Middle[2] Bridger Group, 2000 ft. (? = Claibornian) _Uintatherium.__Encyclopaedia Britannica, 11th Edition, "English Language" to "Epsom Salts" Volume 9, Slice 6

Lower [3] Wind River Group, 800 ft. _Bathyopsis._

[4] Wasatch Group, 2000 ft. (? = Chickasawan) _Coryphodon._

Basal [5] Torrejon Group, 300 ft. _Pantolambda._

[6] Puerco Group, 500 to 1000 ft. _Polymastodon._

[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.

+--------------+-------------------------+----------------------+-------------------+---------------+------------------+------------------+

| | | | | sea. | | |

+--------------+-------------------------+----------------------+-------------------+---------------+------------------+------------------+

+--------------+-------------------------+----------------------+-------------------+ | | |

+--------------+-------------------------+----------------------+-------------------+ | Alpine Flysch | |

+---+----------+ | +-------------------+ and shales. | | |

| n | | Limestones of Rilly and | | | | | and |

| | | | | | | | |

+---+----------+-------------------------+----------------------+-------------------+---------------+------------------+------------------+

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.

See A. de Lapparent, _Traite de geologie_, vol. iii. (5th ed., 1906),

which contains a good general account of the period, with numerous

references to original papers. Also R.B. Newton, _Systematic List of

the Frederick E. Edwards Collection of British Oligocene and Eocene

Mollusca in the British Museum_ (_Natural History_) (1891), pp.

299-325; G.D. Harris, "A Revision of our Lower Eocenes," _Proc.

Geologists' Assoc._ x., 1887-1888; W.B. Clark, "Correlation Papers:

Eocene" (1891), _U.S. Geol. Survey Bull. No. 83._ For more recent

literature consult _Geological Literature added to the Geological

Society's Library_, published annually by the society. (J. A. H.)

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