A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 263.—Skull of a †sabre-tooth tiger (†Machairodus palmidens) from the Miocene of France. (After Filhol.) P. 4, fourth upper premolar, sectorial tooth.

On account of the very incomplete preservation of the material so far collected, little is known of the †sabre-tooth series in North America during the Pliocene and Miocene epochs. Remains of very large cats have been found in the lower Pliocene and upper Miocene, but it is uncertain whether they belong to the feline or the †machairodont subfamily. Some of the species have been referred to the genus †Machairodus, which ranged from the lower Pleistocene to the middle Miocene of Europe, and the reference may be correct, but is uncertain. However, the European representatives of that genus, which are much better known, will serve to show the developmental stage from which †Smilodon was undoubtedly derived. The dental formula was the same as in the American genus, though there were generally two premolars in the lower jaw and in †Smilodon generally but one; the individual teeth were formed on the same plan as in the latter, but were relatively smaller, and the very small, rudimentary upper molar was visible externally and was not overlapped and concealed by the great carnassial; the sabre-like tusk had not attained such great proportions. The skull of †Machairodus, the only part of the skeleton which is definitely known, was like that of †Smilodon on a much smaller scale, but more primitive in several respects. It was longer and had a less capacious brain-case and less prominent sagittal and occipital crests. The large tympanic bullæ were conspicuous in the side-view of the skull, as the processes for the attachment of the neck-muscles had no such development as in †Smilodon. The descending flanges of the lower jaw were larger than in the latter.

The upper Oligocene (John Day) contained a large variety of cat-like forms, of which no less than five genera have been described; one of them (†Pogonodon), nearly as large as a Lion, would seem to have died out here without descendants, and two others, to which we shall return later, so combined the characters of true felines and †machairodonts as to be of uncertain reference. Two other genera, which are much commoner and better known, from the White River, will be described from specimens of that stage.

The White River, or lower Oligocene, had three highly interesting genera of †machairodonts, two of them known from nearly or quite complete skeletons. One of these (†Hoplophoneus), which was, it can hardly be doubted, the direct ancestor of the later typical †machairodonts, had several species, which are found in the various levels of the White River beds. The largest of these species was considerably smaller than †Machairodus, and the smallest and most ancient was inferior to the modern Wild Cat. The number of teeth was variable, but normally greater than in the genera above described, being i 3/3, c 1/1, p 3-2/3-2, m 1/1, × 2 = 28-32. The foremost premolar in each jaw was very small and often absent. The upper canine was a long and curved, but very thin, scimitar, finely serrate on both edges, while the lower canine was but little larger than the incisors. The carnassial teeth had a significant likeness to those of other fissipede families; the upper one, the fourth premolar, was relatively smaller than in †Machairodus and its blade less effectively trenchant; the accessory antero-external cusp was present, though extremely small, and the internal cusp, which in †Smilodon had almost disappeared, was quite large. The lower sectorial, the first molar, though already cat-like and consisting of two thin, broad and trenchant cusps in line, yet had vestiges of the heel and sometimes of the inner cusp. These vestiges were a connecting link between the highly specialized sectorial of the cats and the type usual among the Fissipedia, which is exemplified by the dogs. The small upper molar was less reduced than in the Miocene and Pliocene genera and plainly consisted of a larger external and smaller internal cusp.

Fig. 264.—White River †sabre-tooth tiger (†Hoplophoneus primævus). Restored from a skeleton in the American Museum. †Oreodonts (†Merycoidodon) in the background.

Compared with that of other Fissipedia, the skull was short and broad, but in comparison with that of the modern cats and of †Smilodon, it was decidedly longer and narrower and the face was less abbreviated; the resemblance to †Smilodon was very marked in the form of the cranium, but, of course, the skull of †Hoplophoneus was distinctly more primitive in many respects. Thus, the orbit was much more widely open behind, the tympanic bullæ were but imperfectly ossified, and the perforations, or foramina, in the base of the skull, by which the nerves and blood-vessels communicated with the brain-chamber, were quite different and had more resemblance to those of the ancient dogs (e.g. †Daphœnus). In the classification of the Fissipedia much stress is laid upon the number and arrangement of these cranial foramina, and it is very significant to find the primitive dogs and cats agreeing so much more closely than do the modern members of these families. The lower jaw was relatively much stouter than in †Smilodon and the anterior flanges much more prominent, projecting downward so far that, when the jaws were closed, the points of the tusks did not extend below the flanges. The animal could have made no use at all of the sabre-tusks unless the mouth could have been opened so widely as to clear their points.

With close general resemblance, allowing for the very inferior size, the skeleton of †Hoplophoneus had many significant differences from that of †Smilodon. The neck was shorter and the body, especially the loins, longer, lighter and more slender and the tail very much longer, equalling that of the Leopard in relative length and surpassing it in thickness. The limbs were much less massive and somewhat differently proportioned, the upper arm being shorter and the fore-arm longer. The humerus, though far more slender than that of †Smilodon, was remarkable for the great development of the deltoid and supinator ridges, the latter, together with the shape of the radius, indicating very free rotation of the fore paw. The very prominent internal epicondyle was pierced by a foramen, and the femur had a distinct remnant of the third trochanter. The five-toed feet were comparatively small, but the claws were as completely retractile and as fully hooded as in any of the subsequent genera.

That †Hoplophoneus was a fierce destroyer, is made evident by every part of its skeleton, and, like other cats, it no doubt subsisted upon warm-blooded animals, which it killed for itself, the size of the prey being determined by the size and power of the particular species of the †sabre-toothed genus. In view of the probable extent of the Oligocene forests, the restoration ([Fig. 264]) gives the animal a spotted coat and the general aspect is that of one of the modern spotted cats, but the protruding ends of the tusks and the relatively long head distinguish it from any existing cat. “The presence of long, knife-like canines is correlated with powerful grasping feet possessing highly developed retractile claws. With its powerful feet the animal clung to its prey, while it struck repeatedly with its thin, sharp sabres” (J. C. Merriam).

In the latter part of the White River stage lived one of the most highly specialized of the †machairodonts, so far, at least, as the dentition is concerned, for only the skull is known. This genus, †Eusmilus, which also occurred in the Oligocene of Europe, was apparently an example of premature specialization which led to nothing, for none of the subsequent genera could have been derived from it. The teeth were reduced to a minimum in number: i 3/2, c 1/1, p 2/1, m 1/1, × 2 = 24, one lower incisor and at least one premolar less in each jaw than had †Hoplophoneus. The canine tusk was very large and the flange of the lower jaw for its protection correspondingly elongated, being more prominent than in any other †machairodont. The American species, †E. dakotensis, was the largest carnivore of its time and not greatly inferior in size to the Lion.