Still another White River †machairodont, †Dinictis, differed in many interesting ways from its contemporary †Hoplophoneus, being more primitive and departing less from the ordinary fissipede type of structure. This is shown by the greater number of teeth, which was normally, i 3/3, c 1/1, p 3/3, m 1/2, × 2 = 34. The upper carnassial had a considerably larger internal cusp and the trenchant blade did not have the accessory anterior cusp, which is present in almost all other cats and was thus more dog-like than cat-like. The lower carnassial was more feline, but retained a remnant of the heel and of the inner cusp, but the latter was variable, being sometimes present in one side of the jaw and not in the other, a sign that it was on the point of disappearance. The upper molar was plainly a reduced form of the tritubercular tooth, in plan like that of the dogs, while the second lower molar was a very small, single-rooted tooth. No other American cat has such a primitive dentition as this, and, aside from the sabre-tusk, which was not nearly so long as in †Hoplophoneus, and the lower carnassial, it might almost as well have belonged to a dog or musteline.
Fig. 265.—Primitive †sabre-tooth (†Dinictis felina) from the White River. Restored from specimens in the American Museum and Princeton University.
The skull was very like that of †Hoplophoneus, but was still longer and somewhat different in shape, owing to the higher forehead and lower occiput. The primitive features of the cranial base, such as the foramina, the imperfectly ossified tympanic bullæ, etc., were repeated in †Dinictis, but the lower jaw had much less prominent flanges for the protection of the tusks. The limbs differed considerably from those of †Hoplophoneus in being relatively longer and more slender and retaining more primitive features, such as the larger third trochanter of the femur. The five-toed feet were decidedly small and weak, and the claws, though retractile, were less so than in the other genus and were not hooded. The gait was probably plantigrade or semi-plantigrade.
Fig. 266.—Skull of †Dinictis squalidens, White River. (After Matthew.) p. 4 = fourth upper premolar, sectorial.
The relationships of †Dinictis and †Hoplophoneus are rather puzzling; none of the known species of the former could have been ancestral to the latter, for the two genera were contemporaneous. †Dinictis was apparently the somewhat modified survivor of the ancestral stage and represented very nearly the common starting point of both the feline and †machairodont subfamilies. Dr. Matthew has propounded the bold theory that this genus was the actual ancestor of the felines, continuing the series through †Archælurus and †Nimravus of the John Day to the unmistakable felines of the middle Miocene. This view runs contrary to the supposed “law of the irreversibility of evolution,” a rule which many authorities look upon as well established. The theory postulates a different mode of development from anything that we have so far encountered in the series previously described and supposes that the upper canine first lost its original form, becoming a thin, elongate and scimitar-like tusk, while the lower canine was reduced almost to the proportions of an incisor and the lower jaw acquired a straight, flat chin and inferior flanges for the protection of the tusks. Then, after specialization had advanced so far, it was reversed and the original condition regained. This interesting hypothesis may possibly turn out to be true, though personally I cannot accept it, and, should it do so, it would necessitate a thoroughgoing revision of current opinions as to the processes of mammalian development.
Fig. 267.—Left pes of †Dinictis felina. Cal., calcaneum. As., astragalus. Cb., cuboid. Princeton University Museum.
The only John Day cat which was assuredly derived from †Dinictis was the large †Pogonodon, previously mentioned.