This ocean, already diminished in area, retreated after Oligocene times from the Iranian plateau, Turkestan, Asia Minor and the region of the north-west Alps.
Fossil insects referable to the order have been found in Tertiary beds as old as the White River Oligocene of North America, and the Baltic amber, but nothing is known as to the previous history of the group.
Fossil remains of members of this family have also been found in Europe in strata of the Oligocene period.
From the Oligocene deposits of France and southern England have been obtained numerous remains of opossums referable to the American family Didelphyidae.
In this valley were laid down, either in Eocene or Oligocene times, a great series of lake beds and thick accumulations of brown coal.
The Baltic amber or succinite is found as irregular nodules in a marine glauconitic sand, known as "blue earth," occurring in the Lower Oligocene strata of Samland in East Prussia, where it is now systematically mined.
Oligocene Elornis and, allied, Palaelodus.
Coming to the Tertiary we find the Oligocene beds of Aix, of east Prussia (amber) and of Colorado, and the Miocene of Bavaria, especially rich in remains of beetles, most of which can be referred to existing genera.
Oligocene, quite similar to that of N.
Ancient animal), a name applied by Cuvier to the remains of ungulate mammals recalling tapirs in general appearance, from the Lower Oligocene gypsum quarries of Paris.
Numerous fossil insects preserved in the amber of the Baltic Oligocene have been described by G.
Mayr and others, while Scudder has studied the rich Oligocene faunas of Colorado (Florissant) and Wyoming (Green River).
Remains of spiders from the Baltic amber beds of Oligocene age and from nearly coeval fluviatile or lacustrine deposits of North America belong to forms identical with or closely related to existing genera, thus proving the great antiquity of our present spider fauna.
6 a Oligocene of both hemispheres appears Protapirus, which ranges well into the Miocene, and is essentially a tapir, having lost the third lobe of the last lower molar, and being in process of acquiring molar-like upper premolars, although none of these teeth have two complete inner columns.
Finally, in the Oligocene Colodon the last three upper premolars are like the molars, and the first pair of lower incisors is lost.
The most primitive group is that of the American Hyracodontidae, !represented in the Oligocene by Hyrachyus, Hyracodon and Triplopus.
In the Lower Oligocene of Europe we have Ronzotherium and in that of America Leptaceratherium (Trigonias), which were primitive species with persistent upper canines and three-toed fore-feet.
Possibly they belonged to the Amynodontidae, but they may have been related to the Upper Oligocene Diceratherium, in which the nasal bones formed a transverse pair; this genus being common to Europe and North America.
Hornless rhinoceroses, with five fronttoes, ranging from the Oligocene to the Lower Pliocene in Europe, represent the genus Aceratherium, which may also occur in America, as it certainly does in India.
The latter, like the similar deposits in other West Indian islands, is probably of Oligocene age.
The shells which have been found in them indicate that they belong for the most part to the Oligocene period.
No noteworthy fossil spiders are known; the best-preserved are in amber of Oligocene age.
The flat summit is formed by a succession of limestones - all deposited in shallow water - from the Eocene (or Oligocene) up to recent deposits in the above-mentioned atoll with islands on its reef.
The geological sequence of events appears to have been the following: - After the deposition of the Eocene (or Oligocene) limestone - which reposes upon a floor of basalts and trachytes - basalts and basic tuffs were ejected, over which, during a period of very slow depression, orbitoidal limestones of Miocene age - which seem to make up the great mass of the island - were deposited; then elapsed a long period of rest, during which the atoll condition existed and the guano deposit was formed; from then down to the present time there has succeeded a series of sea-level subsidences, resulting in the formation of the terraces and the accummulation of the detritus now seen on the first inland cliff, the old submarine slope of the island.
Jukes-Browne and Harrison ascribe the Scotland beds to the Eocene or Oligocene period, the Oceanic series to the Miocene, the Bissex Hill marls to the Pliocene, and the coral limestones partly to the Pliocene and partly to the Pleistocene.
The Oligocene and Miocene formations are present, but the Upper Miocene is confined to the coast.
Geologically they are known to date back to the Oligocene period, and wings believed to be referable to them have been found in Liassic and Jurassic beds.
The Lower Limestone probably belongs to the Tongarian stage of the Oligocene series, and the Upper Coralline Limestone to the Tortonian stage of the Miocene.
- Very little is known of the history of the Hymenoptera previous to the Tertiary epoch, early in which, as we know from the evidence of many Oligocene and Miocene fossils, all the more important families had been differentiated.
It includes the two families Anoplotheriidae and Dichobunidae, of which the first died out with the Oligocene, while the second may have given origin to the Tragulina and perhaps the Pecora.
The most interesting genera are, however, the Upper Oligocene and Lower Miocene Gelocus and Prodremotherium, which have perfectly selenodont teeth, and the third and fourth metacarpal and metatarsal bones respectively fused into an imperfect cannon-bone, with the reduction of the lateral metacarpals and metatarsals to mere remnants of their upper and lower extremities.
The genus has a very wide distribution, extending from Europe through Asia to North America, and occurring in strata which are of Oligocene and Miocene age.
In the above genera, so far as is known, the feet were four-toed, although with the lateral digits relatively small; but in Elotherium (or Entelodon), from the Lower Miocene of Europe and the Oligocene of North America, the two lateral digits in each foot had disappeared.
Tapirs are an ancient group with many of the original characters of the primitive Ungulates of the Oligocene period, and have undergone but little change since the Miocene.
There is no proof of their having lived in the Oligocene epoch, but in deposits of Miocene and Pliocene date remains undistinguishable generically and perhaps specifically from the modern tapirs (though named priscus, T.
The Cainozoic system is represented by Oligocene, Miocene, Pliocene and Pleistocene beds.
The best-known Oligocene rocks.
In southern Otago the Oligocene beds are brown coals and lignites with oil shales, which, at Orepuki, contain 47% of oil and gas, with 8% of water.
The Cainozoic volcanic history of New Zealand begins in the Oligocene, when the high volcanic domes of Dunedin and Banks Peninsula were built up. The Dunedin lavas including tephrites and kenytes correspond to the dacite eruptions in the volcanic history of Victoria.
The Eocene beds are nummulitic. There is a lacustrine group which has usually been placed in the Lower Eocene, but the discovery of Anthracotherium magnum in the interbedded lignites proves it to be Oligocene, in part at least.
The Oligocene period consists of a marine phase confined to the littoral zone of Kabylia, and of a continental phase occupying vast areas composed of lacustrine, alluvial, gypsiferous marls, sandstones and conglomerates.
Rhineura of Florida, and also known from the Oligocene of South Dakota; Lepidosternum of South America; and Anops in America and Africa; Blanus cinereus, Mediterranean countries.
The Oligocene lake basin of Florissant, Colorado, has been reconstructed similarly by Samuel Hubbard Scudder and T.
Oligocene (White river formation).
For example, the famous bone-beds of the Oligocene of South Dakota have been analysed by W.
The classification of the Eocene (and Oligocene) formations in the Gulf region, especially east of the Mississippi, is as follows:
This classification is based almost wholly on the fossils, for there seems to be little physical reason for the differentiation of the Oligocene anywhere on the continent.
The oil of Texas and Louisiana is from the Miocene (or possibly Oligocene) dolomite.
In the European Miocene we have Hyotherium and Palaeochoerus, and in the Upper Oligocene Propalaeochoerus, which have square molars without any tendency to a selenodont structure in their cusps.
Choeropotamus is a European Oligocene genus with bunodont molars which show a conspicuous basal cingulum in the lower dentition; the first premolar is absent.