A history of land mammals in the western hemisphere

by William Berryman Scott

A second family of Condylarthra was represented in the lower Eocene by the genus †Meniscotherium and was in some respects considerably more advanced than the †phenacodonts. These were small animals, in which the molars had acquired a crescentic pattern, recalling that seen in the early horses and in the †titanotheres and †chalicotheres, and other perissodactyl families. In the upper molars the two external cusps had been so extended as to form a continuous outer wall, each of the cusps having a concave external face and the two uniting in a prominent median ridge. The lower molars had two crescents, one behind the other, as in several families of both perissodactyls and artiodactyls. The body and tail were long, the limbs relatively longer and lighter than those of †Phenacodus and the five-toed feet were so like those of the modern conies, or klipdasses, of Africa and Asia Minor, that by some investigators the family has been referred to the same order, the Hyracoidea, but the suggestion is not a probable one. It is much more likely that these problematical little †meniscotheres were merely a short-lived branch of the †Condylarthra.

Fig. 234.—Lower Eocene †condylarth, †Meniscotherium terræ-rubræ. Restored from a skeleton in the American Museum.

The †condylarths were quite abundantly represented in the Paleocene, where the genus †Euprotogonia was the forerunner of the Wasatch †Phenacodus, but had an even more primitive type of dentition. The upper molars were not quadritubercular, but tritubercular, the three cusps arranged in a triangle, the two outer ones forming the base and the single inner one the apex. This type of upper molar was, or is still, common to the primitive and unspecialized members of a great many mammalian orders, marsupials, insectivores, rodents, carnivores, lemurs, artiodactyls, etc., and there is strong reason to believe that the tritubercular molar was the common starting point for almost all types of mammalian dentition. However that may be, †Euprotogonia is of great interest as materially helping to close the gap between the clawed and the hoofed mammals, belonging, as it did, to the latter and yet retaining in dentition, limbs and feet so many characteristics of the former.

†Condylarthra were probably present in the lowest Paleocene (Puerco stage), but the material so far obtained is so fragmentary that there can be no certainty on this point.

It is not at all probable that any of the North American †Condylarthra should be regarded as ancestral to any of the more advanced ungulate groups; on the contrary, they would appear to have come to an end in the Wind River, leaving no descendants behind them. It is further true, as was mentioned above, that the presence of †Condylarthra in other continents, while very probable, cannot be positively asserted, because the evidence is incomplete. Yet it would be a great mistake to assume, for this reason, that these most primitive of ungulates were devoid of evolutionary importance and interest. As is so often the case, where, in the absence of the direct ancestry, the collateral relations afford very valuable information as to the course of descent and modification, the †Condylarthra throw useful light upon the origin of the ungulate groups. It is extremely probable that the †condylarths, or some very similar series of primitive hoofed mammals, had a very wide and perhaps cosmopolitan range at the end of the Cretaceous and beginning of the Tertiary period, and that, in the still unidentified region, where the artiodactyls and perissodactyls arose, it was from a condylarthrous ancestry. Possibly, all the other ungulate orders may yet be traced back to the same stock, but it is rather more likely that the ungulates include several series of quite independent origin. At all events, it is quite certain that the clawed mammals long antedated the hoofed types and that the latter arose, either once or at several separate times, from the former. The †Condylarthra show how one, at least, of these transitions was effected, and thus, in principle, how all were accomplished.

CHAPTER XII
HISTORY OF THE †TOXODONTIA (OR †NOTOUNGULATA)

It is a regrettable circumstance that, while the successive Tertiary faunas are very fully represented in South America, approximately complete skeletons have, as yet, been obtained from only a few of the various stages; from the others the known material is very fragmentary and largely made up of teeth and jaws. No doubt, the history of fossil-collecting in North America will, in due course of time, be repeated in the southern continent and more and more complete and satisfactory specimens be obtained. At present, however, it is not possible to trace the modifications of structure in any given series with such detail as in those which were developed within the limits of Arctogæa. No such story as that of the horses, the rhinoceroses or the camels, can yet be told of the South American groups, whatever future exploration may teach us. Nevertheless, much has already been learned concerning the strange creatures that once inhabited the Neotropical region and long ago vanished completely, leaving no trace in the modern world.

As was mentioned in [Chapter VI], on the present geographical distribution of mammals, South America is to-day the richest and, after Australia, the most peculiar zoölogically of all the regions. All of the modern hoofed animals found in that continent at present, the tapirs, peccaries, llamas and deer, are immigrants derived at a comparatively late date from the north, but throughout the Tertiary and the Pleistocene there were several indigenous types of ungulates, and of these the largest and most varied assemblage was that included in the order †Toxodontia. The most important and best known of the families and genera are listed in the table: