MR. OWEN'S MEMOIR ON THE MYLODON ROBUSTUS AND THE HABITS

OF THE MEGATHERIOID ANIMALS.

THE all but complete skeleton of a gigantic animal was discovered near Buenos Ayres in 1841, and purchased for the College of Sur- geons, through the medium of Sir WOODBINE PARISH, our Chargé d'Affaires. On its arrival in England, the skeleton was put to- gether and placed in their Museum ; and the Conservator, Mr. OWEN, has written a minute description and elaborate memoir upon the subject, which is now published by direction of the Coun- cil of the College.

To convey an idea of the form of the extinct gigantic sloth, the Mylodon Robustus, to which this skeleton originally belonged, is not possible without plates. A notion of its magnitude may be attempted. The body, from the fore part of the skull to the end of the tail, is 11 feet in length, being rather shorter than the hippopotamus ; the circumference of the trunk round the pelvis is 9 feet 9 inches, equalling that of the elephant in breadth and ex- ceeding it in depth ; the breadth of the largest caudal vertebra is up- wards of 10 inches in the mylodon, and but 7 inches in the elephant ; whilst the tail of this singular animal, as long as the bind-limbs, "and proportionally thick and strong, assists in supporting rather than depends from the broad sacral termination of the pelvis." In fact, as exhibited in conjectural action by Mr. OWEN'S plate, it answers the purpose of a third leg, forming with the two hind-limbs a species of tripod. The inferred utility of this formation will be seen presently.

The skeleton is not only curious in itself, as the specimen of an extinct race, but important from the light it appears to cast on several disputed questions respecting the habits of the family. In the Museum at Madrid is a skeleton of the Megatherium, "which in certain dimensions surpasses all known quadrupeds, existing or extinct." From the description and the plates of this creature, CUYIER inferred from its teeth that it "lived on vege- tables; and its robust fore-feet, armed with sharp claws, make us believe that it was principally their roots which it attacked,"— an opinion extended by his followers to the other classes of extinct Edentata, of which the mylodon robustus is one. Dr. BUCKLAND and Professor DE BLAINVILLE, however, reject the conclusion of CUVIER, and, from certain anatomical features, asso- ciate a bony armour with the internal skeleton of the megatherium ; whilst Professor DE BLAINVILLE concludes that it had the manners and habits of armadillos, and consequently fed upon flesh, and per- haps also on roots, and that it dug up the earth with its enormous claws, if not for concealment at least to obtain ants. Dr. LUND, a Danish naturalist, "after a close examination and extensive com- parison of Megatherioid animals, discovered by him in the lime- stone caverns of Brazil, has bean led to the inference that they climbed trees, like the sloths of the present day, in order to feed upon their leaves" ; a conclusion that, if true, involves two start- ling considerations touching the enormous bulk and clumsy frames of the animals, (which appear to unfit them for climbers,) and the still more enormous size of the trees they were to climb. Both these points are fairly met by Dr. LUND; who observes upon the latter- " In truth, what ideas must we form of a scale of creation, where, instead of our squirrels, creatures of the size and bulk of the rhinoceros and hippopotamus climbed up trees! It is very certain that the forests in which these huge monsters gambolled could not be such as now clothe the Brazilian mountains : but it will be remembered, that in the former communication which I had the honour of submitting to the Society, I endeavoured to show, that the trees we now see in this region are but the dwarfish descendants of those loftier and nobler forests which originally covered these highlands ; and we may surely be permitted to suppose that the vegetation of that primeval age was on a no less gigantic scale than the animal creation."

After a minute examination, bone by bone and processus by processus, a survey of the complete skeleton, and a consideration of its more remarkable features in reference to comparative anatomy, Mr. OWEN differs from all these conclusions. He con- siders that the Mylodon Robustus, Owen, was of the existing sloth species, though of a gigantic size, but that it overthrew trees to procure the leaves on which it fed, instead of climbing them. The anatomical reasons for this inference would occupy much space without being intelligible to general readers, but we may say that on this hypothesis some peculiarities of formation are reconcileable and explainable. The enormous claws of the fore-arm (popularly leg) enabled it to loosen the tree by digging up the earth round the roots ; the same weapons enabled it to seize the branches or trunk ; the power of the fore-arms produced the to and fro motion by which it endeavoured to overthrow the tree ; which the peculiar formation of the hind-quarters assisted to complete.

" The enormous pelvis of the mylodon proclaims itself the centre whence muscular masses of unwonted force diverged to act upon the trunk, the tail, and the hind-legs. Those muscles originating from the sacrum and the broad and extended lip of the ilium, as the saerolumbalis, the longissimus and lads- shoos dorsi, &c., and which pass forwards to extend the trunk and retract the anterior limbs, have left the most marked evidence of their size and energy of action in the long and strong spinal crest of the sacrum, and in the broad, rugged, and anteriorly-produced margin of the ilium. The fore-limbs being well adapted for grasping the trunk or larger branches of a tree, the forces con- centrated upon them from the broad posterior basis of the body are manifestly adequate, and are precisely such as might be expected to have cooperated in the act of uprooting the tree or of wrenching off the branch so seized. But in order that the pelvis should possess stability and resistance equivalent to the due effect of the forces acting from it and so applied, it was necessary that it should be bound down as it were, and supported by members of corresponding strength.

"Accordingly, we find a thigh-bone, which, though surpassing the humerus in length, is yet not less than half as broad as it is long, and provided with tro- chanters and ridges, the fit attachments of the tendinous insertions of muscular masses which expanded upon the back part and on the fore part of the broad and capacious pelvis, and have there left, in strong and numerous interfascicniar bony crests, unequivocal evidence of the power by which they resisted the efforts of the antagonizing muscles attached to the trunk and fore-limbs to draw forwards the pelvis and hind-legs. The preponderating weight of both these parts, and the extraordinary power of the muscles connecting them to- gether, are quite inexplicable on the scansorial hypothesis of the megatherioid animals; since, if they attained their food by climbing, the fore-legs would be the fixed point when the muscles attaching these to the pelvis were called into action, and the hind-extremities, needing only the requisite prehensile power, ought to have had their bulk reduced as far as was consistent with such power, in order to facilitate their being drawn upwards towards the fore-legs."

To the fossil or comparative anatomist Mr. OWEN'S work is one of great interest, from its broad arguments to its minute de-

scriptions. To the general reader, its learning, acuteness, and ingenuity, would be unappreciated, from the scientific character of the subject-matter; but some of the more general arguments must excite attention from the sagacity and ingenuity they display. Of this kind we may instance the following. Existing animals of the sloth kind, exposed to falls from trees, are guarded against injury to the brain by the strong double bony wall of the cranial cavity. This formation extends to the mylodon ; and in the skeleton in question there are two fractures of the skull which did not destroy the animal, as one is entirely and the other partially healed. This would seem

to favour the climbing hypothesis : to which Mr. OWEN thus re- plies, whilst he answers objections that might attribute the blows to other sources—

"The liability of the mylodon, in the habitual practice of uprending and prostrating large trees, to be struck by the trunk or some of the large branches, must have been greater than that of the sloth to a fall from its tree ; and therefore the advantage to the mylodon of having a double brain-case would not be less.

" Certain it is, that the habits of life, or the conditions under which the mylodon existed, did render it obnoxious to violent blows on the head, and that it was owing to the extensive and deep cellular diploe of the skull, that they were not, in the present instance, death-blows.

"It is at least not probable that any large mammiferous animal could have survived so extensive and complicated a fracture and depression of the vitreous table at the back part of the skull, as that which in the mylodon is here con- fined to the outer table. Either of the blows, however, to the force of which that strong plate of bone has yielded, must have stunned and at least have temporarily disabled the animal; and, if inflicted by the paw of some suffi- ciently powerful carnivore, would have left the mylodon its easy and unresist- ing prey. if the skull of an animal so destroyed had been preserved and after- wards discovered in a fossil state, the broken bones would not have presented any of those effects of the reparative processes which are so extensively mani- fested in the very remarkable specimen under consideration. " It is not very probable that the mylodon, if disabled and its skull fractured by a blow received in conflict with another ofits kind, would have been suffered to escape : the victorious assailant would in all likelihood have followed up his advantage by a mortal wound, such as an irate megatherium might easily have inflicted with its sharp and ponderous claw if excited by combative or destruc- tive instincts. Nothing, however, that bas yet reached us of the habits of ex- isting edentata would lead to the supposition that the extinct ones were actuated by these instincts, or were characterized by less peaceful habits than those of the sloths, the ant-eaters, and armadillos of the present day. Only in self-defence against the carnivorous jaguar or puma is the strong-clawed ant-eater (myrmecophaga jubata) reported to use successfully its powerful weapons, with the analogues of which a mylodon or mcgatherium might be conjectured to have produced the injuries in our present fossil, on the com- bative hypothesis of their origin. But in the conflict of the great anteater with the jaguar, the predatory assailant is overcome by the pertinacity of the grasp, not by the force of the blow. The only analogies, therefore, by which

we can test the conjecture that the injuries in question were inflicted by another niegatherioid animal, diminish its probability.

" There is no certain or conclusive evidence that human beings coexisted with the megatherian animals : but, assuming a primaeval race of Indians to have disputed the lordship of the American forests with the edentate giants, and to have waged against them, as against all other inferior animals, a war of extermination, the same difficulty presents itself to the supposition of the recovery and escape of a stunned mylodon from their deadly assaults with clubs and other weapons, as from the claws and teeth of the beast of prey : for the flesh of the leaf-eating megatherian would doubtless he as much prized for food by a human destroyer as that of the sloth is by the Indians of the present day. " With these difficulties, therefore, opposing themselves to these conjectures, which naturally rise in the mind at the first view of the injuries on the skull of the extinct mylodon, and which suggest the hostile attacks of some other animal as their cause, we are compelled to refer those injuries to the effects of some inanimate force, which, having felled the mylodon and temporarily dis- abled it, was spent, and could not follow up the blow. To a huge denizen of the woods, what accident more likely to produce such injuries than the fall of a tree; and what inhabitant of the forest more obnoxious to such an accident than one destined by its organization to be habitually engaged in uprooting, and therefore in danger from the descent of heavy trees ? The form of the healed as well as of the partly-healed fracture, in both of which the fissures diverge from a longitudinal instead of radiating from a central depression, accords better with a blow from a branch or trunk of a tree than with one inflicted by the point of a large claw. It must, therefore, be conceded, that both the in- juries, and the structure of the skull by which their immediate fatal effects have been obviated, accord with the habits assigned to the megatherian animals in the present memoir; while they can receive no elucidation from, nor appear in any way connected with, the acts of digging the earth for roots, or ants, or for concealment, which have been severally conjectured to be the habitual tabour of the megatherioids by Clavier, D'Alton, and De Blainville."

In the getting-up of this quarto volume no expense is spared. It is illustrated by upwards of twenty plates : one of them a sketch of the skeleton, exhibited not exactly in action but pre- paring for action ; whilst opposed to it is a specimen of the nearest allied existing genus, drawn to the same scale, and looking like a dwarf against a Hercules.