have been very great, although the heavier bony
armor indicates a less exclusively aquatic life. They probably lived
more in the shallow waters of the seas near the shores.
Near the close of the Jurassic appeared for the first time, so far as
we now know, broad-headed mesosuchian crocodiles, forms having less
numerous and stronger teeth, and resembling closely modern alligators.
It has been believed that these broad-headed kinds were of later origin
than the more slender-nosed teleosaurs, but a moment’s consideration
will make it evident how improbable such an evolution must be. The
crocodiles must have descended from strictly terrestrial reptiles, and
no terrestrial reptiles have a slender nose. That they should have
acquired a slender face in adaptation to water habits and then returned
to the more primitive land type with a broad face and less strictly
aquatic habits is contrary to all our experience in paleontology. From
this it is altogether probable that broad-faced crocodiles of later
times must have been the descendants of broad-faced kinds that were
in existence during all the Jurassic times, but of which we as yet
have no knowledge. These broad-faced Jurassic crocodiles were, for the
most part, small creatures, much smaller than the teleosaurs even, and
smaller than any species of crocodiles now living. Their remains are
known only from fresh-water or shore deposits, and are, for the most
part, associated with those of land and fresh-water animals. About the
time of their first known appearance in geological history, the small
mammals and birds had also become more or less abundant, and it was
suggested by Owen that these land animals had something to do with the
development of the ancient amphibious crocodiles. Perhaps this was the
case with respect to their greater abundance and development, and with
certain peculiarities of their structure, but that the gavial-like
teleosaurs should have come back to the land and reverted to a more
primitive form seems quite improbable.
During Cretaceous times, especially in America, numerous forms of
these old mesosuchian crocodiles were doubtless in existence, both
slender-nosed and broad-nosed, and some of them must have been of
considerable size, though none known was as large as some of the
late crocodiles. This type, with biconcave vertebrae, continued to
live on, in both North and South America, to the latter part of
Cretaceous times, and it is even possible that some continued on
into the Tertiary. But long before the close of the Cretaceous,
the modern kind appeared, those with concavo-convex vertebrae, and
more posterior internal nostrils. The earliest are known from New
Jersey (_Thoracosaurus_, _Holops_), so like the Borneo gavials of
today that they are properly classified in the same family, the
Tomistomidae or Gavialidae. If all the later, procoelian type, that
is, those with concavo-convex vertebrae, originated from a single form
when the amphicoelian or mesosuchian type became extinct, Huxley’s
classification into the Mesosuchia and Eusuchia would perhaps be
proper, but we have much reason to suppose that the change in the kinds
of vertebra and in the position of the nostrils was only incidental,
and may have occurred in more than one line of descent, that is, it
may have occurred in the broad-headed kinds of the Jurassic to the
broad-headed crocodiles of today, as also in the gavial-like forms of
the Cretaceous to the gavials of the present. And this is the reason
why naturalists no longer recognize the classification of Huxley,
which, partly perhaps because of the prestige of his name, has so long
been accepted in our chief works on natural history.
MARINE CROCODILES, THALATTOSUCHIA
While the ancient crocodiles of which we have spoken resembled the
modern ones so closely in form of body and probably in habits, there
were certain others of the old Jurassic seas which departed so widely
both in structure and in habits, from their associates that they are by
some authors given a place wholly by themselves as a distinct group.
This has been called by Professor Fraas the Thalattosuchia, a word
meaning “sea-crocodiles.” They were a very early side-branch from the
great genealogical tree of the Crocodilia, a branch which departed so
widely from their associates in adapting themselves to a peculiar and
aberrant mode of existence that they cannot be considered as typical
crocodiles, although so closely related to them in other respects
that therecannot be the least doubt regarding their ancestry. None of
the crocodiles which we have considered, whether ancient or modern,
can truthfully be called purely aquatic. They never ceased to use
their limbs for locomotion on land, more or less of the time, or for
the support of the body; and many of them have subsisted, in part at
least, on land animals. How easy it may have been for some of them
to become almost wholly emancipated from land habits we may easily
conjecture. The gavials, as we have seen, are more at home in the
water than upon land; their food is chiefly found in the water; but,
so long as their habits restrict them chiefly to fresh-water, or to
the vicinity of the shores, their limbs continue to be used as much
for crawling as for swimming. Were the gavials to be driven out to
sea by the stress of fresh-water conditions or attracted thereto by a
greater abundance of more easily obtainable or better food, so far from
land that they no longer would have much use for their legs for the
support or propulsion of their bodies, in the course of time they would
doubtless change to a more purely aquatic type. And in that change
there would be material modifications of their structure: their limbs
would become better adapted to movements in the water; the skin would
become smoother, and the bony and horny scales would be lost, since
they would be not only useless in the water, but actually detrimental
to the well-being of the animals; and the tail would develop into a
more powerful organ of propulsion, as a means of increasing their speed
in obtaining food or in escaping their enemies, such as the sharks.
In fact, we can only imagine that in the transformation precisely
those modifications would occur which we actually find in these old
sea-crocodiles, the Thalattosuchia. And they are of especial interest
to us here because nowhere do we find a better example of Nature in the
act of transforming a terrestrial or subterrestrial animal into a truly
aquatic one.
[Illustration: FIG. 107.—_Geosaurus_, an Upper Jurassic thalattosuchian
crocodile, drawn from restoration and figures by Fraas.]
The group comprises only a few forms, so far as known. All were of
modest size among extinct reptiles, from ten to twenty feet in length,
and all are from the Upper Jurassic deposits of Europe. They did
not exist very long, probably because they found it impossible to
discard old habits and old structures entirely and become absolutely
emancipated from the land; their breeding habits possibly were too
deeply impressed into their structure readily to change, as did those
of other sea-reptiles. Some of their remarkable aquatic adaptations
have long been known, but only within a dozen years has our knowledge
of them become at all complete. Three or four genera have been
described, but only a few forms are well known, of which _Geosaurus_
may be taken as most typical. To this we shall confine our descriptions.
[Illustration: FIG. 108.—_Geosaurus_; skull from side and from above.
(After Fraas.)]
The skull of _Geosaurus_ is rather small in comparison with the length
of the body, smaller proportionally than in any living crocodile, but
not much smaller than that of the teleosaurs. The snout is long and
slender, much like that of the teleosaurs and gavials, but the bones
of the whole upper surface are quite smooth, not roughened and pitted
like those of modern forms. The skull of _Dakosaurus_, another genus
of thalattosuchians, is much less elongate than that of _Geosaurus_,
but has the other characteristics of _Geosaurus_. The eyes are provided
with a stout ring of sclerotic bones, with a pupillary opening of less
than one inch. We have seen that all other strictly aquatic reptiles
have similar eye bones, but no other crocodiles have them. The internal
openings of the nostrils are large and long, but they are not situated
far back, as in the modern crocodiles, not even so far back as in the
early teleosaurs. They had no need of the peculiar breathing apparatus
of the amphibious crocodiles, since all their prey must have been
water-breathing creatures. Their eyes were directed laterally, not
more or less upward, as in their nearest relatives. Nearly all other
crocodiles have an opening through the hind end of the lower jaw, but
the thalattosuchians did not. The teeth were about as numerous as in
the modern gavials, but they projected freely only a short distance
above the gums in life, and they were very slender and sharply pointed,
excellently well adapted for catching smooth and slippery fishes.
Their vertebrae, like those of all other reptiles of their time,
were biconcave. Those of the neck resembled those of the teleosaurs,
save that there were only seven, fewer than is the case with any
other members of the order. In becoming adapted to their peculiar
mode of life these crocodiles lost two vertebrae from the neck. All
modern crocodiles have two ribs attached to the first vertebra; the
thalattosaurs had but one, another evidence of primitive characters.
While the number of vertebrae in the neck was reduced, in the back it
was increased to eighteen; all other crocodiles have but fifteen or
sixteen. The trunk was long, another adaptation to water life. There
were two firmly united vertebrae in the sacrum, as in the modern forms.
The reason for the persistence of this terrestrial character we shall
see later.
[Illustration: FIG. 109.—Tail, scapula (_sc_), and coracoid (_c_) of
_Geosaurus_. (After Fraas.)]
The tail was very long and strong, nearly as long as all the remainder
of the body, and relatively much longer than in other crocodiles. It is
of interest to observe that the head, neck, body, and tail had almost
the same relative proportions as those of the great sea-lizards, the
mosasaurs. The terminal bones of the tail are very peculiar, and very
different from the corresponding bones of other crocodiles. While the
spines of the tail bones along the anterior part are only moderately
stout and long, and are directed obliquely backward, near the terminal
part they become suddenly much broader and are directed upward, and, a
little farther along, obliquely forward. The chevron bones on the under
side also here become broader and longer. The end of the tail curves
markedly downward to end in a slender point. It will be remembered that
a similar downward curvature of the end of the tail observed in nearly
all specimens of ichthyosaurs induced in Owen the belief that the
animals had a fleshy terminal fin, a belief which later discoveries of
the carbonized remains of the flesh confirmed. The peculiar structures
observed in various specimens of these sea-crocodiles, even though no
impressions or remains of the fleshy parts have been discovered, is
quite conclusive evidence that these animals also had a broad, fleshy,
terminal fin. No other explanation of the structure is possible.
[Illustration: FIG. 110.—_Geosaurus._ Elongate hind leg, and
paddle-like front leg. (After Fraas.)]
The ribs are not at all stout and are not much curved. They are
directed posteriorly in the known specimens preserved in the matrix
in such a way as to indicate a slender thorax and abdomen, not the
broad body of the modern crocodiles. The abdominal ribs, that is,
those protecting the region on the under side of the body between the
breast bone and the pelvis, are strongly developed in _Geosaurus_. The
sternum, always present in other crocodiles, is wanting in _Geosaurus_.
The shoulder-blades and coracoids, often changed in shape in water
reptiles, are not unlike those of the amphibious crocodiles, but are
smaller and flatter.
The fore limbs, to use Professor Fraas’s words, “are among the most
interesting of all the peculiarities of _Geosaurus_,” and show very
clearly that these animals were excellent swimmers. The humerus is
remarkably short and broad; the two bones of the forearm, the radius
and ulna, are broad, rounded, or angular plates, not long bones, as
in other crocodiles; the wrist bones also are broad and plate-like.
The three bones of the thumb, that is of the digit which received
most strongly the impact of the water in swimming, are also broad and
flat. All of these bones are marvelously aquatic in type, as will be
evident from a comparison of them with the corresponding bones of the
ichthyosaurs and mosasaurs. The bones of the other fingers, however,
were not much changed from the ordinary crocodilian form, as a further
comparison of them with the fingers of a land crocodile will show.
Furthermore the whole limb or paddle was very small in comparison with
the hind leg, and it was attached very near to the head. The relative
proportions of the front and hind limbs in the geosaurs, gavials, and
alligators are instructive as showing the progressive decrease in size
of the front legs from the subaquatic, through the semiaquatic, to
the almost purely aquatic type. The hind legs, strangely enough, were
not very different in size and structure from those of the gavials or
teleosaurs. The thigh bone is long and slender, though the bones of the
leg and ankle are somewhat shortened and flattened, as are also those
of the first toe. There were no claws on the hind feet, a distinctly
aquatic adaptation, and the toes were certainly webbed. The pelvis,
while not especially stout, is of good size, and was firmly attached to
the sacrum.
Perhaps all these characters may best be summed up in the words of
Professor Fraas, as translated:
We recognize in _Geosaurus_ an unusually slenderly
built crocodile, in appearance very different from all true
crocodiles. The smooth, rounded skull, with its greatly
elongated and slender snout, and the deep-lying, small eyes,
reminds one most of the ichthyosaurs. The skull merges into
the slender, elongated trunk without a visible neck, and
the body was provided neither above nor below with horny or
bony armor, but was, probably, as in the whales, covered
with a smooth, soft skin. The anterior extremities, attached
far forward, are developed as paddles, and served rather
as organs of equilibration than as a means of propulsion,
which was the function of the elongated hind legs and the
extraordinarily strong and powerful tail, which supported
at its end a large fin. The entire impression given of
the animal is that of an excellent swimmer, with all the
peculiar aquatic adaptations. In the skeleton, however, all
the characters of the original crocodiles are preserved.
Most remarkable are the laterally placed eyes, protected
by the stout sclerotic bones, and the overhanging bones of
the orbits. So, too, the large temporal openings of the
skull, doubtless due to the absence of the bony plates in
the integument, give to the animal a strangely abnormal
appearance for a crocodile.
We have observed that all the truly aquatic air-breathing animals,
save the plesiosaurs, have either lost the hind legs or else have them
greatly reduced in size, and the disproportionately large size of
these members in _Geosaurus_ seems inexplicable. But an explanation is
not, I think, hard to find. In the adaptation to water life the first
to become modified for the control of the body are the front legs.
The hind legs never have any really important use when the tail is a
powerful propeller. The hind legs of the geosaurs are still essentially
legs and not paddles, and they were doubtless used either occasionally
for propulsion on land, or perhaps for pushing the body about on
the bottom of shallow waters. And the presence of a well-developed
ventral armor of bony ribs possibly also indicates more or less of the
terrestrial crawling habit. As soon as the hind legs cease to be used
for crawling they take on only a feeble use for the equilibration of
the body, and speedily become small, until finally they disappear.
That the hind legs of these creatures were of some use in the water
is certain, because of the modifications in their structure, and
especially because of the loss of the claws; but that they were of
important use as propellers is hardly probable. We may therefore infer
that the thalattosuchians, while distinctively sea-reptiles, had not
entirely lost their land habits. Moreover, it is highly probable that
their egg-laying habits, which would hardly change with a greater
adaptation to water life, compelled the animals recurrently to visit
the shores. To have finally lost their hind legs they must have become
viviparous in habit, since it seems to be impossible for any true
air-breathers to be hatched in water. Perhaps this insurmountable habit
was the final cause of their extinction in competition with the truly
viviparous aquatic flesh-eaters. The thalattosuchians had but a brief
existence in geological history, during the latter part of the Jurassic
period only, so far as certainly known, nor did they become widely
dispersed over the earth; they are known from Europe, possibly from
Brazil.