modifications of this kind in the most remarkable degree is the cervical
division of the vertebral column. In one edentate species it is
lengthened out by two additional vertebræ more than in any other mammal;
in another it is reduced by anchylosis to as great an extent below the
regular number of moveable pieces: and these, the two most opposite
conditions of the cervical vertebræ which are to be met with in the
mammiferous class are related to equally diverse and opposite habits of
life.
With respect to the _Ai_, or three-toed Sloth, “an animal, great part of
whose life, when not engaged in eating, is spent in sleeping on
trees,—an easy attitude for repose is most essential to its comfortable
existence; and accordingly we find, that the auxiliary vertebræ at the
base of the neck contribute to produce that flexibility of this organ
which allows the head of the animal to incline forwards and rest upon
its bosom.” Dr. Buckland, from whose Paper on the “Adaptation of the
Structure of the Sloths to their peculiar Mode of Life,”[48] the
preceding judicious physiological remark is quoted, adduces the
authority of Mr. Burchell in proof that the Sloth can in a remarkable
manner and with great facility twist its head quite round, and look in
the face of a person standing directly behind it, while at the same time
the body and limbs remain unmoved. A single glance at the length and
slenderness of the cervical region of the spine, and of the feeble
condition of the transverse and spinous processes in the vertebræ
composing that part of the skeleton of the Sloth, is enough to show its
adaptation to increase the rotatory motion and flexibility of the neck.
In describing the skeleton of a species of Armadillo (_Dasypus
6–cinctus_, Linn.)[49] I was led in like manner to point out the
subserviency of the peculiarities of the cervical vertebræ to the habits
and mode of life of that animal; observing that the “anchylosis of the
cervical vertebræ obtains in the _Cetacea_, as well as in the genus
_Dasypus_, and that as in the aquatic order this firm connexion of the
cervical vertebræ assists materially in enabling the head to overcome
the resistance of the dense fluid through which they perpetually move,
so in the Armadillos a like advantage may be derived from this structure
during the act of displacing the denser material in which they excavate
their retreats.”[50]
Having in view these well-marked examples of the subserviency of the
structure of the bones of the neck to the habits of existing species of
the order _Bruta_, I proceeded to investigate the structure of the
corresponding part of the skeleton in the _Scelidotherium_, hoping
thereby to gain a new and useful element in the determination of the
problem at present under discussion, as to the affinities and habits of
the extinct Megatherioid quadrupeds.
The fossil, in its original state, yielded a view of so much of the
anterior part of the bodies of the cervical vertebræ as proved that they
were neither so numerous as in the Sloth, nor anchylosed together as in
the Armadillos: after a long and careful chiselling at the hard matrix
in which they were imbedded, the transverse and spinous processes were
exposed to view, as they are represented in Plates XX. and XXIV. The
description of these processes has already been given.
On comparing the cervical vertebræ of the Scelidotherium with those of
the existing _Bruta_, the closest resemblance to them was found in the
skeleton of the Orycterope. Now this quadruped, though not so rapid a
burrower, or so strictly a subterranean species as the Armadillos,
participates, nevertheless, to a certain extent, in their fossorial
habits, and is closely allied to them in general structure: it differs
from them, indeed, mainly in a modification of the dental system, in the
absence of dermal armour, and of anchylosis of the cervical vertebræ.
But the advantages which, as a burrower, it would have derived from the
latter structure, are compensated for by the shortness of the cervical
vertebræ, and by the great development and imbricated or interlocking
co-adaptation of the transverse and anterior spinous processes of the
cervical vertebræ. The analogous quadruped in the South American
Continent—the great ant-eater (_myrmecophaga jubata_) which uses its
powerful compressed fossorial claws for breaking through the hard walls
of the habitations of its insect prey, but which does not excavate a
subterraneous retreat for itself, presents the cervical vertebræ of a
more elongated form, and without that development of the spinous and
transverse processes which tend to fix the neck and increase the size of
the muscles which move the head: and, if we could conceive that its
fore-feet were employed to scratch up vegetable roots, instead of
disinterring termites, there would be no reason to expect any
modification of the cervical vertebræ as a direct consequence of such a
difference in the application of its fossorial extremities: when,
therefore, we find that the cervical vertebræ do actually differ in two
myrmecophagous species, to the extent observable in the Cape and South
American ant-eaters, we arrive legitimately at the conclusion that such
difference relates to fossorial habits of the one species, in which
habits the other does not participate.
Now, therefore, if this conclusion be just in regard to the Orycterope,
it must bear with more force upon the question of the habits of the
Scelidotherium as the mechanism for strengthening the connection of
cervical vertebræ, and for augmenting the surface of attachment of the
muscles which worked the head and neck, is more strongly wrought out in
that extinct species.
The great size and strength of the spinous process of the dentata, and
the mode in which it is interlocked with the spinous and oblique
processes of the third cervical, together with the imbricated
disposition of the transverse processes of this and the succeeding
vertebræ, and the remarkable height of the dorsal spines, all combine to
indicate in a very striking manner, if not to demonstrate, that the
conical head of the present species, which is comparatively small and
slender, and for its own mere support requiring therefore no such
mechanism, was used in aid of the fossorial actions of the extremities.
As the cervical vertebræ of the Megatherium have their processes
comparatively weaker than in the Scelidotherium, and the anterior dorsal
spines are relatively shorter, it may be concluded, that whatever were
the extent or nature of the fossorial labours of the enormous claws with
which it was provided, the head did not co-operate with the digging
implements in their especial task in the same degree as in the
Scelidothere and Orycterope. At the same time there is no modification
of the cervical region of the spine of the Megathere corresponding with
those which we have seen to be subservient to the arboreal habits of the
sloth, a remark which will not be deemed superfluous by those who have
perused the acute observations and arguments adduced by M. Lund in
favour of the scansorial character of the extremities of the Megatherium
and Megalonyx.
The fragments of the dorsal vertebræ and ribs of the Scelidotherium,
which are figured in Plate XX., offer no modifications which need detain
our attention; they closely conform, excepting in the greater relative
height of the anterior dorsal spines, already noticed, with the
Megatherioid type. The sacrum manifests in its vast expanse, the great
development of the posterior transverse processes to join the ischium,
the capacious medullary cavity, and wide nervous foramina, a like
conformity with the Megatherium, and a corresponding harmony with the
disproportionate bulk of the hind legs.
BONES OF THE EXTREMITIES.
The Scapula in its double spine, the osseous arch formed by the
confluence of the acromion with the coracoid process, and the
substitution of a distinct foramen for the suprascapular notch, agrees
with that of the Megatherium: but the span of the acromial arch is
relatively wider, and the surface for the articulation of the clavicle
is better marked. This articular surface, which is distinctly shewn upon
the acromion of both the scapulæ in Pl. XX. is the more interesting, as
being the only evidence of the clavicle of the Scelidothere which we at
present possess; but it is enough to prove that this quadruped enjoyed
all the advantages in the actions of the fore extremity, which arise out
of the additional fixation of the shoulder-joint afforded by the
clavicle—a bone which the extinct Megatherioids are the largest of the
mammiferous class to possess in a completely developed state. The form,
position, and aspect of the glenoid cavity for the humerus closely
correspond with the condition of the same part in the Megatherium. The
limits of the acromial and coronoid portions of the arch were still
defineable in the present skeleton, which indicates the nonage of the
individual in the unanchylosed condition of most of the epiphysiaes.
In regard to the presence of a clavicle in the Megalonyx M. Lund has
deduced certain conclusions, which, if well founded, would be equally
applicable to the present allied species, and to the great Megatherium.
I am induced, therefore, to offer a few physiological observations on
that bone, which appear to me to lead to a more correct interpretation
of its uses and relations in the great mammiferous animals now under
consideration.
When the anterior extremities in mammalia are used simply for the
purpose of progressive motion on dry land, as in the Pachyderms and
Ruminants, or in water, as in the Cetaceans, there is no clavicle; this
bone is introduced between the sternum and acromion, in order to give
firmness and fixity to the shoulder-joint when the fore-leg is to
discharge some other office than that of locomotion. In these cases,
however, the clavicle exists in various degrees of development, and even
its rudiment may be dispensed with in some of the actions which require
a considerable extent of lateral or outward motion, and of freedom of
rotation of the fore-limb. When, therefore, we find the clavicle fully
developed in the skeleton of an extinct mammiferous animal, and so
placed as to give the humeral articulation all the benefit of this
additional mechanism, we may confidently expect that it will afford an
insight into the habits and mode of life of such extinct species. M.
Lund[51] has argued from the clavicle of the Megalonyx, that it climbed
like a Sloth. “Animals,” says Sir C. Bell,[52] “which fly or dig, or
climb, as Bats, Moles, Porcupines, Squirrels, Ant-eaters, Armadillos,
and Sloths, have this bone; for in them, a lateral or outward motion is
required.” But in regard to the present problem, we have to enquire
whether the clavicle manifests any modifications of form, of strength,
or development in relation to the special differences of these several
actions, with which its presence is asserted to be associated?
In mammals which fly, the clavicle is always complete: the rabbit, the
fox, and the badger are instances of burrowing animals in which the
clavicle is absent or rudimental. The presence of a perfect clavicle is
not more constant in climbing quadrupeds. The Ai, for example, has an
incomplete clavicle, which is attached to the acromion process, and
terminates in a point about one-fourth of the distance between the
acromion and the top of the sternum, to which the clavicular style is
attached by a long slender ligament: the advantage, therefore, which a
perfect clavicle affords in the fixation of the shoulder-joint, is lost
to this climber _par excellence_. Again, the Bears, which are the
bulkiest quadrupeds that are gifted with the faculty of climbing, and
this in so perfect a degree that the Sun-bears of the Eastern Tropics
may be termed arboreal animals,—these scansorial quadrupeds are
destitute of even the smallest rudiment of a clavicle, as I have
ascertained by repeated careful dissection.
Since, therefore, a clavicle in any degree of development is not
essential to a climbing quadruped, we must seek for some other relation
and use of that remarkably strong, and perfect bone, as it exists in the
Megathere, Megalonyx, and Scelidothere. The absence of ‘dentes primores’
or of anterior or incisive teeth in these quadrupeds at once sets aside
any idea of its connection with an action of the fore extremities, very
common in the mammals which possess clavicles, viz., that of carrying
the food to the mouth, and holding it there to be gnawed by the teeth.
Flying is of course out of the question, although our surprise would
hardly be less at seeing a beast as bulky as an elephant climbing a
tree, than it would be to witness it moving through the air. If now we
restrict our comparison to the relations of the clavicle in that order
of Mammalia to which the extinct species in question belonged, we shall
see that it is most constant, strongest, and most complete in those
species which make most use of their strong and long claws in displacing
the earth, as the Armadillos and Orycteropus: and, as the clavicle is
incomplete in one climbing Edental, we are naturally led to conclude
that its perfect development in an extinct species must have been
associated with uses and relations analogous to those with which it
coexists in other genera of the same order. Thus it will be seen, that,
in rejecting the conclusion drawn by M. Lund from the presence of a
clavicle, I concur in the opinion expressed by Dr. Buckland[53] that the
Megatherium—and with it the Megalonyx and Scelidotherium—had the
shoulder-joint strengthened by the clavicle, in reference to the office
of the fore-arm, as an instrument to be employed in digging roots out of
the ground. Not, however, that these gigantic quadrupeds fed on roots,
but rather, as the structure of the teeth would show, on the foliage of
the trees uprooted by the agency of this powerful mechanism of the
fore-legs, and of the otherwise unintelligible colossal strength of the
haunches, hind legs, and tail.
The humerus presents a large convex oval head, on each side of which is
a tuberosity for the implantation of the supra- and sub-scapular
muscles: these tuberosities do not rise above the articular convexity,
so as to restrict the movements of the shoulder-joint, as in the Horse
and Ruminants, but exhibit a structure and disposition conformable to
those which characterize the proximal extremity of the humerus in other
mammalia which enjoy rotatory movements of the upper or fore-limb. The
tuberosities are, however, relatively more developed, and give greater
breadth to the proximal end of the humerus in the Scelidothere than in
the Megathere. The distal end of the humerus, although mutilated,
clearly indicates that it had the same characteristic breadth of the
external and internal condyles, as in the Megatherium. In fig. 1. Pl.
XXV. which gives a front view of the left humerus, the broad internal
condyle, with its extremity broken off, is seen projecting to the left
hand; both in this figure and in fig. 2. in which the internal side of
the humerus is turned towards the observer, the wide groove, with its
two osseous boundaries, is shewn, which plainly indicates that the left
condyle was perforated for the direct passage of the artery or median
nerve, or of both, to the fore-arm. The groove for the musculo-spiral
nerve on the outer side of the humerus is over-arched at its upper part
by a strong obtuse process; which is comparatively less developed in the
Megatherium. The trochlear or inferior articular surface of the humerus
presents, as in the Megatherium, two well-marked convexities, with an
intervening concavity: this indication of the rotatory power of the
fore-leg is confirmed by the form of the head of the radius.
In Pl. XXV. fig. 4. a view is given of this articular surface: it
presents the form of a subcircular gentle concavity, which plays upon
the outer convexity of the humeral articular surface: immediately below
the upper concavity the radius presents a lateral smooth convex surface,
which rotates upon a small concavity on the ulna, analogous to the
‘lesser semilunar,’ in human anatomy, in which the mechanism for
rotation, so far as the upper joint of the radius is concerned, is not
more elaborately wrought out than in the present extinct edentate
quadruped. The radius expands as it proceeds to the elbow-joint, where
it attains a breadth indicative of the great power and size of the
unguiculate paw, of which it may be called the stem, and to the
movements of which it served as the pivot.
All the bones of the fore-limb just described—the scapula, the humerus,
and the radius,—indicate by the bold features and projections of the
muscular ridges and tubercles the prodigious force which was
concentrated upon the actions of the fore-paw, and the ulna, in its
broad and high olecranon (of which a side view is given in fig. 2. Pl.
XXV.) gives corresponding evidence. The great semilunar concavity is
traversed by a sub-median smooth ridge, which plays upon the interspace
of the two humeral convexities. The body of the bone is subcompressed,
straight, and diminishes in size as it approaches the carpal joint: the
immediate articulating surfaces are wanting in both the radius and ulna,
the epiphyseal distal extremities having become detached from their
respective diaphyses.
Of the terminal segment of the locomotive extremities, the only evidence
among the remains of the skeleton of the Scelidothere is the ungueal
phalanx figured at Pl. XXVII. 3, 4, and 5; but as it is uncertain
whether it belongs to the fore or hind-foot, it will be described after
the other bones of the extremities have been noticed.
Of these bones the femur is the most remarkable, both for its great
proportional size, and its extreme breadth, as compared with its length
or thickness: but in all these circumstances the affinity of the
Scelidothere with the Megathere is prominently brought into view. There
is no other known quadruped with which the Scelidothere so closely
corresponds in this respect. In proceeding, however, to compare together
the thigh-bones of these two extinct quadrupeds, several differences
present themselves, which are worthy of notice: of these the first is
the presence in the Scelidothere of a depression for a ‘ligamentum
teres’ on the back part of the head of the femur, near its junction with
the neck of the bone: this is shewn in the posterior view of the femur
given in Pl. XX. The head itself forms a pretty regular hemisphere: the
great trochanter does not rise so high as in the Megatherium, but,
relatively, it emulates it in breadth: the small trochanter is
proportionally more developed: the external contour of the shaft of the
femur is straighter in the Scelidothere than in the Megathere, and the
shaft itself is less bowed forwards at that part. The articular condyles
occupy a relatively smaller space upon the distal extremity of the femur
in the Scelidothere, and they differ more strikingly from those of the
Megathere, in being continued one into the other: the rotular surface,
for example, which is shewn in fig. 5. Pl. XXV. is formed by both
condyles, while in the Megatherium it is a continuation exclusively of
the external articular surface.
The patella, which works upon the above-mentioned surface, is a thick
strong ovate bone, with the smaller end downwards: rough and convex
externally, smooth on the internal surface, which is concave in the
vertical and convex in the transverse directions.
Of the bones of the leg only the proximal end of the tibia is preserved;
but this is valuable, as shewing another well-marked difference between
the Scelidothere and Megathere; for whereas in the latter the fibula is
anchylosed with the tibia, this bone, in the Scelidothere, presents a
smooth flat oval articular surface, which is shewn in fig. 2. Pl. XXVII.
below the outer part of the head of the bone; from the size and
appearance of which, I infer, that the fibula would not have become
confluent with the tibia, even in the mature and full-grown animal.
The relative length of the fore and hind extremities cannot be precisely
determined from the present imperfect skeleton of the Scelidothere; but
there is good evidence for believing, that the fore extremity was the
shortest. The humerus is shorter than the femur by one-ninth part of the
latter bone; and the radius, which wants only the distal epiphysis, must
have been shorter than the humerus. Now the relative development of the
fore and hind legs is one of the points to be taken into consideration
in an attempt to determine the habits and nature of an extinct mammal.
In climbing animals the prehensile power is more essential to the hinder
than to the fore parts or extremities. In the leech the principal sucker
is in the tail; and higher organized climbers, in like manner, depend
mainly on their posterior claspers in descending trees, and hold on by
means of them whilst selecting the place for the next application of
those at the fore part of the body, whether their place be supplied by
the beak, as in the Maccaws, or the fore-feet or hands in the Mammalia.
But, although we perceive the hinder limbs to be the last to lose the
advantageous structure of the hand in the Quadrumanous species, and
notwithstanding that the tail is for this purpose sometimes specially
organized to serve as a prehensile instrument, yet we find that the
power of grasping the branches of trees by either legs or tail is never
maintained at the expense of undue bulk and weight of those organs. On
the contrary, as the fore-limbs are the main instruments in the active
exertions of climbing, so they are the strongest as well as the longest
in all the best climbers, and the weight of the body which they have to
drag along is diminished by dwarfish proportions of the hinder limbs, as
in the Orangs and the Sloths.
Can those huge quadrupeds have been destined to climb that had the
pelvis and hinder extremities more ponderous and bulky in proportion to
the fore parts of the body than in any other known existing or extinct
vertebrate animals?
M. Lund argues for the scansorial character of the Megalonyx, because
its anterior extremities are longer than the posterior ones; but if they
somewhat exceed the hind legs in length, how vastly inferior are they in
respect of their breadth and thickness. The prehensile faculty of the
hinder limbs of the best climbers, as the Sloths, Orangs, and Chameleons
is by no means dependent on the superior mass of muscle and bone which
enters into their conformation, but is associated with the very reverse
conditions.
It is impossible to survey the discrepancy of size between the femur and
the humerus of the Scelidothere, as exhibited in Pl. XX., without a
conviction that it relates to other habits than those of climbing trees.
The expanse of the sacrum, the evidence of the muscular masses employed
in working the hind legs and tail, which is afforded by the capacity of
the cavity lodging the part of the spinal marrow from which the nerves
of those muscles were derived, both indicate the actions of the hind
legs and tail to have been more powerful and energetic than would be
required for mere prehension: and the association of hinder extremities
so remarkable for their bulk, with a long and powerful tail, forbids my
yielding assent to the speculation set forth by M. Lund, as to the
prehensile character of the tail of the Megalonyx.
_Astragalus._—In the examination of this characteristic bone I have kept
in view the question of the habits of the Megatherioid quadrupeds in
general, and the especial affinities of the Scelidotherium, in
illustration of which I shall notice at the same time the peculiarities
of the astragalus of the Sloth, Megatherium and Armadillo.[54]