| 1 South of the Uinta Mts. in Utah. 2 Fort Bridger Basin. 3 Wind river in Wyoming. | 4 Wasatch Mts. in Utah. 5 Torrejon in New Mexico. 6 Puerco river, New Mexico. |
The Fort Union beds of Canada and parts of Montana and N. Dakota are probably the oldest Eocene strata of the Western Interior; they are some 2000 ft. thick and possibly are equivalent to the Midwayan group. But in these beds, as in those known as Arapahoe, Livingston, Denver, Ohio and Ruby, which are now often classed as belonging to the upper Laramie formation, it is safer to regard them as a transitional series between the Mesozoic and Tertiary systems. There is, however, a marked unconformity between the Eocene Telluride or San Miguel and Poison Canyon formations of Colorado and the underlying Laramie rocks.
Many local aspects of Eocene rocks have received special names, but too little is known about them to enable them to be correctly placed in the Eocene series. Such are the Clarno formation (late Eocene) of the John Day basin, Oregon, the Pinyon conglomerate of Yellowstone Park, the Sphinx conglomerate of Montana, the Whitetail conglomerate of Arizona, the Manti shales of Utah, the Mojave formation of S. California and the Amyzon formation of Nevada.
Of the Eocene of other countries little is known in detail. Strata of this age occur in Central and S. America (Patagonia-Megellanian series—Brazil, Chile, Argentina), in S. Australia (and in the Great Australian Bight), New Zealand, in Seymour Island near Graham Land in the Antarctic Regions, Japan, Java, Borneo, New Guinea, Moluccas, Philippines, New Caledonia, also in Greenland, Bear Island, Spitzbergen and Siberia.
Organic Life of the Eocene Period.—As it has been observed above, the name Eocene was given to this period on the ground that in its fauna only a small percentage of living species were present; this estimation was founded upon the assemblage of invertebrate remains in which, from the commencement of this period until the present day, there has been comparatively little change. The real biological interest of the period centres around the higher vertebrate types. In the marine mollusca the most noteworthy change is the entire absence of ammonoids, the group which throughout the Mesozoic era had taken so prominent a place, but disappeared completely with the close of the Cretaceous. Nautiloids were more abundant than they are at present, but as a whole the Cephalopods took a more subordinate part than they had done in previous periods. On the other hand, Gasteropods and Pelecypods found in the numerous shallow seas a very suitable environment and flourished exceedingly, and their shells are often preserved in a state of great perfection and in enormous numbers. Of the Gasteropod genera Cerithium with its estuarine and lagoonal forms Potamides, Potamidopsis, &c., is very characteristic; Rostellaria, Voluta, Fusus, Pleurotoma, Conus, Typhis, may also be cited. Cardium, Venericardia, Crassatella, Corbulomya, Cytherea, Lucina, Anomia, Ostrea are a few of the many Pelecypod genera. Echinoderms were represented by abundant sea-urchins, Echinolampas, Linthia, Conoclypeus, &c. Corals flourished on the numerous reefs and approximated to modern forms (Trochosmilia, Dendrophyllia). But by far the most abundant marine organisms were the foraminifera which flourished in the warm seas in countless myriads. Foremost among these are the Nummulites, which by their extraordinary numerical development and great size, as well as by their wide distribution, demand special recognition. Many other genera of almost equal importance as rock builders, lived at the same time: Orthophragma, Operculina, Assilina, Orbitolites, Miliola, Alveolina. Crustacea were fairly abundant (Xanthopsis, Portunus), and most of the orders and many families of modern insects were represented.
When we turn to the higher forms of life, the reptiles and mammals, we find a remarkable contrast between the fauna of the Eocene and those periods which preceded and succeeded it. The great group of Saurian reptiles, whose members had held dominion on land and sea during most of the Mesozoic time, had completely disappeared by the beginning of the Eocene; in their place placental mammals made their appearance and rapidly became the dominant group. Among the early Eocene mammals no trace can be found of the numerous and clearly-marked orders with which we are familiar to-day; instead we find obscurely differentiated forms, which cannot be fitted without violence into any of the modern orders. The early placental mammals were generalized types (with certain non-placental characters) with potentialities for rapid divergence and development in the direction of the more specialized modern orders. Thus, the Creodonta foreshadowed the Carnivora, the Condylarthra presaged the herbivorous groups; but before the close of this period, so favourable were the conditions of life to a rapid evolution of types, that most of the great orders had been clearly defined, though none of the Eocene genera are still extant. Among the early carnivores were Arctocyon, Palaeonictis, Amblyctonus, Hyaenodon, Cynodon, Provivera, Patriofelis. The primitive dog-like forms did not appear until late in the period, in Europe; and true cats did not arrive until later, though they were represented by Eusmilus in the Upper Eocene of France. The primitive ungulates (Condylarths) were generalized forms with five effective toes, exemplified in Phenacodus. The gross Amblypoda, with five-toed stumpy feet (Coryphodon), were prominent in the early Eocene; particularly striking forms were the Dinoceratidae, Dinoceras, with three pairs of horns or protuberances on its massive skull and a pair of huge canine teeth projecting downwards; Tinoceras, Uintatherium, Loxophodon, &c.; these elephantine creatures, whose remains are so abundant in the Eocene deposits of western America, died out before the close of the period. The divergence of the hoofed mammals into the two prominent divisions, the odd-toed and even-toed, began in this period, but the former did not get beyond the three-toed stage. The least differentiated of the odd-toed group were the Lophiodonts: tapirs were foreshadowed by Systemodon and similar forms (Palaeotherium, Paloplotherium); the peccary-like Hyracotherium was a forerunner of the horse, Hyrochinus was a primitive rhinoceros. The evolution of the horse through such forms as Hyracotherium, Pachynolophus, Eohippus, &c., appears to have proceeded along parallel lines in Eurasia and America, but the true horse did not arrive until later. Ancestral deer were represented by Dichobune, Amphitragulus and others, while many small hog-like forms existed (Diplopus, Eohyus, Hyopotamus, Homacodon). The primitive stock of the camel group developed in N. America in late Eocene time and sent branches into S. America and Eurasia. The edentates were very generalized forms at this period (Ganodonta); the rodents (Tillodontia) attained a large size for members of this group, e.g. Tillotherium. The Insectivores had Eocene forerunners, and the Lemuroids—probable ancestors of the apes—were forms of great interest, Anaptomorphus, Microsyops, Heterohyus, Microchaerus, Coenopithecus; even the Cetaceans were well represented by Zeuglodon and others.
| Stages. | Paris Basin. | England. | Belgian Basin. | Mediterranean regions and Great Central sea. | Flysch Phase. | North America. | |
| Bartonien.[1] | Limestone of Saint-Ouen. Sands of Mortefontaine. Sands of Beauchamp. Sands of Auvers. | Barton beds. Upper Bagshot sands. | Sands of Lede. | Nummulitic limestones, sandstones and shales. | Upper part of the Alpine Flysch and Vienna and Carpathian sandstones. Macigno of the Apennines and Maritime Alps. | Unita Group and Jacksonian. | |
| Lutétien. | Calcaire grossier. | Bracklesham and Bournemouth beds. Lower Bagshot sands. | Laekenien. Bruxellien. Panisélien. | Bridger Group and Claibornian. | |||
| Yprésien. | Nummulitic sands of Soissons and Sands of Cuise and Aizy. | Alum Bay leaf beds. | Sands of Mons en Pévèle. Flanders Clay. | Wind River Group. Wasatch Group and | |||
| Landé- nien. | Sparn- acien. | Plastic Clay and lignite beds. | London Clay. Oldhaven beds. Woolwich and Reading beds. | Upper Landénien sands. Sands of Ostricourt. | Chickasawan. | ||
| Thane- tien. | Limestones of Rilly and Sézanne. Sands of Rilly and Bracheux. | Thanet sands. | Landénien tuffeau. Marls of Gelinden. | Torrejon Group and Midwayan. Puerco Group. | |||
The non-placental mammals although abundant were taking a secondary place; Didelphys, the primitive opossum, is noteworthy on account of its wide geographical range.
Among the birds, the large flightless forms, Eupterornis, Gastornis, were prominent, and many others were present, such as the ancestral forms of our modern gulls, albatrosses, herons, buzzards, eagles, owls, quails, plovers. Reptiles were poorly represented, with the exception of crocodilians, tortoises, turtles and some large snakes.
The flora of the Eocene period, although full of interest, does not convey the impression of newness that is afforded by the fauna of the period. The reason for this difference is this: the newer flora had been introduced and had developed to a considerable extent in the Cretaceous period, and there is no sharp break between the flora of the earlier and that of the later period; in both we find a mixed assemblage—what we should now regard as tropical palms, growing side by side with mild-temperate trees. Early Eocene plants in N. Europe, oaks, willows, chestnuts (Castanea), laurels, indicate a more temperate climate than existed in Middle Eocene when in the Isle of Wight, Hampshire and the adjacent portions of the continent, palms, figs, cinnamon flourished along with the cactus, magnolia, sequoia, cypress and ferns. The late Eocene flora of Europe was very similar to its descendant in modern Australasia.