Cambridge Natural History Mammalia/Chapter XIV
CHAPTER XIV
CARNIVORA (CONTINUED)—PINNIPEDIA (SEALS AND WALRUSES)—CREODONTA
Sub-Order 2. PINNIPEDIA
This group includes the Seals, Sea-Lions, and Walruses,[1] all aquatic and, for the larger part, marine creatures. Being aquatic they have to some extent acquired a fish-like form, though not so completely as have the Whales and even the Sirenia. This is most complete so far as the group is concerned in the Seals, where the hind-limbs have become soldered to the tail and are inefficient as walking legs, where the external ears have vanished, and where the general shape of the body is tapering and thus fish-like. The Walruses and Sea-Lions are less modified in this direction; in the latter (not in the former) the external ear, though small, is persistent, and the hind-limbs are capable of being used as organs of progression upon dry land. The general characters applicable to the Carnivora, given upon a previous page, apply to the Pinnipedia.
The characters confined to the Pinnipedia as a whole are mainly these:—The greater part of the limbs are enclosed within the skin, the hands and feet are fully webbed, and there is a tendency for the nails to disappear, and for the phalanges to increase in number—characters which are clearly not diagnostic of the order but correlated with an aquatic life, since they reappear, and are indeed exaggerated, in the Cetacea. The teeth are peculiar in that the milk dentition is feeble and is early shed. This, as it were, undue emphasis upon one of the two sets of teeth is another likeness to the Whales, where, however, it is the milk dentition that is most pronounced, the "permanent" being feeble and very early shed.
Fig. 227.—Skeleton of Seal. Phoca vitulina. (After de Blainville.)
But the dentition of the Pinnipedes presents other likenesses to the Cetacea, which are, it must be remembered, regarded by some as a modification of the Carnivorous stock, in which case, of course, the likenesses may be genetic rather than due to adaptation in the two cases. There is a distinct tendency towards a homodont series, the grinding teeth being often very simple, and the very distinct carnassial tooth of many terrestrial Carnivores being absent. Finally, the number of the back teeth shows some signs of being on the increase; and Professor Kükenthal has found that this increase is due to the division of existing teeth. Here is a point of likeness to the many teeth of the typical Toothed Whales. Dr. Nehring found in several examples of Halichoerus grypus the normal five back teeth increased to six, and the additional molar was at the end of the series, thus suggesting a lengthening of jaw coupled with an increase in number of teeth.
The incisor teeth of the Pinnipedia differ from those of the land Carnivora in that there are nearly always fewer than 3/3, at least in the adult animal. In possessing lobulated kidneys the Pinnipedia differ from all terrestrial Carnivores except the Otters and Bears—a significant fact.
In the characters of the skeleton the Pinnipedia show many peculiarities. The cranial part of the skull is proportionately to the facial part greater than in terrestrial Carnivora; there is no lachrymal bone, and the orbit is to some extent defective in ossification. The alisphenoid canal, so important a feature in the Carnivora, may be present or absent. It is present, for example, in Otaria jubata.[2] This genus also has the more primitive small and rugged tympanic bullae, which are inflated and more Cat-like in others. The vertebrae show an interesting Creodont peculiarity in the complex interlocking arrangements of the zygapophyses of the dorsal vertebrae. The ossicula auditus differ from those of their terrestrial allies in their large size and massive growth. In this they have come to be like those of the Whales and Sirenians.
Fig. 228.—Patagonian Sea-Lion. Otaria jubata. × 1⁄20.
There is no doubt about their close resemblance to the terrestrial Carnivora, but the question is, to which group of Carnivora have they the most likeness. The semiaquatic Otter, and the still more thoroughly aquatic (marine) Enhydris, suggest an affinity in that direction. The long body and short legs of the Otter, which is more thoroughly at home pursuing fish in the streams than in waddling clumsily upon the banks of the streams, seem to require but little external change to convert it into a small Seal, while the long and completely webbed hind digits of Enhydris are even more like those of a Pinniped. The Sea-Lions, in which the external ear has been preserved, and in which the limbs have not become so entirely useless for progression on the land as they have in the Seals, seem to be the intermediate step in the evolution of the latter. This, however, is not the opinion of Dr. Mivart, who, without definitely committing himself on the point, presents some evidence for the assumption that the marine Carnivora are diphyletic. This double origin, however, is not from two groups of the terrestrial Carnivora. Dr. Mivart, in common with many others, holds that the Pinnipedia as a whole are undoubtedly nearer to the Arctoidea than to either of the two remaining sections of the sub-order. One of the most striking structural characters in which they show this resemblance is the brain; the peculiar Ursine lozenge, already treated of as so distinctive a character of the Arctoidea, is repeated in the Pinnipedia.
There are, however, other points of likeness which seem rather to point to a Creodont origin. Patriofelis is a genus that from more than one side may be looked upon as a possible ancestor of these animals. The Creodont peculiarity of the vertebrae has already been referred to. It may be added that the facial part of the skull is small in Patriofelis, which appears, moreover, to have had an alisphenoid canal. A very remarkable resemblance lies in the structure of the astragalus. This is not deeply grooved on the tibial facet as it is in Fissiped Carnivora. This might be held to be an instance of degeneration in the aquatic Seals, which do not use their limbs as walking organs. But Professor Wortman[3] has pointed out that in the Sea-Otter, which is entirely aquatic, the groove exists and is plain. The likeness offered to the Seals by the spreading feet of Patriofelis is noticed under the description of that genus.[4]
Fig. 229.—Cape Sea-Lion. Otaria pusilla. × 1⁄16.
Fam. 1. Otariidae.—The family Otariidae[5] is no doubt the least modified of the aquatic Carnivora. It is rational, therefore, to commence the survey of the group with this family. They have preserved, as already noted, the independence of the hind-limbs; the external ear is present, though small; there is an obvious neck, and the nostrils are at the end of the snout, as in terrestrial creatures generally. The nails are small and rudimentary, save those upon the three middle digits of the foot. It is a singular fact that among the Otaries the angle of the lower jaw is "inflected as much as in any Marsupial." The literature relating to this family is great, and it seems difficult to reconcile the very varying opinions as to how many genera ought to be admitted. Mr. Allen arranged the nine species which he allowed in six genera; but more generic names have been proposed. At the other extreme stands Dr. Mivart, who speaks of only one genus, Otaria; of this genus the number of species is by no means agreed upon. There can, however, be no doubt of the distinctness of the Northern Fur Seal, O. ursina (the "Seal" of commerce and the cause of international complications), of the Patagonian Maned Sea-Lion, O. jubata,[6] of O. pusilla of the Cape, of the Californian O. gillespiei, of O. hookeri from the Auckland Islands, and of four or five others. The range of the genus is wide, but is mainly Antarctic. It is usual to speak of "Hair Seals" and "Fur Seals," the latter being the species which produce the "sealskin" of commerce. The difference is that in the Fur Seals there is a dense, soft under-fur, which is wanting in the other group. It is, however, impossible to make this character the basis of a generic subdivision. There is a Fur Seal, O. nigrescens, in South America as well as the more widely-known northern form.
Fam. 2. Trichechidae.—This family contains but one genus, Trichechus, the Walrus or Morse, or Odobaenus, as the more correct term seems to be. It is a tiresome result of accurate conformity with the rules of priority in nomenclature that the name Trichechus should be applied to the Manatee. There is but one species of Walrus, though it has been attempted to show that the Pacific and Eastern forms are different. The animal is Arctic and circumpolar. The Walrus is characterised by the enormous canines of its upper jaw, which form the well-known tusks and reach a length of 30 inches. The animal can progress on land like the Sea-Lions; but, as in the Seals, there are no external ears, though there is a slight protuberance above the meatus auditorius. The strong bristles upon the upper lip are as thick as crow quills. The pectoral limb has nails, but these are small, as in the Sea-Lions. The under surface of the manus has a warty pad, which cannot but assist[7] in maintaining a foothold upon slippery ice. The hind-limbs have longer nails, which are still diminutive and subequal in size. There is no free tail. The liver of this animal is much furrowed, but not so much so as in Otaria, though more so than in Phoca. The kidneys are of course lobulate, as in the other aquatic Carnivores. The milk dental formula appears to be I 3/3 C 1/1 Pm + M 5/4. In the adult the formula[8] is I 1/0 C 1/1 M 3/3.
Fam. 3. Phocidae.—The true Seals have no external ears, and the nostrils are quite dorsal in position as in other aquatic animals, such as the Crocodile. There is obviously an approach to the conditions characteristic of the Whales. The hind-limbs are useless for locomotion on land. They are bound up with the tail, and form functionally merely a part of the tail. In this family there are, at any rate, eight genera.
Fig. 230.—Common Seal. Phoca vitulina. × ⅛. (From Parker and Haswell's Zoology.)
Phoca and Halichoerus are not very wide apart from each other. In both there are five well-developed claws on feet and hands. They are British, and generally Arctic and temperate in range. For some reason or other the late Dr. Gray placed Halichoerus in the same sub-family with the Walrus! Phoca is not only marine, but is found in the Caspian and in Lake Baikal. Their existence in those inland seas is believed to be a vestige of a former connexion with the sea. Halichoerus grypus is a large seal 8 feet in length when full grown. Its colour is yellowish grey, with darker grey spots and blotches. It is not uncommon on the shores of our islands, particularly of the Hebrides and Argyllshire. The commonest Seal is Phoca vitulina, not more than 4 to 5 feet long, and of the same spotted coloration as the last. This Seal has, however, a much wider distribution, being Arctic as well as British, American, and North Pacific. A curious fact about this Seal is that it is not impatient of fresh water; not only will it ascend rivers, but it will live in inland lakes. It is said to be especially sensitive to musical sounds. P. hispida is British, but a rare visitor to our islands. It is essentially an Arctic species. The Harp Seal, P. groenlandica, is so called on account of a harp-shaped black bar in the males, which starts at the shoulders and extends to the thighs. Like the other Seals mentioned, the young are white when first born. As may be inferred from its scientific name this species is also Arctic in range. It is also a rare visitor to these shores.
The genus Cystophora is the only other genus of which there is a British representative. It is called the Hooded Seal on account of an inflatable sac upon the face, with which it is said to attempt to terrify its enemies. The genus has an incisor less in each half of each jaw than Phoca and Halichoerus. Its formula is I 2/1 while these genera are both 3/2. C. cristata is a large species reaching a length of 10 feet. The colour of the back is dark grey with deeper coloured spots. A few individuals only have been recorded from our coasts.
Stenorhynchus ( = Ogmorhinus) is an Antarctic genus. The hind-feet are clawless. The incisors are 2/2. The molars have an additional cusp, i.e. three in all.
The genus Leptonyx with but one species, L. weddelli, is purely Antarctic in range. Like the last genus it has two incisors, and has but rudimentary claws upon the hind-feet; the first and fifth toes moreover are the longest. The genus chiefly differs from the last in the simple conical crowns of the molars, which have not the additional cusps of Stenorhynchus.
Ommatophoca is another Antarctic genus with but a single species, O. rossi. In this genus the hind-feet have no claws, and the first and fifth toes are longer than the others. The claws of the fore-feet are rudimentary. The immense size of the orbits gives the name to the genus. There are two incisors, and the molars are all very small.
Monachus is a northern genus inhabiting the Mediterranean and the Atlantic in the vicinity of Madeira and the Canary Islands. It has rudimentary nails upon both pairs of feet. The first and fifth toe of the hind-feet are longer than the others. As with the preceding genera, the incisors are two in each jaw. The species are M. albiventer, the Monk-Seal, and M. tropicalis, the Jamaica Seal.
Allied to Cystophora is the genus Macrorhinus, with (possibly) two species, of which one is Antarctic, the other frequents or frequented the coast of California. The incisors are two in the upper jaw, and but one in the lower. The premolars are four and the molar one; all the teeth are small and simple, but have long roots. The nose of the male has a dilatable proboscis. The southern Elephant Seal is M. leoninus, and reaches a length of some 20 feet. It occurs on the shores of Kerguelen and some other more or less remote islands. Its habits have been studied and described by several observers, beginning with Anson in the last century. The late Professor Moseley gave a good account of this marine monster in his book on the voyage of the "Challenger." When the animal is enraged, the end of the snout is dilated; but when this happens there is no long and hanging proboscis such as has sometimes been described. The inflation affects the skin on the top of the snout, which thus rises rather upwards during inflation. The inflated region, according to Mr. Vallentin, quoted by Mr. J. T. Cunningham, is about 1 foot long in an individual of 17 feet. It has been stated that this proboscis is a temporary structure, only appearing in the breeding season; but recent observations have shown that this statement is inaccurate; it persists all the year round. The males fight greatly during the breeding season, and produce a roar which has been compared to the "noise made by a man when gargling." The females and the young males bellow like a bull. The males fight of course with their teeth, literally falling upon one another with their whole weight. Mr. Cunningham thinks that the use of the proboscis is to protect the nose from injury; or that it may be merely the result of "emotional excitement." In any case the Bladder-nosed Seal, Cystophora, is undoubtedly protected from injury by the possession of a corresponding hood. The nose is the most vulnerable place, and the existence of this hood would stave off the effects of a blow in that region. Moseley, however, has said of Macrorhinus that it cannot be stunned by blows on the nose as other Seals can; but he attributes this, not to the dilated snout, but to the bony crest on the skull, and to the strength of the bones about the nose. This Seal crawls with difficulty on the land, and as the animals move "the vast body trembles like a great bag of jelly, owing to the mass of blubber by which the whole animal is invested, and which is as thick as it is in a whale."[9] When lying on the shore, these animals scrape sand and throw it over themselves, apparently to prevent themselves from being incommoded by the direct rays of the sun, to the effects of which they are very susceptible. The Elephant Seal is mild and inoffensive, unless enraged, and, of course, during the breeding season.
Order VIII. CREODONTA.
This entirely extinct group of Mammalia may be thus characterised:—Small to large carnivorous mammals, with skull on the whole like that of the Carnivora and with trenchant teeth; digits with unguiculate phalanges; tail long; extremities usually with five, sometimes with four digits. In the carpus a centrale is present, and the scaphoid and lunar are separate. Interlocking of posterior dorsal and lumbar zygapophyses very perfect. Brain small but convoluted.
This group, which corresponds with the Carnivora Primigenia of Mr. Lydekker, is not easy to separate absolutely from the existing and more especially from some of the extinct members of the Carnivora Vera. They also come exceedingly near the Condylarthra, the presumed ancestors of the Ungulata, and like them begin in the earliest Tertiary deposits. Their likeness to the carnivorous Marsupials has also been insisted upon; but it would seem that the succession of teeth in the Creodonta is typically Eutherian.
The characteristics of the group may be exemplified by an account of the genus Hyaenodon, after which some of the more important deviations in structure shown by other genera will be referred to.
Hyaenodon is both American and European, and ranges through the Eocene and the Upper Miocene. It is a much-specialised Creodont, and therefore exhibits well the distinctive characters of the group. About a dozen species have been described. One of the best-known is the American H. cruentus, and the following description refers to it. The back part of the skull is low and broad, and is compared by Professor Scott (who has described this and other species) as being "somewhat like that of an opossum."[10] The whole skull is long, and the top has a great sagittal crest. The paroccipital processes are short and are closely applied to the mastoid processes. The mesethmoid is larger than in the carnivorous Marsupials, and the frontals are very large. The palate has a peculiar structure; in most species the hinder ends of the palatines are separated by a narrow fissure which broadens gradually, thus forming the posterior nares. In H. leptocephalus the posterior nares are brought very far back by the meeting of the alisphenoids. The presphenoid, contrary to what we find in the Dog, for example, is chiefly concealed by the vomer, which covers it. The mandible has a long and strong symphysis, and its angle is not inflected. The fore-limb is described as being "weak when compared with the modern Carnivora." The scaphoid and lunar are separate, and there is a centrale. The teeth present us with nearly the typical formula. There is only one molar missing in the upper jaw. The canines are enlarged. It has been suggested from a consideration of its palate that Hyaenodon was a semiaquatic animal; the deep cleaving at the extremities of the phalanges seems to point in the same direction, since they resemble in this the genus Patriofelis, which there are other reasons to regard as aquatic. This latter genus has a fore-limb which is very like that of the Pinnipedia, the digits are much spread out, and would seem to have supported a kind of paddle. In any case it certainly fed upon aquatic tortoises, for their remains have been found in its coprolites. The name Limnofelis, also applied to what appear to have been members of this genus, is suggestive of their habits. Patriofelis, at least one species, seems to have been of about the size of a Lion.
Mesonyx has a brain case which is actually smaller than that of the Marsupial Thylacinus. The lachrymal bone is very large, and extends a little way over the face, as is also the case with Hyaenodon; this condition is also found in Insectivora and in Thylacinus. The axis vertebra has a curiously-shaped spine, which is very different from the hatchet-shaped process of that vertebra usual in the Carnivora, but is not unlike what exists in the Arctoid genera Meles and Mydaus. The limbs show much disparity in length, and seem to argue a much-arched back when the creature progressed. The carpus is stated to be strikingly like that of the Insectivora. There is as in other Creodonts a separation between the scaphoid and lunar; the centrale appears to be present. The pelvis "is most like that of the bear," the metacarpals and the tibia, and some other bones, resemble those of the Hyaena. In fact this animal shows those combined characters which are common in archaic forms.
- ↑ For the genera of Pinnipedia see Mivart, Proc. Zool. Soc. 1885, p. 484.
- ↑ Murie, Trans. Zool. Soc. viii. 1874, p. 501.
- ↑ Bull. Amer. Mus. Nat. Hist. vi. 1894, p. 129.
- ↑ P. 456 below.
- ↑ See especially Allen, North American Pinnipedes, 1880.
- ↑ Murie, Trans. Zool. Soc. vii. 1894, p. 411.
- ↑ Cf. the Dugong, p. 336.
- ↑ Kükenthal, Jen. Zeitschr. xxviii. 1894, p. 76.
- ↑ Cunningham, "Sexual Dimorphism in the Animal Kingdom," London, 1900; see also Flower, Proc. Zool. Soc. 1881, p. 145.
- ↑ Journ. Ac. Sci. Philadelphia, ix. 1886, p. 175.