The Geographical Distribution of Animals, Volume 1 / With a study of the relations of living and extinct faunas as elucidating the past changes of the Earth's surface

by Alfred Russel Wallace

Far back in the Triassic coal of North Carolina has been found the jaw of a small mammal (Dromotherium), the teeth of which somewhat resemble those of the Australian Myrmecobius, and may belong either to the Insectivora or Marsupials; if indeed, at that early period these orders were differentiated.

Carnivora.—The most ancient forms of this order are some remains found in the Middle Eocene of Wyoming, and others recently described by Professor Cope (1875) from the Eocene of New Mexico, of perhaps earlier date. The former consist of three genera, Patriofelis, Uintacyon, and Sinopa,—animals of large size but which cannot be classed in any existing family; and two others, Mesonyx and Synoplotherium, believed by Mr. Cope to be allied to Hyænodon. The latter consist of four genera,— Oxyæna, consisting of several species, some as large as a jaguar, was allied to Hyænodon and Pterodon; Pachyæna, allied to the last; Prototomus, allied to Amphicyon and the Viverridæ; and Limnocyon, a civet-like carnivore with resemblances to the Canidæ.

In the Miocene formations we find the Feline type well developed. The wonderful Machairodus, which in Europe lived down to Post-Pliocene times, is found in the Upper Miocene of Dakota; and perfect crania have been discovered, showing that the chin was lengthened downwards to receive and protect the enormous canines. Dinyctis was allied both to Machairodus and to the weasels. Three new genera have been lately described by Professor Cope from the Miocene of Colorado,—Bunælurus, with characters of both cats and weasels; Daptophilus, allied to Dinyctis; and Hoplophoneus, more allied to Machairodus. The Canidæ are represented by Amphicyon, which occurs in deposits of the same age in Europe; and by Canis, four species of which genus are recorded by Professor Cope from the Miocene of Colorado, and it also occurs in the Pliocene. The Hyænodon is represented by three species in the Miocene of Dakota and Colorado. It occurs also in the European Miocene and Upper Eocene formations, and constitutes a distinct family Hyænodontidæ, allied, according to Dr. Leidy, to wolves, cats, hyænas and weasels. The Ursidæ are represented by only one species of an extinct genus, Leptarchus, from the Pliocene of Nebraska. From the Pliocene of Colorado, Prof. Cope has recently described Tomarctos, as a "short-faced type of dog;" well as species of Canis and Martes.

Ungulata.—The animals belonging to this order being usually of large size and accustomed to feed and travel in herds, are liable to wholesale destruction by floods, bogs, precipices, drought or hunger. It is for these reasons, probably, that their remains are almost always more numerous than those of other orders of mammalia. In America they are especially abundant; and the number of new and intermediate types about whose position there is much difference of opinion among Palæontologists, renders it very difficult to give a connected summary of them with any approach to systematic accuracy.

Beginning with the Perissodactyla, or odd-toed ungulates, we find the Equine animals remarkably numerous and interesting. The true horses of the genus Equus, so abundant in the Post-Pliocene formations, are represented in the Pliocene by several ancestral forms. The most nearly allied to Equus is Pliohippus, consisting of animals about the size of an ass, with the lateral toes not externally developed, but with some differences of dentition. Next come Protohippus and Hipparion, in which the lateral toes are developed but are small and functionless. Then we have the allied genera, Anchippus, Merychippus, and Hyohippus, related to the European Hippotherium, which were all still smaller animals, Protohippus being only 2½ feet high. In the older deposits we come to a series of forms, still unmistakably equine, but with three or more toes used for locomotion and with numerous differentiations in form, proportions, and dentition. These constitute the family Anchitheridæ. In the Miocene we have the genera Anchitherium (found also in the European Miocene), Miohippus and Mesohippus, all with three toes on each foot, and about the size of a sheep or large goat. In the Eocene of Utah and Wyoming, we get a step further back, several species having been discovered about the size of a fox with four toes in front and three behind. These form the genus Orohippus, and are the oldest ancestral horse known. Prof. Marsh points out the remarkably perfect series of forms in America, which, beginning with this minute ancient type, is gradually modified by gaining increased size, increased speed by concentration of the limb-bones, elongation of the head and neck, the canine teeth decreased in size, the molars becoming longer and being coated with cement—till we at last come to animals hardly distinguishable, specifically, from the living horse.

Allied to these, are a series of forms showing a transition to the tapirs, and to the Palæotherium of the European Eocene. In the Pliocene we have Parahippus; in the Miocene Lophiodon, found in the same formation and in the Eocene of Europe, and allied to the tapir; and in the Eocene, Palæosyops, as large as a rhinoceros, which had large canines and was allied to the tapir and Palæotherium; Limnohyus, forming the type of a family Limnohyidæ, which included the last genus and some others mentioned further on; and Hyrachyus, allied to Lophiodon, and to Hyracodon an extinct form of rhinoceros. Besides these we have Lophiotherium (also from the Eocene of Europe); Diplacodon allied to Limnohyus, but with affinities to modern Perissodactyla and nearly as large as a rhinoceros; and Colonoceras, also belonging to the Limnohyidæ, an animal which was the size of a sheep, and had divergent protuberances or horns on its nose. A remarkable genus, Bathmodon, lately described by Professor Cope, and of which five species have been found in the Eocene of New Mexico and Wyoming, is believed to form the type of a new family, having some affinity to Palæosyops and to the extinct Brontotheridæ. It had large canine tusks but no horns.

The Rhinocerotidæ are represented in America by the genus Rhinoceros in the Pliocene and Miocene, and by Aceratherium and Hyracodon in the Miocene. Both the latter were hornless, and Hyracodon was allied to the Eocene Hyrachyus, one of the Lophiodontidæ. In the Eocene and Miocene deposits of Utah, and Oregon, several remarkable extinct rhinoceroses have been recently discovered, forming the genus Diceratherium. These had a pair of nasal horns placed side by side on the snout, not behind each other as in existing two-horned rhinoceroses, the rest of their skeleton resembling the hornless Aceratherium. They were of rather small size.

Next to these extinct rhinoceroses come the Brontotheridæ, an extraordinary family of large mammalia, some of which exceeded in bulk the largest living rhinoceros. They had four toes to the front and three to the hind feet, with a pair of large divergent horns on the front of the head, in both sexes. Professor Marsh and Dr. Leidy have described four genera, Brontotherium, Titanotheium, Megacerops, and Anisacodon, distinguished by peculiarities of dentition. Though most nearly allied to the rhinoceroses, they show some affinity for the gigantic Dinocerata of the Eocene to be noticed further on. Professor Cope has since described another genus, Symborodon, from the Miocene of Colorado, with no less than seven species, one nearly the size of an elephant. He thinks they had a short tapir-like proboscis. The species differ greatly in the form of the cranium and development of the horn-bearing processes.

We commence the Artiodactyla, or even-toed Ungulates, with the hog tribe. These are represented by species of peccaries, (Dicotyles) from the Pliocene of Nebraska and Oregon; and by an allied form Thinohyus, very like Dicotyles, but having an additional premolar tooth and a much smaller brain-cavity. From the Miocene are three allied genera, Nanohyus, Leptochœrus, and Perchœrus. Professor Cope, however, thinks Leptochœrus may be Lemuroid, and allied to Menotherium. The Anthracotheridæ, a family which connects the Hippopotamidæ and Ruminants, and which occurs in the Miocene of Europe and India, are represented in America by the genus Hyopotamus from the Miocene of Dakota, and Elotherium from the Miocene of Oregon and the Eocene of Wyoming; the latter genus being sometimes classed with the preceding family, and lately placed by Professor Marsh, in the new order, Tillodontia. Professor Cope has since described three other genera from the Eocene of New Mexico: Meniscotherium, having resemblances to Palæosyops, Hyopotamus, and the Limnotheridæ; Phenacodus, the size of a hog, of doubtful position, but perhaps near Elotherium; and Achænodon, as large as a cow, but more hog-like than the preceding. Another new genus from the Miocene of Colorado—Pelonax—is said by Professor Cope to come between Elotherium and Hippopotamus.

The Camelidæ are very abundant, and form one of the most striking features of the ancient fauna of America. Procamelus, Homocamelus, and Megalomeryx, are extinct genera found in the Pliocene formation; the first very closely allied to the Old World camel, the last smaller and more sheep-like. In the Miocene two other genera occur, Pœbrotherium and Protomeryx, the former allied to both the camel and the llama.