2. Felidæ. Cats
The only other fissipede group whose development in North America may be followed for a long period is that of the †Sabre-Tooth Tigers, the subfamily †Machairodontinæ, which have been extinct since the Pleistocene; the history of the True Cats (Felinæ) is much more obscure. In most respects the two subfamilies agreed closely and, as they became separate at least in the early Oligocene, they furnish instructive parallel series. The †sabre-tooth cats were terrible beasts of prey, which in most of the Tertiary period ranged over the whole northern hemisphere and in the Pleistocene or late Pliocene extended throughout South America.
Fig. 261.—Skull of the Pleistocene †sabre-tooth tiger (†Smilodon californicus, after Matthew). P. 4, fourth upper premolar, sectorial.
The Pleistocene genus †Smilodon (Frontispiece) belonged to nearly the whole western hemisphere and its various species were distributed from California and Pennsylvania on the north, to the Argentine Pampas on the south. The most obvious and striking peculiarities of †Smilodon were in the teeth, which were much reduced in number, the formula being: i 3/3-2, c 1/1, p 2/2-1, m 1/1. The upper canine was a great, curved, scimitar-like blade, eight inches or more in length, with broad inner and outer faces, but quite thin transversely, and with finely serrate posterior edge. It is difficult to understand how these great tusks, which would seem to have blocked the entrance to the mouth, could have been effectively used, unless the creature could open its mouth much more widely than any existing mammal, so as to clear the points of the tusks, and would then strike with them as a snake does with its fangs. There are great anatomical difficulties in the way of accepting this explanation and the problem, which is the same as that presented by the †uintatheres ([p. 446]), is still unsolved. It is, however, quite certain that no arrangement which was disadvantageous, or even inefficient, could have persisted for such vast periods of time. The lower canine was much diminished and hardly larger than an incisor. The two upper premolars were the third and fourth of the original series; the third was small, but the fourth, the sectorial, was a very large and efficient shearing blade. In addition to the two external trenchant cusps of the blade, which are present in the Carnivora generally, the cats have a third small, anterior cusp which in †Smilodon was large; the internal cusp had almost disappeared. The single upper molar was very small and so overlapped by the great carnassial as to be invisible from the side. The third lower premolar was small and unimportant and most specimens had lost it, leaving only the fourth, which was larger and evidently of functional value. The single molar was the sectorial, a large, thin, flattened blade, consisting of only two cusps, one behind the other, the trenchant edges of which met at nearly a right angle, and there was no trace of a heel.
Fig. 262.—Upper teeth of †Smilodon, left side. P. 4, fourth premolar. m. 1, first molar. (After Matthew.)
The skull was in appearance closely similar to that of one of the great modern cats, such as the Lion or Tiger; with extremely shortened face, heavy and widely expanded zygomatic arches and very prominent sagittal crest. The tympanic bullæ were large and inflated, each divided by a septum into two chambers, but were not visible from the side, being covered externally by very large processes, which served for the attachment of some of the great muscles of the neck. The short, rounded, bullet-head of the true cats was thus repeated, but there were in the skull several interesting differences of detail, which it is not worth while to enumerate here. Suffice it to say, that some of these differences were due to the retention of primitive characters in the skull of †Smilodon, which have been lost in the modern felines, and others to special developments, in which the true cats did not share. The lower jaw had on each side a small, descending flange for the protection of the tusks, which, however, projected well below these flanges when the jaws were shut. The neck was heavy and the structure of its vertebræ was such as to suggest the presence of unusually powerful muscles; the back and loins were also uncommonly stout, in the larger species heavier than in the Lion or Tiger, but, in marked distinction from those modern forms, the tail was short. The limbs were shorter and much heavier in relation to the size of the body than in the great existing cats and must have been extremely powerful. The humerus usually had no epicondylar foramen, which all the true felines possess, though it was sometimes present. The feet also were very stout and armed with large retractile claws; the base of each claw was covered by a thin bony hood, an outgrowth of the ungual phalanx, which is very characteristic of the entire family. The hind foot had five digits, whereas no existing cat has more or less than four. The appearance of these animals must have been very much like that of the Lion or Tiger, aside from the unknown factors of mane and colour-markings, but differed in the great tusks, the short tail and the shorter and more massive legs and feet.
