The Cambridge Natural History Part 20

FAM. 4. t.i.tANOTHERIIDAE.--These Oligocene Ungulates, often attaining to large dimensions, are nearly peculiar, so far as is at present known, to the North American Continent, and are at least most abundant in it.[174]

Many generic names, such as _t.i.tanotherium_, _Brontotherium_, _Brontops_, _t.i.tanops_, and _Menodus_, have been given to them; but a recent study of the entire material accessible for description or already described has led Professor Osborn to the opinion that there was but a single genus, to which the name _t.i.tanotherium_ must be applied. Of this genus there are some thirty well-characterised species, of which the gradual evolution can be traced from the lowest strata of the White River beds where their remains occur. An entire skeleton of _T. robustum_ enables us to understand the osteology of these forms and to compare them with other Perissodactyles.

This animal was more than 13 feet long, standing some 7 feet 7 inches in height. It seems to have presented during life the aspect of a Rhinoceros with perhaps a touch of Elephant. The skull is not unlike that of a Rhinoceros in general dimensions and shape; but there are a pair of apparent horn cores anteriorly, which are smaller in the more ancient forms and acquire a large size, a forward direction with a divergence of the two in the later forms. A glance at the accompanying figures of skulls (Fig.

137) of early and later t.i.tanotheres will exhibit the changes in this particular which the skulls underwent in the lapse of time occupied by the deposition of these Oligocene beds. The nasals are short in the later, longer in the more early species, such as _T. heloceras_ and _T.

coloradense_. The zygomatic arch projects much, and is "shelf-like" in the later forms, the skull thus getting an immense breadth, which, {266} together with the long and divergent horn cores, must have given to the living animal a most bizarre appearance. It is an interesting fact that this animal, though a Perissodactyle, agrees with the Artiodactyla in the nineteen dorso-lumbar vertebrae, of which seventeen bear ribs.

[Ill.u.s.tration]

FIG. 137.--Three figures showing the cranial evolution of _t.i.tanotherium_.

Upper figure, _T. trigonoceras_; middle figure, _T. elatum_; lower figure, _T. platyceras_. (After Osborn.)

The genus further agrees with the Artiodactyles in the structure of the carpus. The toes of the fore-limb are four, those of the hind-limb three; but while the hind-limb is undoubtedly Perissodactyle in the arrangement of its component parts, the fore-limb shows a hint of an Artiodactyle mode of structure. This limb is paraxonic, the axis of the limb pa.s.sing between the two middle digits. It may be that this genus represents more nearly than any other Perissodactyle or Artiodactyle the primitive stem from which both have diverged, though, of course, it is not old enough to be very near to the actual ancestor. The molar dent.i.tion is the typical one; the incisors seem to vary as to their presence or absence, and, if present, in their numbers. In comparing the older with the more recent forms it is noteworthy that there has been an increase of size exactly as there has been during the evolution of the Camels and some other groups of Ungulates. As already mentioned, the size of the horn cores also increases until it culminates in the extraordinary species, _T. platyceras_ and _T. ramosum_, in which these are half as long as the skull, flattened in form, and connected at their bases by a "web" of bone. Arrived at this amount of specialisation the genus _t.i.tanotherium_ apparently exhausted its capacities for modification and ceased to be. The many generic names may be explained by s.e.xual differences on the one hand and an incomplete knowledge of connecting links on the other.[175]

_Palaeosyops_ is somewhat like a Tapir in build, the skull especially resembling that of the Tapir. As in _t.i.tanotherium_ the molar teeth, instead of having an outer wall formed by fused cusps, have a [176]-shaped outer wall on one side and two or one cusps on the opposite side. It is, moreover, an Eocene form, and in correspondence with its greater age is more primitive in some points of structure, for example, in the absence of horns and in the full dental formula. The fore-limbs are four-toed, the hind {267} three-toed. It was intermediate between a Tapir and a Rhinoceros in size. It has been shown, too, from casts of the interior of the skull, that the cerebral hemispheres are much less convoluted than were those of _t.i.tanotherium_.

Related to _Palaeosyops_ is another primitive t.i.tanothere, the genus _Telmatotherium_. This is also Eocene, from the Uinta Basin, the uppermost of Eocene strata. The skull of these creatures was rather elongated, and not unlike that of a t.i.tanothere in general aspect. The dent.i.tion was complete and the canines not very large. The horns, which acquire so prodigious a development in the later t.i.tanotheres, are just recognisable in at any rate many species of this genus _Telmatotherium_, the name being thus by no means an apt one. Better was that proposed by Dr. Wortman, of _Manteoceras_ or "prophet horned." The horns are small elevations upon the frontals just at the junction of these with the nasals, and, indeed, lying partly upon the latter bones. In _T. cornutum_ the horns are chiefly borne upon the very long nasals, whose size contrasts with the same bones in the more highly-developed _t.i.tanotherium_. It appears to be quite possible that _t.i.tanotherium_ was evolved from the genus _Telmatotherium_.[177]

SUB-ORDER 9. LITOPTERNA.

Whether the MACRAUCHENIIDAE should be considered as a separate group of Ungulata is a matter of dispute. Cope placed them in a special order of Ungulates which he called Litopterna. Zittel, on the other hand, regards them as definitely Perissodactyles. One curious point of resemblance to existing Horses is shown--that is the presence of a pit in the incisor teeth. This matter seems to be so important as to need a placing of these forms in the neighbourhood of the Perissodactyles, even of the Equidae; it is so peculiar a character, and apparently so little related to any obvious similarity in way of life, that it seems to mark a special affinity. Not so the fact that in _Macrauchenia_ at any rate the orbit was entirely surrounded by bone as in the Horse. We find that condition so frequently acquired in many groups,--a development from an earlier condition where the cavity for the lodgment of the eye is in continuity with the temporal {268} fossa, that it cannot be regarded as anything more than a mark of specialisation. It is, in fact, the case that the Macraucheniidae are in many points specialised, while retaining many primitive features of structure.

The chief primitive features are: the non-alternating positions of the wrist- and ankle-bones; these, of course, interlock in the Perissodactyles of to-day and in many extinct families. Then the absence of a diastema in the tooth series, coupled with the presence in _Macrauchenia_ of a complete dent.i.tion. The small brain may be referred to the same category.

_Macrauchenia_ must have been a strange-looking animal. It walked upon three toes on each limb; the skull was Horse-like in general form, but the nostrils are removed to a point about as far back as in the Whales or nearly so, the nasal bones being correspondingly reduced. This it is thought argues a proboscis. The humerus is particularly compared by Burmeister[178] to that of a Horse. The radius and ulna though both well developed are fused. The neck is long, and, as in the Camel, the vertebral arteries run inside the neural arches. Since the fore-legs seem to have been rather longer than the hind-legs, though only very slightly, and the neck was long, the animal may have presented some likeness to the Giraffe.

It is interesting to note that in the proportions of humerus to ulna this animal is more Lama-like than Horse-like. On the other hand, the proportions of femur to tibia are more Horse-like. The remains of the creature are limited to South America, and to quite superficial deposits.

It is evidently a specialised type, and has pursued a course parallel to that of the Horse. Much nearer to the Horse however, but apparently by convergence only, is the genus _Thoatherium_, usually placed in a separate family, the PROTOROTHERIIDAE. In this creature, which has many archaic characters, the toes are reduced to one in each foot. In an allied form, _Protorotherium_, we have the two lateral toes diminishing just as in _Anchitherium_.

{269}

CHAPTER XI

UNGULATA (_continued_)--ARTIODACTYLA (EVEN-TOED UNGULATES)--SIRENIA

SUB-ORDER 10. ARTIODACTYLA.

[Ill.u.s.tration]

FIG. 138.--Bones of the Ma.n.u.s--A, of Pig (_Sus scrofa_). 1/3. B, of Red Deer (_Cervus elaphus_). . C, of Camel (_Camelus bactria.n.u.s_). 1/8.

_c_, Cuneiform; _l_, lunar; _m_, magnum; _m_^2, _m_^5, second and fifth metacarpals; _R_, radius; _s_, scaphoid; _td_, trapezoid; _u_, unciform; _U_, ulna; _II-V_, second to fifth fingers. (From Flower's _Osteology_.)

The Artiodactyle or "Even-toed" Ungulates are to be distinguished from the Perissodactyla, and from other Ungulate groups, by a number of trenchant characters. The most salient {270} of these, and that which has given its name to the group, concerns the arrangement of the digits. Instead of there being but one prevailing digit--the third, in the hand and foot, through which the axis of the foot pa.s.ses, there are two, numbers three and four, between which the same axis pa.s.ses, and which are perfectly symmetrical with each other. This type of foot has been termed "paraxonic," as opposed to the "mesaxonic" Perissodactyle foot (see Fig. 121 B, p. 235). It has been attempted to prove that the single prevailing digit of the Horse's foot is a fused pair of digits, and the state of affairs which characterises the Camel, where the two metacarpals or metatarsals are to an almost complete extent united, has been urged in proof; so, too, certain abnormalities, such as those called "solid-hoofed pigs."[179] These latter are simply Pigs in which the two central metacarpals and the terminal hoofs are completely fused with one another. In some of such cases there is not the slightest trace of the union of the separate metacarpals and phalanges.

Even the sesamoid bones, attached behind to the toes, are two in number instead of four. And, furthermore, the tendon supplying the bones is single, though showing traces of its double origin. Such Pigs often show the abnormality from generation to generation, and they proved convenient for those whose scruples would not allow them to eat the flesh of a beast "dividing the hoof" and not chewing the cud. More singular still, as showing a pathological approach from another side to the Perissodactyle condition in an Artiodactyle, is a calf, where the foot ended in three equi-sized digits, of which the middle one lay in the longitudinal axis of the limb. From the opposite side cases are known of a Horse with a split hoof and phalanges, thus presenting the most striking likeness to a Camel.

There is, furthermore, in certain groups of Artiodactyles (_e.g._ the Tragulidae) a tendency for the two middle metacarpals to unite, quite apart from such "sports" as those ill.u.s.trated by the cases just set forth. And, as already mentioned, the union of the two middle metacarpals culminates in the Camel, Ox, etc. There is, however, absolutely no trace of such a fusion in the series of Perissodactyle animals known to us; and it would be by fusion rather than dismemberment that, as it would appear on this theory, the modern Ungulate foot has been arrived at. Of course {271} the facts of Ungulate descent are absolutely destructive of any such comparisons.

As is the case with the Perissodactyles, the Artiodactyles show a historical series, the primitive five-toed condition being almost preserved in _Oreodon_, up to the most modern modification exemplified by the Ox, Sheep, etc., in which animals there are not even vestiges of the fourth and fifth toes. It has been stated, however, that the foetal Sheep has traces of those rudiments. The so-called cannon bone (the fused third and fourth metapodia) is accompanied in its fusion by an increase in length. At the same time the functional middle metacarpals push aside the rudiments and, forming a broad surface for that purpose, articulate with the magnum and unciform bones to the exclusion of the rudiments. This has been termed an "adaptive reduction." In the "inadaptive reduction" there is the same reduction of the metacarpals, but the rudiments still articulate as in the primitive Artiodactyle foot, _i.e._ Mc II with trapezium, trapezoid, and magnum; Mc III with magnum and unciform; Mc IV and V with unciform. This would appear to give greater solidity and consequently greater strength to the foot.

[Ill.u.s.tration]

FIG. 139.--Dorsal surface of right tarsus of Red Deer (_Cervus elaphus_).

1/3. _a_, Astragalus; _c_, calcaneum; _c_^3, cuneiform; _cb_, cuboid; _mIII_, _mIV_, metatarsals; _n_, navicular. (From Flower's _Osteology_.)

The carpal bones of the Artiodactyla alternate in their articulation; the primitive state of affairs[180] is not retained even in the earliest types.

The femur has no third trochanter, so prevalent in the Perissodactyles. In the hind-foot the calcaneum has an articular facet for the fibula, which is not characteristic of the Perissodactyla. In the more modern forms, _e.g._ the Cervidae, the navicular and cuboid become fused into one bone; and there are even further fusions which will be referred to later as characteristic features of different groups. It is interesting to notice that the reduction begins earlier and is clearer in the hind-foot than in the fore. One {272} can see how this may be purely adaptive, the push of the hind-legs in running needing a firmer support. In _Hyomoschus_ this is the case. The hind-limbs are provided with a cannon bone, while the metacarpals of the fore-feet are still free.

The number of dorso-lumbar vertebrae is less in the Artiodactyle than in the Perissodactyle Ungulates. Whereas the former have but nineteen, the latter have, as a rule, twenty-three such vertebrae.[181] The number of ribs varies from twelve (_Camelus_, _Hydropotes_) through thirteen (_Cervus_, _Gazella_) to fourteen in _Dicotyles_, _Giraffa_, etc.

The curious form of teeth known as "selenodont" is characteristic of the Artiodactyla, though only found well developed in the modern forms, and of those only in the Pecora. The more primitive forms had "bunodont" teeth with typically four tubercles (if we except the tritubercular and but little-known _Pantolestes_); and the intermediate "buno-selenodont" type characterises such groups as the Anthracotheriidae.

While the stomach of the Perissodactyles is always a simple sac, it is complicated, or shows signs of complication, in the Artiodactyles. That of the Hippopotamus is divided into two chambers; there are three in _Tragulus_, and four in the typical Ruminants such as _Cervus_, _Ovis_, etc.

Had we to deal only with the still living genera of Artiodactyles, it would be easy to sort them into two groups on the characters of the teeth; for the Pigs and Hippopotamus are provided with tubercular molars; they are bunodont. The Deer, Camels, Oxen, Giraffes, etc., have selenodont molars.

Besides, the latter are "Ruminants," and have a more complicated stomach.

The existing Chevrotains forbid a more trenchant division, since they are, as will be pointed out in due course, somewhat intermediate in structure; the feet are more Pig-like, and the stomach is not so typically Ruminant.

In any case such a division is prevented by certain extinct families which are perhaps ancestral to both. They have teeth which are not quite bunodont and not quite selenodont. These teeth have been termed buno-selenodont or buno-lophodont.

The distribution of the living Artiodactyles presents us with some interesting facts. The vast preponderance of species occurs in the Old World--34 in America as against over 250 species {273} in Europe, Asia, and Africa. The Neotropical region has no Oxen, or Sheep, or Antelopes. The latter are confined to Africa, Asia, and certain parts of the Palaearctic region; they are vastly more prevalent in Africa, where they take the place of the totally absent Deer. The Pig tribe is almost entirely Oriental and Ethiopian in distribution, only one form, the European Wild Boar, ranging into the Palaearctic region; and the two species of Peccary are found in both North and South America. Broadly speaking, the Ethiopian region is the headquarters of the Artiodactyla. But the great island of Madagascar has but one form of Artiodactyle, a Pig of the genus _Potamoch.o.e.rus_.[182]

GROUP I.--_SUINA._

FAM. 1. HIPPOPOTAMIDAE.--The family Hippopotamidae contains of existing genera only _Hippopotamus_, for the Liberian dwarf Hippopotamus is not now regarded, as it was formerly, as the type of another genus, _Ch.o.e.ropsis_.

The reasons for its former separation were the loss of the outer pair of incisors and the different proportions of various parts of the skull. This little Liberian animal has, however, been shown by Sir W. Flower[183] to possess the missing incisors occasionally; and as to the proportions of the skull, it is exceedingly common for small animals to vary from larger relatives in this way. Hence, considering the characteristic features of the Hippopotamus and the fewness of species, it seems unnecessary to divide it up further. We shall therefore only recognise one genus.

The Hippopotamus at present is African in range, and confined to that continent. But quite recently it inhabited Madagascar; and further back still in time the existing African species, _H. amphibius_, ranged into Europe; there were also Indian forms, which were contemporary with the Stone-age man. The Common Hippopotamus is a great thick-skinned beast with but few hairs. It has four toes on each foot, a complex stomach, but no caec.u.m. The strong incisors continue growing through life, as do the great canines. The number of incisors is two on each side of each jaw. Some of the extinct species had six in each {274} jaw, and they were distinguished as a genus _Hexaprotodon_, contrasting with _Tetraprotodon_, until intermediate conditions were observed. _Ch.o.e.ropsis_, as already observed, was a still further reduction of the tetraprotodont type. The molars (the formula is Pm 4/4 M 3/3) when worn show a double trefoil pattern. The orbital cavity is encircled by bone. As with many other aquatic mammals the kidneys are lobulated.

[Ill.u.s.tration]

FIG. 140.--Hippopotamus. _Hippopotamus amphibius._ 1/40.

A very singular fact about the Hippopotamus is the production of a "b.l.o.o.d.y sweat," a carmine-coloured secretion, containing small crystals and corpuscles, from the skin. This coloured fluid has of course nothing to do with blood.[184]

The animal grows to a length of at any rate 14 feet. The limbs and the tail are short. Like other aquatic animals the nostrils are on the surface of the head, and can be closed when the animal is under water. When it reaches the surface of the water after a prolonged immersion, it spouts like a Whale. Sir Samuel Baker says that ten minutes is the longest time that the Hippopotamus can remain below the water. It is frequently a dangerous animal to encounter, as it will capsize boats, and even bite large pieces out of their bottoms; with its huge teeth it {275} can and does attack and destroy human beings. The Hippopotamus not only swims, but can walk along the bottom of a river with great rapidity. It occasionally puts out to sea from the mouths of rivers frequented by it; and it is supposed that in this way Madagascar was populated with Hippopotamuses, whose remains are now found in swamps in that island.

[Ill.u.s.tration]

FIG. 141.--Wild Boar. _Sus scrofa._ 1/12.

FAM. 2. SUIDAE.--The Pig family, Suidae, differ from the last in their smaller size, in the terminal nostrils and mobile snout, which is not grooved, except faintly as in _Babirusa_. They are generally hairy, but the Babyroussa is an exception, while _Phacoch.o.e.rus_ is but slightly haired.

Though there are four digits, as in the Hippopotamus, only two reach the ground in walking. The stomach, furthermore, is simple, and (except in _Dicotyles_) there is a caec.u.m. The kidneys are smooth, and the liver is more lobate than in _Hippopotamus_. The orbital cavity is confluent with the temporal fossa. The typical genus, _Sus_, is distributed over Europe, Asia, and the islands of the Malay Archipelago, reaching as far as Borneo and Celebes. The dent.i.tion[185] is complete. A single species, the so-called _S. sennaariensis_, is from Ethiopian Africa, but it is not certain how far this animal may be an escaped species introduced by man. A very large number of "species" of _Sus_ have been described, but Dr.

Forsyth {276} Major is disposed to reduce them to four if not to fewer species. He allows the widely-ranging _S. scrofa_, _S. vittatus_, and the eastern Malayan _S. verrucosus_ and _S. barbatus_.

[Ill.u.s.tration]

FIG. 142.--Pygmy Hog (from _Nature_). _Sus salvania._ 1/6.

The Pygmy Hog of the Bhotans seems to be not ent.i.tled to specific rank, certainly not to generic (in the opinion of some), though it has been termed _Porcula salvania_.[186] The Wild Boar of Europe is _Sus scrofa_. It was formerly quite abundant in this country; not merely are its remains exhumed from fens, caves and peat bogs, but there is ample evidence of its continuance down to a comparatively late historic period. Enactments are on record as to the hunting of these animals; there are places, such as Boarstall, whose names are clearly derived from the name of the animal, presumably once a native of the locality; and various doc.u.ments all show the presence of the Wild Boar {277} in this country down to so late a period as the end of the sixteenth century.