THE SKELETON OF REPTILES
The bony framework, or skeleton, that which gives form and stature
to the body, and which serves for the support of the soft parts and
the attachment of muscles, is, with rare exceptions, all that is ever
preserved of fossil animals. Because, therefore, students of extinct
animals must rely so much, if not exclusively, upon the skeleton
much attention has been given to the study of comparative osteology,
the science of bones. Not only are most of the bones of the skeleton
characteristic of the genus to which they belong, but the more
general plan of the skeleton, or parts of the skeleton, is likewise
characteristic of the larger groups. The paleontologist may become so
expert in deciphering the characters of single bones, or even parts
of bones—often all that are known of animals new to science—that he
is able to hazard guesses as to the general structure of the skeleton
to which they belong. But such guesses usually will approximate the
real truth only in the degree that the bones upon which they are based
approximate like bones of other animals that are better known. Not all
parts of the skeleton are equally characteristic of the type of animal
which possessed them. A tooth of a mammal may positively determine the
species to which it belongs, while the toe bone of the same animal
might not enable one to guess at its family, even. As a rule one can
seldom be quite sure of the species of a reptile unless the larger part
of the skeleton, or at least the skull, is available, although almost
any bone of the skeleton, if one is expert, will permit a decision as
to the family, if not the genus.
One must often depend upon the positions and relations of the bones,
as found in the rocky matrix, for the final determination of many
characters. One can, for instance, never be sure of the number of
bones in the neck, trunk, tail, or feet of a reptile, until specimens
have been found with all such bones in position. It is for this reason
that much care is exercised in the collection of specimens of fossil
animals, and especially of fossil reptiles, to preserve all parts of
the skeleton, so far as possible, in the relations they occupied in the
rocks until they can be studied in the laboratory. Many grievous errors
have been made in the past by hasty inferences from fragmentary and
poorly collected specimens.
[Illustration: FIG. 3.—_Limnoscelis_, a subaquatic cotylosaur, from the
Permocarboniferous of New Mexico.]
Because of the reliance which must be placed upon the skeleton it
will be necessary to speak somewhat in detail of its structure in
the reptiles, and to use not a few terms in its description that are
unfamiliar to the general reader. So far as possible technical terms
will be avoided, though some must be used, as there are no equivalents
in the English language for them. The reader may use this chapter as a
sort of explanatory index or glossary for the better elucidation of the
necessary details of the following chapters.
It is needless to say that the skeleton of a reptile is arranged on
essentially the same plan as that of our own; the bones have the
same names that they have in our own skeleton, but there are more of
them, and the individual bones, as a general rule, are less highly
specialized, that is, are not so well adapted for special functions.
In a word, the skeleton of a reptile for the most part is generalized,
though particular parts may be highly specialized for particular uses.
As a rule, if not as a law, the course of evolution has been to reduce
the number of parts and to adapt those which remain more closely to
their special uses, either by increase in size, or by modifications of
their shape and structure.
SKULL AND TEETH
The skull of reptiles is much more primitive or generalized in
structure than is that of mammals, to such an extent, indeed, that
there is yet much doubt as to the precise homologies of some of the
bones composing it; and, inasmuch as the names were originally given,
for the most part, to the bones of the human skull, there is still
some confusion among students as to the proper names in all cases, a
confusion that doubtless will not be wholly dissipated until we know
much more about the early or more primitive reptiles than we do at
present.
[Illustration: Fig. 4 Fig. 5]
[Illustration: FIG. 4.—_Seymouria_, a primitive cotylosaurian.
Skull, from above: _pm_, premaxilla; _n_, nasal; _l_, lacrimal; _p_,
prefrontal; _f_, frontal; _pf_, postfrontal; _it_, intertemporal;
_st_, supratemporal; _sq_, squamosal; _ds_, dermosupraoccipital; _t_,
tabulare; _j_, jugal; _po_, postorbital; _m_, maxilla; _s_, surangular;
_ang_, angular; _pa_, parietal.
FIG. 5.—_Seymouria_, skull from the side. Explanations as in fig. 4.]
As in other parts of the skeleton, there has been a reduction in the
number of parts of the reptile skull from that of the more primitive
forms, and a better adaptation of those which remain for the special
uses they subserve. This reduction in number has been caused in part
by the actual loss of bones, in part by the fusion of contiguous ones.
The most primitive reptiles had no less than seventy-two separate bones
in the skull;[1] the human skull has but twenty-eight inclusive of the
ear bones. There is but little variation, either in the number or in
the relations of bones, in the mammalian skull. If one knows the human
skull thoroughly he can easily understand the structure of the skull
of any mammal. The same cannot be said of the skulls of reptiles; one
would be greatly puzzled in the comparison of the skulls of turtles
and crocodiles, if he knew nothing about other forms. And it is safe
to formulate another general law in evolution here: Characters which
have been longest inherited are least liable to change. The earliest
reptiles had at least four pairs of bones which have disappeared
in all later reptiles; and they had some bones in pairs which have
fused in later reptiles, either with their mates or with contiguous
bones. The crocodile has at least two pairs of bones which have
disappeared in turtles. On the other hand, the turtle has at least one
pair of free bones which have been fused with adjacent bones in the
crocodiles, and one pair that is fused which is free in the latter.
The lizard has one pair of bones that has been wholly wanting in other
reptiles for millions of years, while on the other hand it has lost
some bones that are present in all other modern reptiles. The four
parts of the occipital bone of mammals, basioccipital, exoccipitals,
and supraoccipital, are almost invariably free and there is a single
occipital condyle, except in the Theriodontia.
[1] Paired maxillae, premaxillae, nasals, prefrontals, lacrimals,
frontals, parietals, dermosupraoccipitals, tabularia, supratemporals,
intertemporals, squamosals, jugals, quadratojugals, postorbitals,
postfrontals, quadrates, exoccipitals, paroccipitals, vomers,
palatines, pterygoids, sphenomaxillae, stapes, transverse, alisphenoids
or orbitosphenoids, epipterygoids, articulars, prearticulars, angulars,
surangulars, coronoids, splenials, dentaries, one supraoccipital, one
basioccipital, one basisphenoid, one ethmoid.
[Illustration: FIG. 6.—_Labidosaurus_, a cotylosaur. Skull from above:
_pm_, premaxilla; _n_, nasal; _m_, maxilla; _l_, lacrimal; _p_,
prefrontal; _fr_, frontal; _pf_, postfrontal; _po_, postorbital; _j_,
jugal; _pa_, parietal; _sq_, squamosal; _ds_, dermosupraoccipital;
_pf_, parietal foramen.]
In this reduction or fusion of parts, or in addition thereto, there has
been a general lightening-up of the whole skull-structure in reptiles
from the rather massive and protected form of the older to the lighter,
less protected, and more fragile type of the later ones, since speed,
greater agility, better sense organs, and doubtless greater brain power
have rendered unnecessary or useless the older kinds, just as modern
methods and modern arms have rendered useless the coat of mail of the
Middle Ages.
[Illustration: FIG. 7.—_Edaphosaurus_, a theromorph reptile from the
Permian of Texas. Skull with single temporal vacuity.]
The old reptiles had a continuous covering or roof for the skull,
pierced only by the openings for the nostrils in front—the nares—the
orbits for the eyes near the middle, and a smaller median opening
back of them for the so-called “pineal eye.” The temporal region,
that is, the region back of the orbits on each side, was completely
roofed over by bone for the support and protection of the jaw muscles.
In later reptiles this region has been lightened, either by holes
that pierce it or by the emargination of its free borders, as in the
turtles. The openings have occurred in different ways, and with the
loss of different bones in various lines of descent. In one large
group of reptiles, comprising the pterodactyls, dinosaurs, phytosaurs,
crocodiles, and rhynchocephalians, there are two openings on each side,
called the supratemporal and lateral temporal vacuities. In another
still larger group there is a single vacuity on each side, all members
of which it has been thought were markedly related to each other.
Some of these, the lizards, snakes, and mosasaurs, the ichthyosaurs,
and probably the proganosaurs, have the single opening high up on
the side, corresponding apparently to the supra temporal vacuity of
the double-arched forms, as those with two openings are called. Many
others, however, like the whole order Therapsida and the Theromorpha,
have the single opening lower down and bounded differently; their
relationships are doubtful, since it is very much of a question how the
single opening has arisen. There have been many theories to account for
the origin of the temporal vacuities, but all are yet speculations.
Notwithstanding these doubts, which more recent discoveries have
intensified, there can be none that the structure of this region of the
skull offers important and reliable characters for the classification
of the reptiles into the larger groups, but, unfortunately, we are
very uncertain yet as to what this classification should be. We are
confident that all those reptiles having two temporal vacuities on each
side are related to each other; we are yet very much in doubt as to the
classification of all other reptiles, or at least all others having
only a single temporal vacuity on each side.
[Illustration: FIG. 8.—_Sphenodon_ (tuatera). Skull from side and
above: _pm_, premaxilla; _n_, nasal; _prf_, prefrontal; _f_, frontal;
_pf_, postfrontal; _p_, parietal; _po_, postorbital; _sq_, squamosal;
_m_, maxilla; _j_, jugal; _qj_, quadratojugal; _q_, quadrate; _c_,
coronoid; _sa_, surangular; _art_, articular; _pa_, prearticular; _d_,
dentary; _an_, angular.]
Better evidences of relationships, or the absence of relationships, are
offered by the presence of certain bones in the skulls in some orders
that are lost in others, since it may be accepted as an axiom that new
bones have not appeared in the skulls of reptiles, birds, or mammals;
and that no bone which has once disappeared has ever been functionally
regained by the descendants of those that lost it. The presence,
then, of an extra bone in the temporal region of the lizards or the
ichthyosaurs is proof that they have had a long and independent descent
from reptiles which possessed it.
The mandible of the earliest reptiles was composed of not less than
seven separate and distinct bones, as shown in the accompanying
figures. The mandible of no modern reptile has more than six, and some
have fewer. The mandible of mammals is composed of a single bone,
the dentary; those reptiles, the Theriodontia, which doubtless were
ancestral to the mammals in Triassic times, have all the bones, except
the dentary, much reduced, or even vestigial. The prearticular bone, as
shown, so far as known, has been absent in all reptiles since Triassic
times, except the ichthyosaurs, plesiosaurs, _Sphenodon_, and turtles,
all reptiles of ancient origin. The coronoid bone primitively extended
the whole length of the teeth on the inner side; in all reptiles,
except the plesiosaurs, since Triassic times it is either reduced to
a small bone back of the teeth or is absent. So also the splenial has
been greatly reduced in size in all later reptiles and may be wanting
as in _Sphenodon_ and modern turtles. The articular of reptiles, it is
now generally believed, is represented in mammals by one of the ear
bones, the quadrate by another.
[Illustration: FIG. 9.—Mandible of _Trimerorhachis_, a stegocephalian
amphibian, ancestrally related to the reptiles: _A_ from within;
_B_ from without. The coronoid is composed of three bones, the true
coronoid (_cor_), the intercoronoid (_icor_), and the precoronoid
(_pc_). The splenial is composed of two, the true splenial (_sp_) and
the postsplenial (_psp_). The prearticular (_pa_) is broad, the dentary
(_d_) is small; and the angular (_an_) is only slightly visible on the
inner side.]
[Illustration: FIG. 10.—Mandible of _Labidosaurus_, a cotylosaur
reptile: _A_ from within; _B_ from without. The coronoid (_cor_) is a
single bone, but extends far forward. The splenial (_sp_) is also a
single bone, replacing the two of the amphibians. The _prearticular_
(_pa_) is narrower, and the angular (_ang_) appears broadly on the
inner side. The dentary (_d_) is much larger and the surangular (_sa_)
is distinct. The articular (_art_) is small.]
[Illustration: FIG. 11.—Mandible of _Alligator_, a modern, highly
specialized reptile, from within. The coronoid (_cor_) is small and
is situated far back; the splenial (_sp_) does not extend to the
symphysis; the prearticular (_pa_) has disappeared, or has fused with
the angular (_an_) or articular (_art_). The dentary (_d_) has become
the chief bone of the mandible.]
The teeth of reptiles are of much less importance, as a rule, in the
determination of relationships than are the teeth of mammals. Rarely
are their shapes of specific, and often not of generic, importance,
though their number and relative sizes may be. The teeth of mammals,
as a rule, are forty-four or less in number, and they are always
inserted in distinct sockets in the jaw bones. Among reptiles they
are indefinite in number, and may be attached to any of the bones
of the palate and sometimes also to the coronoid of the mandibles.
Furthermore, except in those reptiles related to the immediate
ancestors of the mammals, they are alike or nearly alike in the
jaws, that is, homodont, not distinguishable into incisors, canines,
and molars. They may be inserted in separate sockets (thecodont),
in grooves, or simply be co-ossified to the surface of the bone
(acrodont). And they are usually reproduced indefinitely by new teeth
growing at the side of the base or below them. More usually they are
pointed and curved; sometimes they are flattened, with sharp cutting
edges in front and behind in the more strictly carnivorous reptiles;
in those of herbivorous habits they are more dilated and roughened on
the crown, not pointed; in not a few they are low, broad, and flat
and are used only for crushing the hard shells of invertebrates. With
the very few exceptions among certain dinosaurs, they never have more
than one root for attachment. The evolutional tendency for reptiles,
as for the mammals, is to loose teeth, especially those of the palate.
Among living reptiles it is only the most primitive types, such as
the lizards, snakes, and the tuatera, which have teeth on the palatal
bones, and in none are there teeth on the vomers, as was the rule in
the ancient reptiles. The lizards may have them on pterygoids and
palatines, and the tuatera has them on the palatines only. There may be
as many as eighty on each jaw, above and below, and hundreds of smaller
ones on the palate, or they may be reduced in number to five or six,
or even to a single one; some reptiles, like the turtles and later
pterodactyls, have none. The teeth of reptiles are composed of the same
kinds of tissues as are the teeth of mammals, that is, of dentine and
enamel, but the enamel is always thin, perhaps because the teeth are
so easily replaced that a thicker protective covering is not needed.
The arrangement of the dentine in primitive reptiles is complicated,
that is, plicated or folded in labyrinthine figures, like that of
many stegocephalian amphibians, the Labyrinthodontia, especially.
This labyrinthine structure of the dentine persisted longest in the
ichthyosaurs.
VERTEBRAE AND RIBS
The spinal column or backbone of reptiles, as in all air-breathing
vertebrates, is made up of a variable number of separate segments
called vertebrae, permitting flexibility. Each vertebra is composed of
a body, or centrum, and an arch on the dorsal side for the protection
of the spinal cord. Various projections from the vertebra, called
processes, serve for the attachment of ligaments or muscles, for
articular union with adjacent vertebrae, or for the support of ribs,
and these processes have characteristic differences in different
reptiles. The pair in front and behind, for articulation with the
adjoining vertebrae, may become obsolete or even lost in swimming
reptiles, as we shall see; they are called zygapophyses. In not a few
reptiles there is an additional pair for zygapophysial articulation
in front and behind, called zygosphene and zygantrum, for the greater
strengthening of the column; they are especially characteristic of
snakes and certain lizards. In certain other reptiles, especially the
long-necked dinosaurs, there is an additional pair arranged differently
from the zygophene, that have received the names hyposphene and
hypantrum.
[Illustration: FIG. 12.—Procoelous vertebra of snake: _za_, zygantrum;
_zs_, zygosphene; _ps_, posterior zygapophysis.]
On the top of the arch is the spine or spinous process, which may vary
enormously in size and length; sometimes it is flattened or dilated
above for the support of an exoskeleton, or it may be heavy and massive
for the attachment of strong muscles and ligaments. In the modern
basilisk lizards and in the ancient _Dimetrodon_ and _Edaphosaurus_
from the Permian rocks of Texas these spines are of enormous length,
some of them nearly four feet long in reptiles not twice that length.
Slender crawling reptiles usually have no spines, or only vestigial
ones. On the sides of the arch there may be a distinct transverse
process for the articulation of the rib.
In all early reptiles the ends of the body or centrum are concave,
as they are in nearly all fishes. Such a conformation, called
amphicoelous, gives great flexibility to the spinal column, but only
moderate strength, since the intervening spaces are filled with
cartilage in life. In all living reptiles, with few exceptions, the
body is concave, like a saucer, in front and correspondingly convex
behind, and the intervening cartilage has largely disappeared. Such a
mode of union, called procoelous, adds greatly to the strength of the
backbone, enabling it to receive greater shocks or greater pressure
without dislocation; or to sustain the greater strain of muscles used
in running swiftly or in climbing. Among living reptiles, only the
gecko lizards and the tuatera have biconcave vertebrae. Some extinct
reptiles, such as some of the dinosaurs, animals that walked erect upon
their legs, had their vertebrae convex in front and concave behind
(opisthocoelous). Birds, though walking erect, have a very different
and more complicated articulation of the cervical vertebrae, and
certain reptiles, like the turtles, have very complicated cervical
vertebrae.
In the embryos of all vertebrate animals there appears first an
elongated fibrous rod, called the notochord, in the place of the future
spinal column. This rod may persist through life, never ossifying, as
was the case with all the earliest fishes, and is the condition in some
living ones. As the embryo grows, however, the separate segments, or
vertebrae, ossify about this rod in all reptiles, forming bony rings,
perforate at first in the middle for the more or less constricted
notochord. This stage was the permanent condition in all the earliest
reptiles and in some later ones. Such animals are said to have
notochordal vertebrae, the notochord more or less continuous, like a
string of beads, the beads representing the enlargements between the
contiguous vertebrae.
[Illustration: FIG. 13.—Notochordal cervical vertebrae, with
intercentra, of _Ophiacodon_, a primitive theromorph reptile from the
Permocarboniferous of New Mexico: _pa_, proatlas; _an_, arch of atlas;
_o_, odontoid; _ax_, axis.]
In many early amphibians, and probably in all the earliest ones, as
well as in the fishes from which they were derived, the vertebra
is more complicated in that it is composed of at least three pairs
of separate bones, two of which united with each other, the third
finally disappearing in modern animals, or at the most represented
by a mere vestige called the intercentrum. The dorsal pair of these
bones, called the neurocentra, forms the arch of the vertebra. The
ventral posterior pair, called the pleurocentra, increases in size and
unites to form the centrum or body of the vertebra; while the ventral
anterior pair, early united with each other, is called the hypocentrum
or intercentrum, persistent in all early reptiles as a vestige between
the centra on the ventral side. This divided condition of the vertebra
is persistent in the first vertebra, the atlas of all higher animals,
in which the so-called body is the hypocentrum or intercentrum, the
arch is the neurocentrum, while the pleurocentra have fused more or
less with the anterior part of the next vertebra, the axis, to form the
so-called odontoid. That this is the real explanation of the structure
of the atlas is proved by the various stages of its evolution in the
reptiles, from the earliest (Fig. 15) in which it scarcely differs
from rhachitomous—as this structure is called—vertebrae of an early
amphibian, to the modern in which the structure is nearly like that of
mammals.
[Illustration: FIG. 14.—Rhachitomous dorsal vertebra of _Eryops_:
_n_, neurocentrum or arch; _pl_, pleurocentrum; _i_, intercentrum or
hypocentrum; _az_, anterior zygapophysis; _pz_, posterior zygapophysis;
_d_, diapophysis, for tubercle of rib; _p_, parapophysis, for head of
rib.]
In front of the atlas, that is, between it and the skull, there was, in
all early reptiles, as well as in some later ones, like the crocodiles
and tuatera, the remnant of what is believed to have been another
vertebra, of which only the arch remains, and which is called the
proatlas. In its earliest condition it articulated with the skull in
front and the arch of the atlas behind.
As in mammals, the vertebrae of the different regions have received
distinctive names, cervical, dorsal, lumbar, sacral, and caudal. The
numbers of each region are far more variable than they are among
mammals, the total number of vertebrae in the column varying from
about thirty to more than five hundred, in certain snakes. Nor are the
different regions always easily distinguishable, especially those in
front of the sacrum. In the earliest reptiles there was practically
no neck, and only two vertebrae, the atlas and axis, that properly
can be called cervical. Very soon, however, the reptiles developed a
longer neck with seven vertebrae, a number that has remained singularly
constant in higher animals, especially in the mammals. In most modern
reptiles there are from seven to nine; in a few lizards, five. But the
number was much more inconstant among the older reptiles; some of the
plesiosaurs had as many as seventy-six cervical vertebrae; some of the
older lizards even had as many as eighteen.
Ordinarily the cervical vertebrae differ from those behind them only
in the small size or fusion of their ribs; sometimes, however, as in
the Protorosauria and Pterosauria, the vertebrae may be much elongated.
The dorsal vertebrae of reptiles vary in number from ten in turtles
and some dinosaurs to forty-three in _Pleurosaurus_; and under the
name dorsal we include the so-called lumbar, as there is seldom any
real distinction between the two series, save the smaller size or the
co-ossification of the ribs of the latter.
[Illustration: FIG. 15.—_Ophiacodon_, a primitive theromorph reptile:
proatlas, atlas, and axis, with ribs.]
[Illustration: FIG. 16.—Sacrum of _Chelone_.]
The sacrum in reptiles primitively consisted of a single vertebra,
which bore a large rib on each side for the support of the pelvis. Very
early, however, a second or even a third vertebra was added to it from
behind. The number two is the rule among reptiles, both ancient and
modern; among crawling reptiles the number never exceeds three, but
among ambulatory and flying reptiles the number may be as great as in
any mammal.
The number of caudal vertebrae in reptiles is exceedingly variable,
from a dozen or fifteen up to a hundred and fifty or more. In snakes
but two regions are distinguishable, the caudal and precaudal, and the
number altogether may reach nearly five hundred. With the exception
of the first few basal caudal vertebrae (pygals) and the minute ones
at the extreme tip, all caudal vertebrae of reptiles bear a slender,
usually =Y=-shaped bone below in the interval between the centra,
for the protection of the vessels and nerves. Because of their shape
they have been called chevrons, and are really outgrowths from the
intercentra.
The ribs of reptiles are of more importance in classification than
one would suppose. The primitive rib was a slender, curved bone,
with the vertebral end dilated to articulate continuously with the
intercentral space—that between the centra and the anterior part of
the arch. And this is the condition still remaining in the tuatera.
Very soon, however, the lower end of the articular surface (capitulum)
became separated from the upper (tubercle) by a notch, and the ribs
became distinctly double-headed. And this mode of articulation is
the rule among mammals. Among later reptiles, however, there were
many modifications. In nearly all the head migrated a little backward
on the centrum. By the loss of the tubercle in lizards, the head
became truly single-headed, and attached solely to the body; and this
condition is characteristic of the order Squamata. In another large
group the head of the rib gradually migrated up on the arch and on
the transverse process (diapophysis), so that both head and tubercle
are attached to the diapophysis; and this condition is equally
characteristic of the orders of reptiles known collectively as the
Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs.
In the Sauropterygia, the ribs are single-headed and attached to the
end of the diapophysis. Finally in most ichthyosaurs the capitulum and
tubercle both articulate with the body of the vertebra.
[Illustration: FIG. 17.—_Ostodolepis_, a primitive theromorph reptile.
Vertebrae from in front and side, with primitive double-headed rib and
intercentrum.]
Ribs primitively were probably attached to all the vertebrae to the end
of the tail. In the earliest reptiles that we know they are present on
all vertebrae as far back as the tenth or twelfth caudal only, those of
the caudal for the most part co-ossified with the centra. The ribs of
the neck vertebrae more quickly disappeared, or became fused with the
vertebrae, and only in the crocodiles among living reptiles are there
ribs on the atlas. The sacral ribs, on the other hand, became much
larger and stouter and developed an articulation at their outer ends
for the support of the ilium (Fig. 16).
The so-called ventral ribs are slender ossifications in the connective
tissue under the skin, on the under side of the body, and are
characteristic of most reptiles. The anterior ones doubtless fused
together more or less to form the sternum or breast bone, which was
otherwise absent in the early reptiles.
PECTORAL OR SHOULDER GIRDLE
Those bones which form the framework for the support of the anterior
extremity in vertebrate animals are known collectively as the pectoral
girdle. In our own skeleton there are but two on each side, or four in
all, the scapula or shoulder-blade, and the clavicle or collar-bone. A
third bone, however, is represented in all mammals by a mere vestige
which early unites with the scapula and is called the coracoid
process. In the lowest forms of mammals, the Monotremata, of which the
_Ornithorhynchus_ and _Echidna_ are the only examples, not only is
this coracoid bone largely developed, articulating with the sternum
or breast bone, but there is an additional coracoid bone in front of
this; and there is also an interclavicle. Indeed, the pectoral girdle
in these mammals is more primitive or generalized in structure than it
is in any living reptiles, composed of scapula, coracoid, metacoracoid,
and clavicle on each side and an interclavicle in the middle. No living
reptiles have the metacoracoid, and, as is the case with many mammals,
some reptiles have no clavicles.
Primitively, that is, in all the old reptiles, the girdle is composed
of scapula, coracoid, metacoracoid, clavicles, and interclavicle,
while in some of the very oldest there is yet another bone, more or
less of a vestige, derived from the ancestral amphibians and called
the cleithrum or supraclavicle. The scapula is more or less elongated
in crawling and climbing reptiles; more slender and bird-like in those
which walked erect after the manner of birds and mammals; shorter and
more fan-shaped in the swimming reptiles, as we shall see. In some
pterodactyls, unlike all other known animals, the scapula articulated
at its upper end with the backbone, giving a much firmer support
for the anterior extremities. Only in those reptiles allied to the
ancestors of the mammals has the scapula ever had a spine or projection
on its dorsal side.
[Illustration: FIG. 18.—_Cacops_, a Permian stegocephalian, ancestrally
allied to the primitive reptiles, with rhachitomous vertebrae and large
cleithrum above the scapula.]
Of the two coracoid bones in the original pectoral girdle the posterior
one began to disappear early and is entirely lost in all reptiles that
lived later than Triassic times, though it still persists in the lowest
mammals, as we have seen. In most later reptiles the remaining coracoid
has become less firmly attached to the scapula than it was in the older
reptiles. It usually has a small foramen piercing it near the middle of
the upper border or end, the supracoracoid foramen. The clavicle, while
more constant among reptiles than among mammals, has been lost in some,
the Crocodilia, for instance, as also the dinosaurs and pterodactyls.
The interclavicle is more constant in reptiles, a more or less
=T=-shaped bone underlying the coracoids where they join, or the breast
bone; but there were some reptiles that lost it, the dinosaurs and
pterodactyls, for instance. In the turtles both the clavicles and the
interclavicle form a part of the under shell or plastron.
[Illustration: FIG. 19.—Scapula (_sc_), coracoid (_cor_), and
metacoracoid (_mcor_) of _Dimetrodon_]
The cleithrum is known in only a few of the old reptiles; it is a
more or less slender bone which lies along the upper front margin of
the scapula, articulating at its lower end with the upper end of the
clavicle on each side.
The breast bone or sternum, while not properly a part of the pectoral
girdle, may be mentioned here. In reptiles it is rarely well developed
or even ossified, the flying reptiles known as the pterodactyls being
the most notable exceptions. It was a comparatively late development in
this class, the earliest ones not possessing it even in a cartilaginous
condition. It was doubtless evolved from the more or less numerous and
slender ossifications on the under side of the body called ventral
or abdominal ribs, after the coracoids had become reduced and more
slender. Whenever it is present the coracoid articulates with it
on each side in front. In most lizards it remains as a cartilage
throughout life.
[Illustration: FIG. 20.—Clavicles and interclavicle of _Ophiacodon_, a
theromorph reptile from the Permocarboniferous of New Mexico.]
ANTERIOR EXTREMITY
The upper arm bone, or humerus, like most other bones of the
extremities, has been greatly modified by the habits of the different
reptiles. In running and climbing reptiles it is always slender, while
in burrowing reptiles it is short and stout and much expanded at the
extremities, like the humerus of the mole among mammals. And we shall
also see how greatly modified it was among the swimming reptiles.
The humerus of flying reptiles has an enormous process on the side,
corresponding to the attachment of the deltoid muscle. The head of the
humerus, for articulation with the glenoid cavity of the scapula, is
rounded in all reptiles, except the pterodactyls, and the articulation
is always at the extremity. At the lower extremity the protuberance
at the outer or radial side is called the ectocondyle; that on the
inner or ulnar side, the entocondyle. Between the two at the end are
the articular surfaces for the radius and ulna, the capitellum and
trochlea. A little above each of these condyles there is usually, on
one side or the other or on both, a foramen or hole for the passage of
arteries or nerves. That on the inner side, which is characteristic of
all early reptiles and of many mammals, is called the entepicondylar
foramen; that on the outer side, the ectepicondylar foramen; the latter
is present in the lizards, and both are found in the tuatera and some
of the early reptiles.
[Illustration: FIG. 21.—Anterior extremity of _Ophiacodon_.]
The radius and ulna are always distinct bones in reptiles, and always
freely movable on each other; they are usually shorter than the
humerus, but in some springing and climbing reptiles they are quite as
long.
The carpus or wrist of reptiles consists primitively of eleven
distinct, irregularly shaped bones, which articulate more or less
closely with each other in three rows. Those of the first row, all true
carpals, are known usually as the radiale, intermedium, ulnare, and
pisiform, corresponding quite with the bones of the human wrist known
as the scaphoid, lunar, cuneiform, and pisiform. The second row has
but two bones, on the radial side, known as the centralia; while the
third row has a bone to correspond to each of the metacarpals, five
in number, and collectively known as the carpalia. Some or indeed all
of these bones may be either absent or unossified, that is, remaining
through life as nodules of cartilage. Seldom, however, are there less
than nine bones in the carpus of reptiles.
The metacarpals, like the digits, primitively were five in number, and
seldom are there less, though the fifth is sometimes lost, and rarely
also the first. They are more or less elongate bones, increasing in
length from the first to the fourth, with the fifth usually shorter.
The first and the fifth are usually more freely movable on the wrist
than are the other three.
The number of joints or phalanges in the fingers of all primitive
reptiles is that of the modern lizards and the tuatera, that is, two
on the first finger or thumb, three on the second, four on the third,
five on the fourth, and three on the fifth. The crocodiles have one
less phalange on the fourth digit; the turtles have usually two less
on the fourth and one less on the third, that is, with precisely the
same arrangement that is found in our own fingers and that of mammals
in general, two on the thumb and three on each of the other fingers.
As exceptions the river turtles have four bones in the fourth digit.
And this mammal-like and turtle-like arrangement of the phalanges was
that of those early reptiles, the Theriodontia, from which the mammals
arose. The last or ungual phalange of reptiles is usually claw-like,
that is, sharp, curved, and pointed, but sometimes it is more nail-or
hoof-like.
PELVIC OR HIP GIRDLE
The pelvic girdle or pelvis in reptiles and higher animals consists
of three bones on each side, often closely fused in adult reptiles
and together known as the innominate bone. The upper or dorsal one of
these three bones—that to which the sacrum is attached—is the ilium;
the one on the lower or ventral side in front is the pubis; and that on
the ventral side behind is the ischium. On the outer side, where these
three bones meet, is a cup-like depression, sometimes a hole, called
the acetabulum, for the articulation of the head of the thigh bone,
homologous with the glenoid articulation of the pectoral girdle, which,
as we have seen, was originally formed by three bones, the scapula,
coracoid, and metacoracoid, the two latter bones, like the pubis and
ischium, meeting in the middle line below. In all the primitive and
early reptiles the pubis and ischium form a continuous plate of bone
without holes in it, except a small one just below the acetabulum in
the pubis, called the obturator foramen, and corresponding to the
supracoracoid foramen of the coracoid. One may almost always recognize
these two bones by the presence of the foramen. This “plate-like”
condition of the pelvis has been lost in all late and modern reptiles
by the appearance of a larger or smaller vacuity between the pubis
and ischium, either paired, when it corresponds quite with the
so-called obturator opening of mammals, or singly in the middle. This
old-fashioned character, like the old-fashioned type of pectoral
girdle, disappeared entirely about the close of the Mesozoic period,
the Choristodera, described in the following pages, being the last of
the kind.
[Illustration]
[Illustration: FIG. 22.—Pelvis of _Ophiacodon_: _A_ from side; _B_ from
above; _pu_, pubis; _il_, ilium; _is_, ischium.]
The ilium in reptiles usually has a more or less prolonged process
or projection turned backward by the side of the anterior caudal
vertebrae, but in those animals which walked erect on the hind legs,
the dinosaurs and pterodactyls, as also some of the more erect-walking
reptiles ancestral to the mammals, this process was directed forward,
as in birds and mammals. The crocodilia, unlike all other known
reptiles, have the pubes excluded from the acetabulum, and they do not
meet in a median symphysis. This character alone will distinguish any
crocodilian from all other reptiles. But there is some doubt as to the
homology of the bones usually called pubes in the crocodiles. Some of
the bipedal dinosaurs have the pubis forked, the anterior part directed
downward and forward, and not meeting its mate in a symphysis, the
posterior process long and slender, lying below the long ischium, as
in birds. Indeed, when this peculiarity of the dinosaurian pubis was
first discovered, it was thought to be an evidence of the immediate
relationship of birds; its structure is now interpreted differently.
POSTERIOR EXTREMITY
The thigh bone or femur in reptiles, like the humerus, is variable
in size and shape. Only in those reptiles that walked erect is the
articulation of the head set off from the shaft of the bone by a
distinct neck. In others the articulation is at the extreme top of
the bone, since the thigh bones are habitually turned more or less
directly outward from the acetabulum and the long axis of the body.
The more or less pronounced rugosities at the upper end of the femur,
for the attachment of muscles, called trochanters, are not easily
distinguishable into the greater and lesser, as in mammals. Sometimes,
as in the erect-walking dinosaurs, there is a more or less pronounced
process on the shaft lower down, called the fourth trochanter, for the
attachment of caudal muscles. On the back part of the shaft there is a
ridge or line for the attachment of muscles, corresponding to the linea
aspera of the mammalian femur. The projections at the lower end on the
sides are called condyles.
The two bones of the leg, or shin, are usually shorter than the thigh
bones, though in running and leaping animals they may be quite as long
or even longer. That on the inner or big toe side is called the tibia,
and articulates with the distal end of the femur, but chiefly with its
inner condyle. It has a more or less well-developed crest in front
above for the attachment of the extensor muscles directly, since there
never is a patella in reptiles, and only rarely sesamoid bones of any
kind. The fibula, at the little-toe side of the leg, is usually more
slender than the tibia, though it may be larger in swimming reptiles
and even in some running forms. It disappeared in some of the later
pterodactyls. Its upper articulation has a more gliding and somewhat
rotary motion on the outer condyle of the femur, turning the foot
outward in extension of the leg.
[Illustration: FIG. 23.—Right hind foot of _Ophiacodon_: _a_,
astragalus; _c_, calcaneum; _c1_, _c2_, centralia; 1, 2, 3, 4, 5,
tarsalia.]
The tarsus of reptiles differs from that of mammals, in that the
chief movements of extension and flexion of the foot upon the leg
occur within the tarsus rather than between the tarsus and leg bones.
Primitively the tarsus of reptiles consisted of nine bones, two in
the first row, two in the second, and five in the third, but in all
modern reptiles the bones of the middle row and the fifth one in the
third row have disappeared; in some lizards and turtles the two of
the first row are fused. The two bones of the proximal row correspond
quite to the astragalus and calcaneum, the astragalus articulating with
both tibia and fibula proximally, the calcaneum with the latter only.
The oldest known tarsus of any vertebrated animal, one from the Coal
Measures of Ohio, has this structure, while in all the early amphibians
there were three bones, the tibiale, intermedium, and fibulare. Some
of the later swimming reptiles, like the ichthyosaurs and plesiosaurs,
have apparently this amphibian structure, with three bones that are
usually called tibiale, intermedium, and fibulare, but it is very
doubtful indeed whether they are homologically the same. In the middle
row two centralia are known in one or two very ancient reptiles, but
for the most part there is only a single centrale, and even that is
usually lost in later reptiles. The third row, like the third row of
the carpus, had a distinct bone for each digit originally, but the
fifth one was very soon lost and has never reappeared. The structure
of the digits and number of bones are quite like those of the hands,
except that the fifth toe has four bones instead of three, that is,
the phalangeal formula was 2, 3, 4, 5, 4. As a rule in terrestrial
reptiles, as in terrestrial mammals, the hind foot is more specialized
than the front ones.
Most reptiles have an external covering or exoskeleton of horny plates
or scales or bony scutes. Horny scales are of course not preservable
as petrifactions, though in many instances their actual carbonized
remains or their impressions have been detected. Such information comes
only rarely, though doubtless in the course of time we shall obtain
it for most extinct reptiles. In the mosasaurs, for instance, very
perfect impressions showing the detailed structure of the scales have
been frequently found. Similar impressions were long since observed
by Lortet in _Pleurosaurus_, and in not a few dinosaurs impressions
of most wonderful perfection have been found. It is only in the water
reptiles, probably, that all external coverings tended to disappear.
Bony dermal plates or scutes are less common among reptiles, though by
no means rare. The turtles, as is well known, are almost completely
inclosed in such an exoskeleton, bones which have coalesced more or
less to form a box or carapace within which the head and limbs may be
withdrawn for protection. In the modern crocodilians also the body is
more or less protected by small bone plates forming rows on the back
and sometimes on the under side. The ancient phytosaurs had similar
plates. Not a few of the dinosaurs were more or less covered with bony
scutes and sometimes with large bony plates or spines. Some modern
lizards have bony plates over the body instead of horny scales.