CHAPTER XII

HISTORY OF THE †TOXODONTIA (OR †NOTOUNGULATA)

It is a regrettable circumstance that, while the successive Tertiary faunas are very fully represented in South America, approximately complete skeletons have, as yet, been obtained from only a few of the various stages; from the others the known material is very fragmentary and largely made up of teeth and jaws. No doubt, the history of fossil-collecting in North America will, in due course of time, be repeated in the southern continent and more and more complete and satisfactory specimens be obtained. At present, however, it is not possible to trace the modifications of structure in any given series with such detail as in those which were developed within the limits of Arctogæa. No such story as that of the horses, the rhinoceroses or the camels, can yet be told of the South American groups, whatever future exploration may teach us. Nevertheless, much has already been learned concerning the strange creatures that once inhabited the Neotropical region and long ago vanished completely, leaving no trace in the modern world.

As was mentioned in Chapter VI, on the present geographical distribution of mammals, South America is to-day the richest and, after Australia, the most peculiar zoölogically of all the regions. All of the modern hoofed animals found in that continent at present, the tapirs, peccaries, llamas and deer, are immigrants derived at a comparatively late date from the north, but throughout the Tertiary and the Pleistocene there were several indigenous types of ungulates, and of these the largest and most varied assemblage was that included in the order †Toxodontia. The most important and best known of the families and genera are listed in the table:

Suborder †TOXODONTA. †Toxodonts Proper

I. †TOXODONTIDÆ.

_†Toxodon_, up. Plio. and Pamp. _†Xotodon_, do. _†Trigodon_, Monte Hermoso. _†Nesodon_, Santa Cruz. _†Adinotherium_, do. _†Pronesodon_, Deseado. _†Proadinotherium_, do.

II. †NOTOHIPPIDÆ.

_†Notohippus_, Patagonian. _†Rhynchippus_, Deseado. _†Morphippus_, do.

III. †LEONTINIIDÆ.

_†Leontinia_, Deseado. _†Colpodon_, Patagonian.

Suborder †TYPOTHERIA. †Typotheres

I. †TYPOTHERIIDÆ.

_†Typotherium_, Plioc. and Pleist. _†Eutrachytherus_, Deseado.

II. †INTERATHERIIDÆ.

_†Interatherium_, Santa Cruz. _†Protypotherium_, do.

III. †HEGETOTHERIIDÆ.

_†Hegetotherium_, Santa Cruz. _†Pachyrukhos_, Santa Cruz to Pampean.

IV. †NOTOPITHECIDÆ.

_†Notopithecus_, Casa Mayor. _†Adpithecus_, do.

V. †ARCHÆOPITHECIDÆ.

_†Henricosbornia_, Casa Mayor.

VI. †ARCHÆOHYRACIDÆ.

_†Archæohyrax_, Deseado.

Suborder †ENTELONYCHIA. †Homalodotheres

I. †NOTOSTYLOPIDÆ.

_†Notostylops_, Casa Mayor.

II. †ISOTEMNIDÆ.

_†Isotemnus_, Casa Mayor. _†Pleurocœlodon_, Deseado.

III. †HOMALODONTOTHERIIDÆ.

_†Homalodontotherium_, Santa Cruz. _†Asmodeus_, Deseado. _†Proasmodeus_, Astraponotus Beds. _†Thomashuxleya_, Casa Mayor.

Suborder †PYROTHERIA. †Pyrotheres

†PYROTHERIIDÆ.

_†Pyrotherium_, Deseado. _†Propyrotherium_, Astraponotus Beds. _†Carolozittellia_, Casa Mayor. _†Paulogervaisia_, do.

SUBORDER †TOXODONTA. †TOXODONTS PROPER

Among the remarkable animals which Charles Darwin found in the Pampean deposits of Argentina and took with him to England, was a skull of one which Sir Richard Owen named _†Toxodon_, or “Bow-Tooth,” from the strongly curved grinding teeth, those of the opposite sides almost meeting in the median line above the hard palate. For many years _†Toxodon_, of which hardly anything was known, save the skull and teeth, was a zoölogical puzzle and no one was able to reach any satisfactory conclusion as to its systematic position and relationships, as all the attempts made to force it into one of the known ungulate groups were obvious failures. The discovery of complete skeletons, two of which are mounted in the La Plata Museum, showed the necessity of making a new group for its reception, as Owen had originally proposed. Through the exploration of Argentina and its Patagonian provinces, the history of the suborder was followed far back into the Tertiary period and its indigenous character demonstrated. This and all the other subdivisions of the †Toxodontia were exclusively Neotropical in distribution, and none have been found farther north than Nicaragua and there only in the Pleistocene.

The suborder was represented in the Pampean beds by several genera, which differed in size and in the complexity of the grinding teeth, but only of _†Toxodon_ is the skeleton at all fully known. The Pampean species of this genus were massive, elephantine creatures, rivalling the largest rhinoceroses in bulk, but not equalling them in height. The teeth were all thoroughly hypsodont and apparently continued to grow throughout life without forming roots; the dental formula was: _i_ 2/3, _c_ 0/0, _p_ 3/3, _m_ 3/3, × 2 = 34. The first upper incisor was broad and chisel-shaped, the second more tusk-like, but in some species these proportions were reversed; the lower incisors were procumbent, pointing straight forward, and of these the third was the largest. The canines were lost and there was a long, toothless gap behind the incisors. The premolars were smaller and simpler than the molars, and the anterior ones were very small and were frequently shed at an early stage, making the number of these teeth variable in different specimens. The upper molars also were of quite simple pattern; the broad and smooth external wall showed no distinct signs of a division into cusps, and from it arose two oblique transverse ridges; the deep cleft or valley which separated these ridges was divided and made Y-shaped on the grinding surface by a prominent spur from the outer wall between the two principal crests. The lower molars were composed of two crescents, one behind the other, of which the posterior one was very much longer, and both were very narrow transversely.

The skull had shortened nasal bones, an indication that some sort of a proboscis or prehensile upper lip was present. There was no trace of a horn, and the general aspect of the skull was not unlike that of one of the hornless rhinoceroses, except for its great vertical depth; the sagittal crest was very short and had almost disappeared. The auditory apparatus was very extraordinary, though it can hardly be described without an undue employment of anatomical terms; suffice it to say that in addition to the usual outer ear-chamber, formed by the inflated tympanic bone, there was a second chamber in the rear wall of the skull, communicating with the first by a canal. This arrangement would seem to imply an unusual keenness in the sense of hearing. The external entrance to the ear was placed very high up on the side of the head, as in the pigs and in many aquatic mammals, suggesting that _†Toxodon_ was more or less amphibious. The anterior, or symphyseal, region of the lower jaw was very broad, flattened and shovel-like, hardly projecting at all below the plane of the lower incisors.

The neck was short and stout, the body long and extremely bulky, having an immense, almost hippopotamus-like girth; the spines of the anterior dorsal vertebræ were very long, making a high hump at the shoulders. The limbs were short and very heavy, the bones very massive and with large projections for muscular attachments. The fore leg was much shorter than the hind, depressing the neck and head in very curious fashion. The shoulder-blade was rather narrow, the spine without acromion or distinct metacromion; the hip-bones were greatly expanded and turned outward, quite in elephant-like fashion, a character which almost invariably accompanies great increase in bodily mass. The thigh-bone was also very elephantine in appearance, a likeness due to its shape and proportions, to the loss of the third trochanter and the flattening of the shaft, so that the width much exceeded the antero-posterior thickness. All of these characters are, as a rule, associated with greatly augmented weight and have been independently acquired in several series of large and massive animals, elephants, †uintatheres, †titanotheres, and to this list should be added the †toxodonts. In the fore-arm the bones were separate and the ulna was quite unreduced and very stout, but in the lower leg, which was very short in comparison with the thigh, the tibia and fibula were coössified at the upper end, but not at the lower, a most exceptional arrangement. The feet were surprisingly small and had but three digits, the reduction from the original five having proceeded to that extent before the process was arrested by augmenting weight. The heel-bone (calcaneum) was so articulated with the other bones of the tarsus as to project almost straight backward, nearly at a right angle to the position normal in a digitigrade foot, a feature which is not known to occur in any other mammal. The hoof-bones were so small and nodular that the foot must have been of the columnar type, the weight resting upon the usual elastic pad.

The restoration (Fig. 121, p. 217) shows _†Toxodon_ as a very heavy, slow-moving, water-loving animal; the aquatic habits are, of course, conjectural, but the general proportions are accurately given by the skeleton.

From the Pleistocene, _†Toxodon_ may be followed back without notable change to the Pliocene, but there it was in association with the last of a curious phylum, the genus _†Trigodon_ (Fig. 138, p. 263), as yet known only from the skull. In these animals a very prominent bony knob or boss on the forehead clearly demonstrates the former presence of a large, rhinoceros-like, frontal horn. But very few of the indigenous South American ungulates possessed horns, or horn-like protuberances of the skull, and all of these so far discovered belonged to the suborder †Toxodonta. _†Trigodon_ was, from present knowledge, the only horned creature of its time and region, for the deer and antelopes which had probably arrived in South America had not advanced so far south as Argentina. Another very peculiar feature of this genus was that the lower incisors were present in uneven number, two on each side and one in the middle. Nothing has been found of the skeleton, but it was doubtless that of a smaller and somewhat lighter _†Toxodon_.

[Illustration: FIG. 235.—Skull of _†Toxodon_, Pampean formation, the upper molars much broken. La Plata Museum.]

The material from the lower Pliocene adds nothing to our knowledge of this suborder, but in the Santa Cruz time of Patagonia, which was Miocene, it was very abundantly represented and preponderatingly by the genus _†Nesodon_, which was the first discovered member of the marvellous Santa Cruz fauna, named nearly 70 years ago by Sir Richard Owen. It also chanced that Owen’s specimen was the imperfect lower jaw of a young animal with the milk-teeth, which were mistaken for the permanent dentition, and when the latter was found long afterwards, it was naturally supposed to belong to a different animal and received a different generic name. Nor was this all; the changes which took place in the appearance and relative size of the permanent teeth within the life-time of the individual were so remarkable, that the successive stages of development were by several investigators supposed to be distinct genera and species and named accordingly. In this way nearly 30 different names have, at one time or another, been assigned to the common species, _†N. imbricatus_; and it was not until the late Dr. Ameghino had brought together a complete series of skulls and jaws illustrating these changes, and showing the gradual transition from one to the other, that the confusion could be cleared up.

[Illustration: FIG. 236.—Skull of Santa Cruz †toxodont, _†Nesodon_; same scale of reduction as Fig. 235.]

There was a long hiatus in time between _†Toxodon_ and _†Nesodon_ and so great was the structural difference between them, that there is much doubt whether the latter was directly ancestral to the former; in any event, _†Nesodon_ so nearly represents what the desired ancestor must have been, as to serve for all practical purposes of the study.

All the species of this Santa Cruz genus were much smaller animals than the species of _†Toxodon_, _†N. imbricatus_ being no longer than a tapir, with considerably shorter legs, and of much slighter and more slender build than _†Toxodon_, though every tooth and every bone proclaims its relationship to the latter.

In _†Nesodon_ the dental formula was unreduced; _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 = 44, though several of the teeth were much reduced in size, so as to have lost their functional importance, and frequently individuals are found in which one or more of these insignificant teeth are lacking. The first upper incisor was a broad, chisel-shaped tooth, which continued to grow for a period, then formed its root, and growth ceased; the second incisor was a pointed, triangular tusk, which grew throughout life, becoming longer with advancing age; while the third, which was lost in _†Toxodon_, was small and unimportant. In the lower jaw the first and second incisors were chisel-like and had a limited growth; being rather narrow, they both bit against the broad first upper incisor; the third incisor was a persistently growing tusk, not so large as the upper one, against the posterior face of which it impinged and was obliquely truncated by wear, so that its length was limited, while the upper tusk continued to elongate and was made narrower and sharper by wear. All the lower incisors were far less procumbent than in _†Toxodon_, and were directed obliquely upward and forward. The remarkable changes of appearance which, as mentioned above, took place within the life-time of the individual, were largely due to the differential growth of the incisors. The milk-incisors were all nearly alike and formed no tusks; when the permanent incisors were first protruded, the first upper and the first and second lower were large and the tusks were not visible, and, when the latter did appear, they were for some time smaller than the other incisors. These, however, formed roots and ceased to grow, actually becoming smaller with advancing age, for the crowns narrowed to the roots and, the more they were worn down, the smaller they became. The tusks, on the other hand, grew throughout life and became larger as the other incisors were reduced by wear, and thus the whole appearance of the anterior part of the jaw was totally changed.

This mode of forming the tusks by the enlargement of the second upper and third lower incisor is an unusual one, though it was repeated in another South American ungulate order, the †Litopterna, and nearly so in the Proboscidea, in which both upper and lower tusks were the second of the three original incisors.

In both jaws, the canines of _†Nesodon_ were insignificant and sometimes absent. The premolars, which were smaller and simpler than the molars, had quite high crowns, but early ceased to grow and formed long roots. The molars were truly hypsodont and formed no roots till late in life; they were constructed on the same plan as those of _†Toxodon_, but were decidedly more complex, the upper ones having several spurs and crests given off inward from the external wall, in addition to the two principal transverse crests, and they had a certain superficial likeness to the teeth of a rhinoceros. As in _†Toxodon_, these upper molars were curved inward, so as almost to meet those of the opposite side above the palate. The lower molars had the same bicrescentic plan as in _†Toxodon_, but were more complicated, and in the concavity of the hinder crescent was a vertical pillar, which was well-nigh universal among the indigenous South American ungulates.

If _†Nesodon_ was really the ancestor of _†Toxodon_, then the development of the grinding teeth must have been a process of completing the hypsodontism, until the teeth grew persistently, never forming roots, and, at the same time, of simplifying the pattern. This is contrary to the usual course of evolution, in which the pattern grew more complex in the successive stages; but such steadily increasing complexity was not invariable, and several instances of undoubted simplification are known among mammals, though not yet in other ungulates. Only the recovery of the intermediate genera will enable us to determine whether _†Nesodon_ was the actual ancestor of _†Toxodon_, or whether it was merely one of a short-lived branch from the main stem, in which the teeth had acquired an unusual degree of complexity.

[Illustration: FIG. 237.—_†Nesodon imbricatus_, Santa Cruz stage. Restored by C. Knight from a skeleton in the museum of Princeton University.]

A few years ago Dr. Ameghino announced the very surprising discovery that, instead of having merely the normal arrangement of two dentitions, the milk and the permanent, _†Nesodon_ developed three successive dentitions, one preceding the milk-series, and therefore called _pre-lacteal_. In certain other mammals traces of a pre-lacteal series had already been found, in the shape of tooth-germs, which never attain full development or even cut the gum; and quite recently Dr. Ameghino has shown that in the tapir at least one functional pre-lacteal premolar is formed. The significance of this fully developed pre-lacteal dentition in _†Nesodon_ is not yet clear, though it seems reasonable to suppose that it was the almost uniquely late retention of a primitive character.

The skull was closely similar to that of _†Toxodon_, on a smaller scale, but there were several minor differences, which were, in part, conditioned by the larger and much more completely hypsodont teeth of the Pampean genus, as well as by its generally increased size and bulk. In _†Nesodon_ the sagittal and occipital crests were much more prominent and the former was much longer, while the thickening of the cranial bones was in only an incipient stage. The nasal bones were considerably longer. The jaws were lower and shallower, in correlation with the less perfectly hypsodont teeth, and in the lower jaw the chin was much more erect and rounded. The entire head of this curious Santa Cruz animal had something remarkably rodent-like in its appearance, though it is quite inadmissible to suppose that the likeness was due to relationship.

The skeleton was far smaller and lighter and otherwise differently proportioned from that of _†Toxodon_, but there was, nevertheless, a close agreement between the two genera. The neck was of moderate length and thickness, the body long and heavy, but with no such relative bulk as in the Pampean genus. The hump at the shoulders, as indicated by the spines of the anterior dorsal vertebræ, though already well defined, was less prominent. The shoulder-blade (scapula) was relatively broader than in _†Toxodon_, its spine had a distinct acromion and two very long and conspicuous processes given off backward from the spine, only one of which, and that a mere vestige, is indicated in _†Toxodon_. The hip-bones were almost parallel with the backbone and were not nearly so broad or so everted as in the latter, a difference which is amply accounted for by the great discrepancy in girth.

[Illustration: FIG. 238.—Left pes of _†Toxodon_. La Plata Museum. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 1_ and _2_, coössified internal and middle cuneiforms. _Cn. 3_, external cuneiform. _Cb._, cuboid.]

The limbs were of nearly equal length and there was no such shortening of the fore-arm or elongation of the thigh as in _†Toxodon_, and so the descent of the backbone forward, which gave such grotesqueness to the skeleton of the latter, was far less pronounced. The limb-bones were rather slender, in size and proportions not unlike those of a tapir, but in structure very like the very much larger and more massive ones of _†Toxodon_. The bones of the fore-arm were separate, but those of the lower leg were coössified in the same exceptional manner as in the Pampean genus, that is, the upper ends, but not the lower, were fused together. The thigh-bone was not flattened, but had the normal cylindrical shaft and a conspicuous third trochanter. The feet, in which the digits were already reduced to three, were extremely small in comparison with the size of the animal; in structure, they were almost identical with those of _†Toxodon_, but were far narrower and more slender. The heel-bone (calcaneum) articulated with the other bones of the tarsus in a normal manner. The digits were well separated and the hoof-bones quite strongly developed, indicating that the hoofs were functional, supporting most of the weight. In short, the difference in the external appearance of the feet between the two genera was much the same as between the tapirs and rhinoceroses.

The species of _†Nesodon_, of which many have been named on very questionable grounds, differed but little in size and were of such variable and fluctuating character that a proper discrimination of them is exceedingly difficult. One of these species (_†N. cornutus_) gives indications of having possessed a small dermal horn on the forehead and was thus a possible ancestor of _†Trigodon_.

[Illustration: FIG. 239.—Left pes of _†Nesodon_, Princeton University Museum. Letters as in Fig. 238 and scale of reduction the same.]

A second phylum of the suborder was represented in the Santa Cruz stage by the genus _†Adinotherium_, the species of which, not equalling a sheep in size, were very much smaller animals than those of _†Nesodon_, but closely like them in other respects. The dentition, including the pre-lacteal series, and the skull were almost identical in the two genera, with the exception that a large proportion of the individuals of _†Adinotherium_ had the small frontal horn, while others had no trace of it. While it is quite possible that the presence or absence of the horn, which was always inconspicuous, may have been a matter of specific distinction, a more probable explanation is that it was a sexual character, the males horned and the females hornless. Much the same thing is to be observed in the modern Javan Rhinoceros (_R. sondaicus_) in which the females have a very small horn, or none at all, and the males a large one.

In the skeleton also there were few differences, other than those of size, between _†Adinotherium_ and _†Nesodon_; the former was not only smaller, but also lighter and more slender proportionately, and there was no hump at the shoulders, the spines of the dorsal and lumbar vertebræ all reaching the same level, so that the back must have been nearly straight in the living animal. From the more general and constant presence of the frontal horn, _†Adinotherium_ was more probably the ancestor of the horned _†Trigodon_ than was _†Nesodon_, but until the intermediate forms shall have been recovered, no definite decision can be made.

[Illustration: FIG. 240.—_†Adinotherium ovinum_, small, horned †toxodont of the Santa Cruz. Restored from a skeleton in the museum of Princeton University.—Note the minute horn on the forehead.]

The same or very nearly the same genera of the family †Toxodontidæ lived in the Patagonian and Deseado stages, but there the record breaks off and can, for the present at least, be followed no farther. It remains to be determined whether the series originated in regions farther to the north, or whether the ancestral types will be found in Patagonia.

The other two families are still very incompletely known, but sufficiently to justify their inclusion in the present suborder. In the †Leontiniidæ, which are known only from the Deseado stage (_†Leontinia_), we have a curious variant of the †toxodont type. The tusks were decidedly smaller than in the Santa Cruz members of the preceding family, the grinding teeth with lower crowns and simpler structure. The skull was much like that of _†Nesodon_, but the anterior nasal opening was of quite a different shape, being carried much farther back on the sides, so that the nasal bones had a far longer portion which was freely projecting and unsupported; these bones were shorter and much thicker than in the Santa Cruz genera and, to all appearances, supported a small, median horn on their anterior ends. The feet, so far as they have been recovered, did not differ in any significant manner from those of the preceding family.

[Illustration: FIG. 241.—Skull of _Adinotherium_, top-view, showing the rugosity on the forehead for the small frontal horn.—Princeton University Museum.]

Another imperfectly known family, that of the †Notohippidæ, occurred in the Patagonian stage, but was most abundant in the Deseado, where several genera of it have been found. These animals had mostly hypsodont teeth, forming roots in old age, and the teeth were in closed series, but there was no tusk-like enlargement of the incisors. In the later genera, those of the Patagonian stage (_†Notohippus_, _†Argyrohippus_), the crowns of the grinding teeth had a thick covering of cement, and those of the lower jaw had some resemblance, though not at all a close one, to the teeth of horses. The skull also had a certain suggestion of likeness to the horses and Dr. Ameghino was persuaded that these animals were ancestors of the horses. The family went back to the Astraponotus stage, but can be traced no farther.

SUBORDER †TYPOTHERIA. †TYPOTHERES

This suborder was composed of much smaller animals than the †Toxodonta and contained no large forms; some, indeed, were exceedingly small, no larger than rabbits. It was much the most diversified of the suborders, as is made evident by the table of families and genera. Two of these families, the †Typotheriidæ and the †Hegetotheriidæ, continued into the older Pleistocene. Of the former there was the genus first named and described, _†Typotherium_, which has given its name to the family and suborder, and the species of which were much the largest of the entire group, almost equalling a large pig in size. At the first glance this genus might easily be mistaken for a large rodent, and indeed it has actually been referred to that order, but the resemblance was a purely superficial one and involved no relationship.

In _†Typotherium_ the teeth were considerably reduced in number, the formula being: _i_ 1/2, _c_ 0/0, _p_ 2/1, _m_ 3/3, × 2 = 24. The first incisor in each jaw was a broad, scalpriform, persistently growing tooth, which much resembled the corresponding tooth in the rodents, but was not, as it is in the latter, worn to a sharp chisel-edge by attrition, but was abruptly truncated. There was a second similar, but much smaller, tooth in the lower jaw; the other incisors and all the canines had been lost and the premolars reduced to two in the upper and one in the lower jaw. The molars were large, persistently growing and thoroughly hypsodont; in pattern they were very similar to those of _†Toxodon_. The skull without the lower jaw was low and the cranial portion broad and flattened, but retaining a long sagittal crest. The eye-sockets were nearly, but not quite, closed behind by the very long and slender postorbital processes of the frontal bones. In front of the eyes the face was suddenly constricted into a long, narrow rostrum, and it is this shape of the skull which, together with the persistently growing, scalpriform incisors, gave such a rodent-like appearance to the head. The auditory region had the same remarkable structure as in the †Toxodonta. The lower jaw had a short horizontal portion and very high vertical portion, which gave the head great vertical depth.

The skeleton, so far as it is known, was decidedly more primitive than that of the contemporary _†Toxodon_, as is shown by the presence of collar-bones (clavicles) and by the larger number of digits, five in the front foot and four in the hind. The hoof-bones, or ungual phalanges, were narrow, pointed and nail-like, though in the hind foot they were broader and more hoof-like.

Little can be done as yet in tracing back the history of this family, the Santa Cruz beds having yielded no member of it. In the Deseado stage, the genus _†Eutrachytherus_ differed surprisingly little from _†Typotherium_, in view of the long hiatus in time between them. The Deseado genus already had thoroughly hypsodont and rootless teeth, and the molar pattern was quite the same as in _†Typotherium_, but the teeth were much more numerous, the formula being: _i_ 3/2, _c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 = 42. Nothing is known of the skeleton. The family arose probably from one of the Eocene families (†Archæopithecidæ or †Acœlodidæ) with low-crowned teeth, but the connection cannot be made out. Presumably, the development of this family ran its chief course in some part of South America far to the north of the fossil-beds of Patagonia.

The second family which was represented in Pampean times was that of the †Hegetotheriidæ, and the sole genus of it which survived so late was _†Pachyrukhos_, a little creature no larger than a rabbit. The genus went back without any noteworthy change to the Santa Cruz stage of the Miocene, from which complete skeletons have been obtained. The dental formula was nearly as in _†Typotherium_: _i_ 1/2, _c_ 0/0, _p_ 3/3, _m_ 3/3, × 2 = 30, and the enlarged, rootless and scalpriform incisors were similar. The grinding teeth were thoroughly hypsodont and had a thin coating of cement; the molar-pattern was fundamentally like that of _†Nesodon_, in simpler form, but can be seen only in freshly erupted and unworn teeth.

The skull was very rodent-like in appearance, its flat top and narrow, tapering facial region, and the gnawing incisors adding much to the resemblance. The very large eye-sockets and the enormously developed auditory region suggest nocturnal habits, and, no doubt, the timid, defenceless little creatures hid themselves by day, perhaps in burrows. The enlargement of the accessory auditory chambers, which all of the †Toxodontia possessed, reached its maximum in _†Pachyrukhos_, and the chambers formed great, inflated protuberances at the postero-external angles of the skull. The neck was short, the body long and the tail very short, much like that of a rabbit. Collar-bones were present, as they probably were in all of the other members of the suborder †Typotheria, though this has not been definitely ascertained in all cases. The limbs were relatively long, especially the hind legs, and very slender; the bones of the fore-arm were separate, but those of the lower leg were coössified at both ends. The feet, which had four digits each, were of unequal size, the posterior pair being much longer than the anterior, and the hoofs were long, slender and pointed, almost claw-like. The entire skeleton suggests a leaping gait and its proportions and general appearance were remarkably like those of a rabbit-skeleton. In the restoration (Fig. 300, p. 639) Mr. Knight has followed these indications and drawn an animal which might readily be mistaken for a curious, short-eared rabbit; and there is every justification for doing this, though the character of the fur and the form of the ears are, of course, merely conjectural. Perhaps the ears are too small.

Associated with _†Pachyrukhos_ in the Santa Cruz stage was another genus of the family, _†Hegetotherium_, which, though it cannot possibly have been ancestral to the former, yet serves to indicate, in general terms, what the ancestor must have been. This is another example of the long-continued survival of the more primitive together with the more advanced and specialized form. _†Hegetotherium_ persisted into the Pliocene, but is not known from the Pleistocene. In this genus one upper and two lower incisors were already enlarged, rootless and scalpriform, but none of the teeth had been lost; it is interesting to note, however, that the teeth which were lacking in _†Pachyrukhos_ were all very small and ready to disappear. The Santa Cruz species of _†Hegetotherium_ were considerably larger and more robust animals than those of _†Pachyrukhos_.

Both of these genera were preceded by very similar, almost identical forms in the Patagonian, Deseado and Astraponotus stages, but the family cannot be definitely traced farther back than the lower Oligocene, but it very probably arose from some one of the groups, with low-crowned teeth, of the Casa Mayor stage.

[Illustration: FIG. 242.—Santa Cruz †typothere (_†Protypotherium australe_) and armadillo (_†Stegotherium tesselatum_). Restored by C. Knight from skeletons in the museum of Princeton University.]

The family †Interatheriidæ was, in most respects, more conservative and underwent less change than either of the preceding groups. A persistently primitive type was the genus _†Protypotherium_, which appeared for the last time in the Pliocene of Monte Hermoso, but was much more abundant and better preserved in the Santa Cruz. The animal was small and had the full complement of teeth, which were arranged in each jaw in a continuous series, and were fully hypsodont and rootless, except incisors and canine, which were rooted. None of the incisors was specially enlarged, but there was a gradual transition of increasing size and complexity from the incisors to the molars. A remarkable feature of this genus was the deeply cleft form of the lower incisors, giving them a fork-like shape, somewhat as in the modern Hyracoidea. The ulna and radius in the fore-arm and the tibia and fibula in the lower leg were separate, but the digits were already reduced to four in each foot. This was one of the few Santa Cruz ungulates which possessed a long and heavy tail. The limbs were relatively long and the feet were armed with such slender hoofs that they looked almost like claws. The restoration shows the animal to have had, like nearly all of the †Typotheria, a very rodent-like appearance, a likeness which may, perhaps, be unduly increased by the form given to the ears.

In the allied genus, _†Interatherium_, from which the family is named, the head was short, broad and deep, almost bullet-like; the first incisor was enlarged and chisel-shaped, and the other incisors and the canines were much reduced in size. It is an interesting fact, observed as yet only in this genus, but probably true also of all the smaller members of the suborder which had hypsodont teeth, that the milk-premolars were rooted and comparatively low-crowned, while their permanent successors were completely hypsodont and rootless. The limbs were considerably shorter than those of _†Protypotherium_ and the tail long and thick, except for which, the general appearance of the skeleton suggests that of the modern “conies” or “klipdases” (Hyracoidea) of Africa and Syria, a suggestion which Mr. Knight has followed in the drawing (Fig. 297, p. 636).

This family was represented in the Deseado stage by a genus (_†Plagiarthrus_) in which the teeth developed roots in old age, but is not known from more ancient formations. Their probable ancestors of the Eocene were very small animals, with brachyodont teeth, the premolars smaller and of simpler pattern than the molars. The upper molars had a continuous external wall, with indication of separate cusps, and two transverse crests, as in the †Toxodonta, and the lower molars were composed of two incomplete crescents. The teeth were present in undiminished number and the anterior incisors were but little enlarged. Nothing is known of the skeleton.

SUBORDER †ENTELONYCHIA. †HOMALODOTHERES

This third suborder of the †Toxodontia was in some respects the most peculiar of all; no representatives of it have been found in formations later than the Santa Cruz, and the group attained its culmination in the still older Deseado stage, in which there were very large members of it. These most extraordinary beasts are still incompletely known, and little can be done as yet in the way of following out the steps of change which led up to their exceptional characters, though the suborder itself may be traced back to the Eocene by means of jaws and teeth alone.

The Santa Cruz genus labours under the portentous name of _†Homalodontotherium_, which may be shortened to the vernacular form of †homalodothere. In this genus the dentition was unreduced in number, and the teeth, though having rather high crowns, were all rooted and placed in continuous series, with a gradual transition in shape from the incisors to the molars. The canines were tusks of very moderate size, which projected but little above and below the plane of the other teeth; the premolars, except the last, which was nearly molariform, were smaller and simpler than the molars, which had a pattern fundamentally the same as in the †Toxodonta. Those of the upper jaw were, however, less complicated by spurs and accessory crests, and they had a somewhat stronger resemblance to the rhinoceros pattern, though the resemblance is demonstrably superficial and not indicative of relationship.

The skull was very like that of the Santa Cruz †toxodonts, _†Nesodon_, etc., and had the same unusual structure of the auditory region as was found throughout the order, but differed in many details, which it is not worth while to enumerate, though it may be said that the nasal bones were so much shortened that some kind of a proboscis or prehensile upper lip was probably present. The head was quite small in proportion to the size of the animal as a whole. Such of the vertebræ as are known were quite similar to those of _†Nesodon_, but the limbs were far longer and quite stout, though not massive. The humerus was remarkable for the great development of the ridges for the attachment of the deltoid and supinator muscles and for the prominence of the epicondyles, all of which gave to the bone the appearance of the humerus of a huge burrower, yet it is impossible to believe that so large an animal could have had burrowing habits. The fore-arm bones were separate and very long, the ulna almost as heavy as the radius; the latter is not known from a complete specimen, but there would appear to have been some power of rotation, a power which is conditioned by the shape of the upper end of the radius, and its mode of articulation with the humerus in the elbow-joint. The thigh-bone was long and heavy and its shaft was much flattened, having lost the normal cylindrical shape, but retained a small third trochanter. The bones of the lower leg were separate and relatively short, and the fibula was uncommonly heavy.

So far, there was nothing very unusual, save in the shape of the humerus, about the skeletal structure of the †Entelonychia, the remarkable characters having been confined to the feet. Were it not for these, the group might be included in the suborder †Toxodonta without difficulty. The feet, which were five-toed, differed notably in size, the manus being more than twice as long as the pes. In the former the metacarpals were very long and, though actually stout, were slender in proportion to their length; there was also a very unusual feature in an ungulate foot, that the heaviest of the digits was the fifth, or external one. The mode of articulation of the metacarpals with the first row of phalanges was very exceptional, indicating an extraordinary mobility of the toes, and the hoofs had been transformed into large, bluntly pointed claws, somewhat like those of the †chalicotheres, those aberrant perissodactyls (see p. 354), but not so large or so sharp. In the pes, the ankle-bone had hardly any groove for the tibia, and its lower end was hemispherical, as in the †Condylarthra and the clawed mammals generally. The toes were quite grotesquely short in comparison with those of the fore foot, and, as in the latter, the fifth was the heaviest of the series. The hind foot was apparently plantigrade, the heel-bone and the entire sole being applied to the ground in walking, while the fore foot was probably digitigrade, the wrist being raised and the metacarpals vertical. The weight was carried upon the metacarpals and one or more pads under the phalanges, as in the digitigrade carnivores, such as dogs and cats. In describing the †chalicotheres, it was pointed out that it was uncertain whether each foot had a single large pad, or whether there was a separate one under the phalanges of each digit, and a larger one, the “ball of the foot,” under the metacarpals collectively. The same doubt applies to the manus of the †homalodotheres.

This is the third instance to be cited of the acquisition of claws by a hoofed mammal and, as in the other two cases, the †chalicotheres and †agriochœrids (p. 383), we are confronted by the seemingly incompatible association of teeth which could have masticated only soft vegetable tissues with feet like those of a beast of prey. As in the other two groups, the problem as to the habits and mode of life of the †homalodotheres is an unsolved one, chiefly because no mammal now living is at all like these extraordinary creatures and one can therefore form but vague conjectures as to the use of such feet to herbivorous animals. Possibly they subsisted largely upon roots and tubers and used the great claws for digging up food, the principal employment that bears now make of their claws. This remarkable transformation of hoofs into claws took place in three unrelated groups of hoofed animals and must have occurred independently among the Artiodactyla, the Perissodactyla and the †Toxodontia. By no possibility, so far as we are able to comprehend the course of evolutionary change, could this common characteristic have been due to inheritance from a common ancestry.

The †homalodotheres were among the largest of Santa Cruz mammals, but they were then already approaching extinction, while in the Deseado stage they were more numerous and varied and some of them very much larger. This is an exception to the more common rule, according to which the successive members of a phylum increased in stature until the maximum was reached and this, in many cases, was followed by extinction. The rule is, however, by no means without exceptions and several have already been referred to. The largest of American proboscideans was the †Imperial Elephant (_Elephas †imperator_) of the upper Pliocene and Pleistocene and in many other phyla the Pleistocene species were much larger than the Recent. So with the †homalodotheres; they reached their culmination in size and importance in the Deseado stage, fewer and smaller forms surviving into the Santa Cruz, after which the entire suborder vanished. The family may be traced back to the Eocene, where it is represented chiefly by a genus (_†Thomashuxleya_) which had larger canine tusks and much more brachyodont teeth, but there is no way of determining when the transformation of the hoofs took place. The other two families (†Notostylopidæ, †Isotemnidæ) flourished chiefly or exclusively in the Eocene and were small animals still very imperfectly understood.

SUBORDER †PYROTHERIA. †PYROTHERES

This suborder was a remarkable group, still incompletely known, of elephant-like animals, which reached their culmination and died out in the Oligocene, their last appearance being in the Deseado stage. The genus _†Pyrotherium_ from the Deseado (also called the Pyrotherium Beds) was the latest, largest and best known of the suborder. The dentition was much reduced as is shown by the formula: _i_ 2/1, _c_ 0/0, _p_ 3/2, _m_ 3/3, × 2 = 28. The upper incisors were two downwardly directed tusks, the first quite small, the second considerably larger; the single lower incisor of each side was a stout, but not very long, horizontally directed tusk, with the enamel confined to a longitudinal band; the other incisors and the canines had disappeared. The premolars, except the foremost one, had the molar-pattern, which very rarely occurred among the indigenous South American ungulates. The grinding teeth were similar above and below and each had two elevated, transverse crests, which, when quite unworn, carried a row of bead-like tubercles. These teeth are decidedly reminiscent of the dentition of the aberrant proboscidean _†Dinotherium_, from the Miocene and Pliocene of Europe (p. 435), and this resemblance, together with the form of the tusks, has led to the reference of this group to the Proboscidea, but the assignment is undoubtedly erroneous, as is shown by the character of the skull and skeleton.

[Illustration: FIG. 243.—Head of _†Pyrotherium_, showing the two pairs of upper tusks. Restored from a skull in the museum of Amherst College.]

The skull, hitherto unknown, was obtained by the Amherst College Expedition to Patagonia and its description by Professor F. B. Loomis is anxiously awaited. In advance of that, he has published a brief account, with a figure. This skull was long and narrow, with very short facial region and nasal bones so shortened that the nasal canal passed almost vertically down through the head, as in the elephants, and there must have been a considerable proboscis. Despite this great modification, the skull was plainly of the †toxodont and not of the proboscidean type. The legs were extremely massive and the fore legs were considerably shorter than the hind, with such a difference in length that the head must have been carried low, as in the Pampean _†Toxodon_. The upper arm and thigh were much longer than the fore-arm and lower leg respectively. The humerus was immensely broadened, especially the lower end, and the processes for muscular attachment were extremely prominent. The femur was long, with broad and flattened shaft, and had no trace of the third trochanter, quite strongly resembling the thigh-bone of an elephant, which, as we have repeatedly seen, is the type more or less closely approximated by all of the very heavy ungulates. In the standing position, the femur was in nearly the same vertical line as the tibia and the whole leg must have been almost perfectly straight, with the knee-joint free from the body. The short and massive fore-arm bones were coössified, at least in some individuals, as were the equally heavy bones of the lower leg, the fibula being exceptionally stout. Little is known of the feet, but that little renders probable the inference that they were short, columnar and five-toed.

The Eocene representatives of the Pyrotheria are known only from very fragmentary material. _†Propyrotherium_, of the Astraponotus Beds, was smaller than the Deseado genus and still smaller was _†Carolozittellia_ of the Casa Mayor, which was not so large as a tapir. In the latter the molars were of the same type as in the succeeding forms and small tusks had already begun to develop. The older Eocene genus _†Paulogervaisia_ was probably a member of this suborder; if so, it shows that the molars with transverse crests were derived from quadritubercular teeth, just as happened in the Proboscidea and several other ungulate groups.