The Cambridge Natural History Part 14

A rude painting in a cavern, in red ochre, seems to Dr. Moreno (whose words we have just quoted) to be somewhat suggestive of a _Glyptodon_. There are some reasons for believing that this quadruped was kept by man as a domestic creature. In the cave are two walls of rough pieces of stone which seem to have dropped down owing to the wearing away of the roof; they also seem to have been loosely piled together to form two walls, within which enclosure an imperfect skull of the animal was found. This skull shows clearly that the so-called "_Neomylodon_" must be referred to _Glossotherium_ or _Grypotherium_, as it is sometimes termed. This skull is perforated on the roof in such a way as could only have been effected (in the opinion of experts) by a weapon in the hand of a man. A hole in the skin has been even compared to a bullet-wound. But this it is perhaps unnecessary to discuss. The skin of _Glossotherium_ is, like that of other extinct "Ground-sloths" (_e.g. Mylodon_), filled with small and irregular ossicles. But in _Mylodon_, the sculptured appearance of the dermal ossicles appears to indicate that they reached the surface of the body and were covered by epidermis alone, which is not the case with the animal now under consideration. The microscopic characters of the ossicles, too, show differences in the two. _Glossotherium_ being "precisely intermediate between _Mylodon_ and the existing Armadillo (_Dasypus_)." Now _Glossotherium_ and _Mylodon_ are regarded as forms which lie between the existing Anteaters and the Sloths of the same part of the world. We have already pointed out the facts of structure which lead to this conclusion.

It might therefore be reasonably surmised that the hair of _Glossotherium_ would be also intermediate, or at least like that of one of the two genera _Myrmecophaga_ and _Bradypus_. But microscopical investigation has {183} negatived this supposition. It has shown that the Armadillos are in this matter the nearest relatives of _Glossotherium_. This result is important as tending further to confirm the close interrelationship of all the American Edentates as contrasted with the Old-World forms--a matter which has already been emphasised. It is suggested, however, that the absence of under fur, which is so well developed in the Sloth, and the difference shown in transverse sections from the hair of _Myrmecophaga_, may be explained by difference in habitat. _Glossotherium_ lived under conditions similar to those under which the Armadillos live to-day. Thus the outer covering of the body became alike in the two cases, the same needs supervening in both genera.

_Lestodon_ is another allied genus, which seems to possess canines. At any rate, in front of the four molars, and separated from them by a diastema, is a smallish, somewhat canine-like tooth, in both jaws.

_Megalonyx_ and its allies are sometimes placed in a distinct family, MEGALONYCHIDAE. _Megalonyx_ itself had a skull very like that of _Bradypus_, being shorter and not so elongated as in the Mylodontidae.

There is a strong tusk anteriorly, which is separated by a considerable s.p.a.ce from the three molars lying behind it. Both pairs of limbs seem to have possessed five toes. This is a North American genus. It differs from the bulk of the American Edentates in having a complete jugal arch.

_Megatherium_ is the type of yet a third family, MEGATHERIIDAE, of the Gravigrade Edentates. This creature is familiar from the many restorations which have been built up, and from its huge bulk, little short of that of an elephant. The skull, which is small for the size of the creature, has a complete jugal arch, from the middle of which depends a downward process as in other allied forms. The teeth grow to an extraordinary depth, and there are five of them in the upper and four in the lower jaw--on each side of course. The fore-limbs of the _Megatherium_ are very much more slender than the enormously bulky hind-limbs, upon which and the equally ma.s.sive tail the animal seems to have supported itself while tearing down branches of trees, upon whose leaves it fed. In the scapula the acromion joins the coracoid as in _Bradypus_; the clavicle is large. The fore-limb is four-toed, and the hind-limb three-toed. The latter has but one clawed digit (the third, _i.e._ the inner). {184} On the ma.n.u.s, the three inner digits have powerful claws. This animal, too, was Pleistocene in time. The Megatheriidae had, however, small as well as gigantic forms.

The genus _Zamicrus_ had a skull no bigger than that of a Sloth, while _Nothrotherium_ was also a comparatively small creature; the teeth of the latter genus are reduced to 4/3.

The extinct group of the GLYPTODONTIDAE comprises large creatures with a dense covering of bony scutes which are arranged in a tesselated fashion, and thus form an immobile armature of immense strength. In correspondence with this ma.s.sive carapace the dorsal vertebrae have fused together, and the lumbar vertebrae form a series ankylosed to each other and to the following sacrals. These creatures are all South American.

[Ill.u.s.tration]

FIG. 106.--_Glyptodon clavipes._ 1/12. (After Owen.)

_Glyptodon_, the genus which gives its name to the family, is known from numerous remains in South America, and also from so far north as Texas and Mexico. It grew to be as long as 16 or 17 feet. In the skull there is an exceedingly long downward process of the zygomatic arch, as in Sloths, the arch itself being complete. The process extends so far down as to reach a point about on a level with the middle of the lower jaw. The nasals are short or rudimentary. As in _Myrmecophaga_, the pterygoids enter into the formation of the bony palate. The lower jaw has a spout-shaped extremity, and, behind, it rises into an enormous vertical branch as high as the front part of the jaw is long. There are eight teeth in each half of each jaw. As in {185} some Armadillos, the cervical vertebrae are at least partly fused.

The atlas is free, but the rest, or at any rate five of them, are united.

The last cervical is sometimes fused with the succeeding dorsals; the latter are twelve in number, and are fused together so far as concerns their centra and neural processes. The succeeding region of the vertebral column includes seven to nine lumbars, which are fused with the eight sacrals; in this region the neural processes are high, and there is thus produced a strong and lofty ridge along the back, which forms a powerful support for the carapace. The fore-limbs are shorter than the hind-limbs, which latter are attached to an unusually ma.s.sive pelvis. The claws of the limbs are blunt and almost hoof-like.

The heavy carapace consists of sculptured, five or six-sided plates, which have no particular arrangement in the middle, but towards the margins show indications of an arrangement in transverse rows. The moderately long tail is also encircled by bony skin-plates which are th.o.r.n.y above, or at least provided each with a blunt upstanding process. It appears that outside this bony system of scutes were h.o.r.n.y epidermic scales, corresponding exactly with the tesserae which they cover. There are apparently a good many species of _Glyptodon_.

In the allied genus _Panochthus_ the tail is rather longer, and the bony rings which surround it, instead of being all movable as in _Glyptodon_, are at first so, but later, _i.e._ towards the end of the tail, become welded into a single and ma.s.sive piece. Both feet are here four-toed, while in _Glyptodon_ the hind-feet are five-toed and the fore-feet four-toed.

_Daedicurus_ shows a further specialisation, in that the feet have three and four digits respectively. The orbit too shows a specialisation in being separated from the temporal fossa. The descending process of the zygomatic arch is not so extraordinarily exaggerated as it is in _Glyptodon_. It has the same terminal tube of osseous scutes upon the tail. This creature seems to have reached a length of about twelve feet.

_Propalaeohoplophorus_ is, unlike the great Armadillos that we have hitherto dealt with, a small animal, not exceeding 2 feet or so in length of carapace. A small alveolus on each side of the premaxillae seems to suggest the former presence of an incisor tooth; and it seems that the animal possesses both true molars and premolars; for the first four of the eight teeth are much {186} simpler in structure than those which follow.

The dorsal vertebrae again are not fused together; the hind-limbs are five-toed. All the plates of the carapace are arranged in definite transverse rows; it has been observed, too, that some of the anterior scutes overlap like those of the Armadillos, to which this animal possesses further likenesses in the exclusion of the maxillae from the border of the nostril (a Glyptodont character), and the comparative feebleness of the scutes.

A primitive genus also appears to be _Peltephilus_, which is perhaps rather an Armadillo than a _Glyptodon_. However, it comes somewhat between the two, like _Propalaeohoplophorus_, with which it may therefore be treated. A most singular feature of this genus has been mentioned on p. 27 in connexion with the skull in the Mammalia generally. That is the fact that a portion of the squamosal surrounding the articular facet for the lower jaw is separated by a suture from the rest of that bone, and is therefore obviously suggestive of the quadrate in the lower Vertebrates. As in certain Armadillos and Glyptodons, etc., the pterygoids appear in this genus to have taken a share in the formation of the hard palate. The plates of the carapace were movable, as is shown by the fact that they sometimes slightly overlap. In view of the possible origin of the Edentates from lowly-organised Mammalia, it is noteworthy that the humerus has been especially compared to that of the Monotreme. _Peltephilus_ differs from other Armadillos in having teeth in the front of the jaws. The total number of teeth is twenty-eight, _i.e._ seven in each half of each jaw.

SUB-ORDER 2. NOMARTHRA.

As already explained, the Old-World Edentates differ from the New-World forms in having normal dorsal vertebrae, that is to say, without additional zygapophyses. That negative feature, however, though combined with the positive fact that both the Old-World forms feed upon ants, is hardly sufficient to outweigh the many structural differences which distinguish the Orycteropodidae from the Manidae; which will be placed therefore in different groups. To that containing the Aard Vark, the name TUBULIDENTATA may be applied. {187}

This group contains but one family, the ORYCTEROPODIDAE, of which there is but a single genus.

The Aard Vark (earth-pig), genus _Orycteropus_, is characterised by its heavy build, the body being covered by rather coa.r.s.e and not very abundant hair; the snout is long and pig-like, with round nostrils at its end; the ears are long, erect, and pointed; the tail is very thick at first, so that it has been aptly described as "a tapering of the body to a point." The fore-limbs are four-toed, the hind five-toed.

[Ill.u.s.tration]

FIG. 107.--Aard Vark, or Cape Anteater. _Orycteropus capensis._ 1/16.

In the skull there is a complete though slender zygoma; the premaxillaries, though small, are not so rudimentary as in the American Edentates. The annular tympanic is not ankylosed to the surrounding bones, a character found in other low mammals. Contrary to what is found in _Manis_, _Orycteropus_ has a huge lachrymal. There are thirteen dorsal and seven lumbar vertebrae. The clavicle is well developed. _Orycteropus_ is peculiar among Edentates in that the ischia do not unite with the vertebral column.

The femur has a third trochanter.

As mentioned on p. 162, the Aard Vark is diphyodont like normal mammals.

The permanent teeth consist of five molars and premolars on each side of each jaw; the first two of these are premolars, and are simpler in their form than the succeeding two teeth, which are partly divided by a median furrow into two halves. These teeth are also peculiar in that they consist entirely of vaso-dentine. They have been compared in minute structure to those of the Ray _Myliobates_. According to Mr. Oldfield {188} Thomas[115]

there are seven milk teeth on each side of the upper jaw (limited to the maxillae, and thus not incisors). An eighth tooth was discovered on one side of one of the specimens examined by Thomas. In the lower jaw there are only four milk teeth on each side. It is interesting to note that the histological structure of these milk teeth agrees with that of the permanent teeth. There are two species of this genus found in Africa: the southern, _O. capensis_, is more hairy than the northern, _O. aethiopicus_.

_O. gaudryi_ is a Pliocene species from the Island of Samos and from Persia, described by Dr. Forsyth Major and Dr. Andrews.[116] It closely resembles the existing _O. aethiopicus_.

[Ill.u.s.tration]

FIG. 108.--Section of lower jaw with the teeth of _Orycteropus_. 2.

(After Owen.)

Of the Scaly Anteaters, Group SQUAMATA or MANIDAE, there is really but one genus, though _Phatagin_, _Pholidotus_, _s.m.u.tsia_, and _Pangolin_ have been used to distinguish various forms. The genus _Manis_ is African and Oriental in range. Dr. Jentink, who has lately revised the species, allows seven.[117] The external form of these animals is fairly well known, the remarkable scales distinguishing the Pangolins from other animals. Between the scales lie hairs, which seem to be absent in the adults of the African species, though present in the young, thus affording a convenient method of distinguishing the Ethiopian from the Oriental forms. The scales have been compared to agglutinated hairs. That they are not "merely mimetic of the Lizards' scales" is held by Weber,[118] who compares them directly with those {189} structures, as he does the scales of other mammals, such as those upon the tail of _Anomalurus_, etc. This, however, is not a universal opinion. It is true that these scales occur chiefly in the lower forms of mammals such as those under consideration, Marsupials, Rodents, and Insectivores; but the fact that the hairs are developed before the scales shows, or seems to show, that the former are the older structures, and to lead to the inference that the scales of mammals are new structures. The scattered hairs of the Pangolin have no sebaceous glands excepting on the snout. This, again, looks as if they were degenerate structures, and emphasises the non-archaic character of the scales. These animals have no trace of teeth except possibly some slight epithelial thickenings which have been interpreted as a last remnant; the tongue is suited for the capture of ants, and is therefore much like that of the not nearly-related American Anteaters. The stomach is of simple form; it is characterised by a large gland, which suggests that of the Koala (see p. 144); the intestine has no caec.u.m. Retia mirabilia occur on the limb arteries. The placenta is non-deciduate and diffuse; it is specially compared by Weber with that of the Horse. Considering the many adaptive resemblances between this genus and the American Anteaters, especially in the mouth cavity, it is remarkable that in _Manis_ the pterygoids are not joined as they are in _Myrmecophaga_. In spite of statements to the contrary, it appears that there is sometimes a distinct lachrymal.

A remarkable feature in the skeleton of _Manis_ is the singular sternum.

The xiphoid cartilage is extraordinarily elongated into thin strips, which reach the pelvis and return. This state of affairs is to be found in the African species only. This structure is not comparable, as it has been said to be, with abdominal ribs such as those of the reptile _Hatteria_.

These animals are mainly anteaters. The j.a.panese have a curious legend as to the method adopted for the capture of ants, which is related by Dr.

Jentink in his monograph of the genus. The Manis "erects his scales and feigns to be dead; the ants creep between the erected scales, after which the anteater again closes its scales and enters the water; he now again erects the scales, the ants are set floating, and are then swallowed by the anteaters"! The same story is related by Mr. Stanley Flower on the authority of the Malays.

Though it seems clear that the likenesses which _Manis_ shows {190} to the Anteaters of the New World are chiefly adaptive and have nothing to do with real affinity, being merely an expression of a similar mode of life, it is curious to note that here and there we do find certain resemblances which do not seem to be susceptible of the latter explanation. The jugal bone, absent in _Manis_, is small in _Myrmecophaga_; the clavicle is absent and again small or rudimentary in the Anteaters; it is large in other Edentates. The third trochanter is absent, as in _Myrmecophaga_ (and the Sloths). There are many scales on the body; in _Myrmecophaga_ there are traces of these structures on the tail, as also in _Tamandua_. In the features mentioned, the Myrmecophagidae differ from either or from both of the two other American families (_i.e._ Dasypodidae, Bradypodidae) and agree with _Manis_. The facts are not a little remarkable.

[Ill.u.s.tration]

FIG. 109.--Manis. _Manis gigantea._ 1/12.

ORDER III. GANODONTA.[119]

Allied to the Edentata, and apparently representing the ancestral forms from which they, at any rate the Xenarthra were derived, is the order of the Ganodonta. Of this order a number of genera are now known, which can be ranged in a series which more and more approaches the Edentata as we pa.s.s from the older to the newer forms. This interesting and transitional series will be made manifest by a description of the characters of the various genera taken in their proper {191} chronological order. The following genera are included by Wortman in his family STYLINODONTIDAE.

The earliest type of the Ganodonta is the genus _Hemiga.n.u.s_, with but one species, _H. otariidens_. This animal lived during the deposition of the lowest Eocene strata, the Puerco beds of North America. It was about as big as a fair-sized Dog, and had powerful jaws. There were at least two pairs of incisors in the upper jaw, together with powerful canines and the full premolar and molar formula. In the lower jaw the canines were also strong, but the incisors are not certainly known to be more than two pairs. The enamel upon the posterior surface of the canine is thin, and in the case of the incisors the enamel seems to be limited to the anterior face. The lower molars are quadritubercular. It is believed from the presence of a suture on the upper surface of the premaxillary that the snout of the creature was tubular. The cervical vertebrae, only known by their centra, are like those of the Armadillos (and for the matter of that of the Whales) in the great transverse as opposed to the antero-posterior diameter. The feet are especially compared with those of the Ground Sloths. The single ungual phalanx is marked by a large subungual process, which is pierced by a considerable foramen. The tibia again is to be compared with that of the Armadillos.

In the Upper Puerco (Torrejon) beds the remains of _Psittacotherium_ are found. This genus, when first discovered, was referred to the Tillodontia by some and to the Ungulates, the latter being a refuge for indeterminate Eocene mammals, just as the "Mult.i.tuberculata" is for similarly-placed Secondary mammals. It is now known to be clearly a member of the order Ganodonta. Wortman thinks that there is but one species, _P. multifragum_.

It seems to have had a general aspect much like that of _Hemiga.n.u.s_--that is judging from the skull--and was not very greatly different in size. The facial portion of the skull is short, and the zygoma is deep. The infra-orbital ca.n.a.l is double, a feature which crops up in the Sloth, and has been mentioned in the later form of Ground Sloth, _Megalonyx_ (but it must be remembered that the same characteristic is not unknown in Rodents).

The dent.i.tion is reduced as compared with that of _Hemiga.n.u.s_, that is to say, as far as concerns the molars and the incisors. There is but a single pair of incisors in each jaw; the canines are strong; the premolar and molar series seem to have been complete in the lower jaw, {192} but reduced by one premolar at least in the upper jaw. It is very important to notice that the incisors have enamel only on their anterior faces, and that the same is the case with the canines, the slender layer present behind the tooth in _Hemiga.n.u.s_ having vanished in this later form. The tooth pattern of the molars is like that of _Hemiga.n.u.s_. The fore-limb is decidedly Edentate-like; but it is the foot which presents the strongest likenesses to that order. "If an anatomist," remarks Dr. Wortman, "had no other part of the skeleton than that of the foot to guide his judgment, and he should fail to detect a most striking similarity between it and that of the Edentata, especially the Ground Sloths, he would not only lay himself open to the criticism of being lacking in the ordinary powers of observation and comparison, but would be suspected of placing the matter upon a basis other than that established by such a method." It is not certain how many toes upon the fore-limbs were possessed by _Psittacotherium_, but the close resemblance to _Mylodon_ is indeed striking, the third digit being in both forms the most p.r.o.nounced. Some vertebrae of this Ganodont have been discovered which do not show the complex articular arrangements of later American Edentates. The sacrum, on the other hand, is very like that of the Sloth, and there is a foreshadowing of the attachment of the ilia to the sacrum by co-ossification which is met with in later Edentates. A still later type is the genus _Calamodon_, which has been shown to occur in Europe as well as in America. _C. simplex_ was a larger beast than either of the genera that have already been treated of, thus affording another example of the increase in size of later as compared with earlier members of the same group, so p.r.o.nounced among the Ungulata. The lower jaw has the same ma.s.sive structure that characterises that bone in _Hemiga.n.u.s_ and _Psittacotherium_. There is but one incisor, but the premolar and molar series are complete. The canine is Rodent-like in appearance, being imbedded throughout the greater part of the lower jaw; it evidently grew from a persistent pulp. It is enamelled upon the anterior face only. The premolar and molar teeth are in this genus commencing to lose their enamel, which is distributed in the form of vertical bands, leaving inters.p.a.ces which are not covered by enamel. These teeth, moreover, are decidedly hypselodont, more decidedly so than in _Psittacotherium_; they are, when unworn, quadricuspidate, with accessory cusps; when more worn, the teeth {193} are double-ridged, and that transversely to the long axis of the jaw; finally, the much-worn teeth have flattish crowns more or less surrounded by a ring of enamel.

A still later form, coming from the Lower and Middle Eocene strata, is the genus _Stylinodon_. _S. cylindrifer_, which is the more archaic of the two described species, is only known from a single molar, fragments of a canine, and "some inconsiderable pieces of the skull." The molar is interesting on account of the fact that the enamel is still further reduced; it is represented only by narrow vertical strips, which are much narrower than those of older forms of Ganodonts. It is also hypselodont, and has a persistent pulp. So, too, the canine which had a thick anterior facing of enamel. The later species, _S. mirus_, is more fully known. The teeth seem to have been much the same as in the last-described species; the premolars and molars were seven in all in the lower jaw, and the canine was imbedded in the bone for a long distance, as in _Calamodon_. The cervical vertebrae have short centra as in _Hemiga.n.u.s_. The clavicles were well developed. The humerus possessed an entepicondylar foramen, and its head displays the pyriform pattern so characteristic of later Edentates. The foot is clearly like that of _Psittacotherium_.

In reviewing the series, therefore, we see a gradual diminution of the incisors, a gradual loss of enamel on the teeth generally, and the production of hypselodont teeth growing from persistent pulps; all of which are features of the later Edentates. The progression is so gradual that the forms enumerated and described seem to have been part of a continuous series culminating in the Ground Sloths of later times. The other points of similarity will be gathered from the facts given in the foregoing pages.

There is another family belonging to the Ganodonta whose position with regard to the Edentata is not so clear. This is the family CONORYCTIDAE, of which two genera are known. The earliest of these, from the Lower Puerco, is _Onychodectes_. In _O. tissonensis_ the skull is long and narrow, thus contrasting with that of the last family. The facial part is also long. The lower jaw is much more slender. The molar formula was complete, but there is some doubt as to the incisors. The molars are tritubercular.

The other known genus is _Conoryctes_. Its skull has a shorter {194} facial portion, and is thus more like that of Stylinodontidae than that of _Onychodectes_. The dental formula is known, and is complete save for the loss of one incisor above and below, and one premolar above. The relationship of these Ganodonts to any later forms is uncertain; but their skeletal structure is as yet by no means fully known.

{195}

CHAPTER IX

UNGULATA--CONDYLARTHRA--AMBLYPODA--ANCYLOPODA--TYPOTHERIA--TOXODONTIA-- PROBOSCIDEA--HYRACOIDEA