VI.Prom Eocene to Pliocene times, the greater part of South America, like Australia, was a hermit animal kingdom which enjoyed little or no communication with other lands. Except for whales to whom broad oceans were no barrier, its mammals, who had established themselves there at a very remote period, were peculiar unto themselves and unknown on other continents. The sloth and armadillo, now degenerates, are notable examples. None of its beasts had contact with those of North America, Europe, Asia or Africa, who had been crossing and re-crossing each others paths with more or less freedom. Throughout Eocene, Oligocene and Miocene times, South America's land-animals were prisoners hermetically sealed and therefore shut off from the rest of the world.
Dibelodon, the South American Mastodon
Throughout all this time, no connection between the two Americas existed, or at least it was impassable. Meanwhile a race of mastodons, originating probably in Asia or thereabouts, had established itself in North America and was drifting southward. This was a comparatively short-jawed race, although the chin-spout retained considerable of the old Long-jawed Trilophodon's peculiarities. In the northern continent, this race is not well understood, but now in the Pliocene period the barrier shutting off South America was being lifted and it is there that we find this new order of mastodons fully developed.
The newly-opened route from north to south led through Central America, and by means of it the mastodon for the first time in ancient animal history was enabled to set foot upon South American soil. In time he spread from the Andes to Argentina, ascending over 12,000 feet into the mountains, and making his home there, as well as descending almost to sea-level, where he lived and thrived. This group, whose enterprise was in keeping with elephantine traditions, is known as Dibelodon, meaning two-tusked, not a particularly descriptive title, for in the Pliocene period, the tendency was for all mastodons to dispense with tusks in the lower jaws. However this animal was a modification of an old-fashioned type, the Long-jawed Trilophodon.
His lower tusks, although absent as a rule, were occasionally present in a rudimentary state. The upper ones were stoutly developed and protected by long wearing strips or enamel bands. His molar teeth were three-ridged and trefoiled, and these tooth characteristics, together with his enamel-banded upper tusks, show his affiliation with the Long-jawed Trilophodon. Unlike the latter, however, his trunk was more or less pendant and probably he made good use of it to pluck his food, although the stout upper tusks must have been excellent food-getters, as well as admirable weapons for defensive purposes. In one species (Mastodon Andium), these were long enamel-banded and with a spiral or gimlet-like twist, a feature unknown in any other Mastodon or Elephant tusk. The tusks and teeth of Dibelodon exhibit many variations in the ten or more species which ranged southward through Argentina and to the north as far as Mexico, but all are classed in one group, the South American Mastodon.
North American animals and possible forerunners of this race have been found in Texas (Mastodon successor). One from the Lower Pliocene of Devil's Gulch, Nebraska, (Eubelodon Morrilli) recently discovered, departs from the typical Dibelodon in having no enamel upon his upper tusks, even though his lower jaw is of moderate length, and he lacks lower tusks. Just what bearing these northern forms had upon their southern relatives, is far from clear and Dibelodon's ancestry remains obscure. Like all others of the primitive mastodon race, he passed slowly away in the Pleistocene period after a long life in the Pliocene. For a time the communication between North and South America remained uninterrupted. Horses, tapirs and other northerners followed the Mastodon into South America and colonized there while, from the latter continent, giant ground sloths and armadillos ventured up into Texas and spread over the southern United States. But in the meantime, Central America was being overgrown with rank vegetation. Forests became jungles with trees and creeping vines so closely interwoven as to be impassable even for mastodons. Travel between the two continents ceased and once more South America was a hermit kingdom. Although doomed to end his days far remote from Asia, the land of his youth, Dibelodon persisted for a long time in the late Pliocene and early Pleistocene period of his captivity, surviving many another of the more fortunate mastodon races, whose lives were unrestricted by natural barriiers, and who roamed without let or hindrance throughout the length and breadth of the outside world.
[from the original, typewritten draft with hand-written corrections - GL,III, ed.]