The remains of species of the family LEPORIDÆ are very abundant in some Post-Pliocene cave deposits on both sides of the Atlantic, and in several cases the species are evidently identical with those now living. Besides these, species of the genus Lepus have been found in Pliocene and Miocene beds in France. In North America three extinct Leporine genera have been recognised, differing from Lepus in certain peculiarities of the molar teeth:—Palæolagus, with three species, from the Miocene of Dakota and Colorado; Panolax, from the Pliocene marls of Santa Fé; and Praotherium, from a bone-cave in Pennsylvania. The last-named genus has the crowns of the molars transversely oval, and without the enamel-band or crest which is seen on the surface of the teeth of other Hares.
The LAGOMYIDÆ are known in a fossil state chiefly from Post-Pliocene deposits, and the bone breccias of caves in various parts of Europe. In Post-Pliocene times the genus Lagomys seems to have been very generally distributed over the South of Europe; and the earliest appearance of the genus is in the Pliocene, three species having been described from deposits of that age at Œningen and Montpellier. The family is, however, carried further back in time by the genus Titanomys, in which the molars differ but slightly in structure from those of Lagomys, but there are only four of them in each series, both above and below. Two species of this genus have been recorded from Miocene deposits in France and Germany.
SIDE VIEW OF SKULL AND LOWER JAW OF MESOTHERIUM CRISTATUM.
DENTITION OF MESOTHERIUM CRISTATUM.
(A) Upper Jaw; (B) Lower Jaw; (C) Incisors.
We have thus passed very briefly in review the fossil Rodents belonging to the two great sections of the order to which all its living species are to be referred; and it will be seen that while a knowledge of their existence is necessary to complete the history of the order, they present none of those peculiar characters which lend such interest to the fossil members of many other orders. There is, however, one fossil South American type to which we have yet to refer, as, by the curious combination of characters which it presents, it has long been somewhat of a puzzle to palæontologists, and although generally placed among the Rodents, its peculiarities are such that Mr. Alston found himself compelled to establish a third primary section of the order for its reception. According to M. Bravard, the first discoverer of this peculiar type, the Pliocene deposits of the Pampas of La Plata contain the remains of three species belonging to it; but the bones which have been sent to Europe, and which represent most parts of the skeleton, seem all to belong to a single species, which has been very fully described by M. Serres under the name of Mesotherium cristatum.[55] What distinguishes it at once from all other known Rodents is the presence in the lower jaw of four incisor teeth, the second pair being very small and placed immediately behind the outer edge of the broad middle pair. The latter are peculiarly widened and compressed from front to back in both jaws, and their summits, instead of being worn to a sharp chisel-like edge as in ordinary Rodents, show an elongated ring of enamel surrounding a slightly depressed surface. Hence Mr. Alston denominated this section HEBETIDENTATA, or BLUNT-TOOTHED RODENTS. The enamel in all the incisors is continuous round the tooth. The molar teeth are rootless and curved, the convex side being directed outwards, contrary to what occurs in other Rodents. They are surrounded by enamel, and show re-entering folds which differ in the two jaws. Their number on each side is five in the upper and four in the lower jaw. The skull is massive, with enormously-developed sagittal and occipital crests, the latter of which run forward so far as to join the zygomatic arches; and these crests rise so high that the upper surface of the actual brain-case is entirely concealed by them when the skull is looked at from the side. The lower jaw in its characters presents some resemblance to the same part in the Leporidæ; but it has the condyle for its articulation with the skull transverse, and fitting into a cavity of corresponding direction, a character which occurs in no other Rodent. Of the remainder of the skeleton we need only state that the animal possessed perfect clavicles; that the shoulder-blade and humerus somewhat resemble those of the Beaver; that the fibula articulated with the heel-bone; and that both front and hind limbs possessed five toes, some of which, judging from the form of the terminal joint, were probably furnished with hoof-like claws.
Thus, as regards its affinities in the order Rodentia, Mesotherium presents resemblances in its lower jaw (as also in some peculiarities of the skull), and in the articulation of the heel with the shank, to the Hares; while in the shortness of the incisors and some other cranial peculiarities, the form of the shoulder-blades, and the probably hoof-like character of the claws, we may notice an approach to the Cavies, which are also South American forms, and especially to the Capybara, which it probably resembled in its habits, although, if the evidence of the Beaver-like shoulder-blade and humerus be taken into account, it would appear to have been still more aquatic.
On the other hand, the resemblance to certain other Mammalia, and especially to some aberrant Ungulates, is unmistakable. The number of incisor teeth is the same as in Hyrax, and in these teeth there is also a certain amount of resemblance to the curious genus Toxodon, in which the incisors are four in the upper and six in the lower jaw, and worn away in somewhat the same fashion. In Toxodon also, the convexity of the curve of the molars is turned outwards. Certain other characters of Mesotherium—such as the mode of articulation of the lower jaw, and the peculiar connection of some of the caudal vertebræ with the ischiatic bones—present resemblances to the Edentata. As Mr. Alston says, “It appears to have been a survivor, to Pliocene times, of a much earlier type, which represented an era at which the Rodents were not yet clearly marked off from their allies. In fact, Mesotherium seems to continue into the order Glires that line of affinity which Professor Flower has pointed out as extending from the typical Ungulates through Hyracodon, Homalodontotherium, Nesodon, and Toxodon.”
The general relationships of Mesotherium to the other Rodents, and of these among themselves, are represented by Mr. Alston in a diagrammatic form, from which the following scheme, which will serve also as a table of the families, is derived:—