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 fluvio-marine deposits of the Upper Eocene and Oligocene formations contain an interesting mammalian fauna, proving that the African continent formed a centre of radiation for the mammalia in early Tertiary times.
The Eocene and Oligocene form a broad belt along the northern coast, very much more continuous than the Mesozoic band, and from this belt a branch extends southwards to Sciacca.
The Eoccne system is unknown in Germany except in the foothills of the Alps; but the Oligocene and Miocene are widely spread, especially in the great plain and in the depression of the Danube.
The Oligocene forms a band stretching from Antwerp to Maastricht, and this is followed towards the north by a discontinuous strip of Miocene and a fairly extensive area of Pliocene.
Except in Limburg, where, in the neighbourhood of Maastricht, the upper layers of the chalk are exposed and followed by Oligocene and Miocene beds, the whole of Holland is covered by recent deposits of considerable thickness, beneath which deep borings have revealed the existence of Pliocene beds similar to the " Crags " of East Anglia.
The Molasse, in the neighbourhood of the mountains, consists chiefly of conglomerates and sandstones, and the Flysch consists of sandstones and shales; but the Molasse is of Miocene and Oligocene age, while the Flysch is mainly Eocene.
Remains of several of the existing genera have been found in Oligocene and later beds of Europe, Sumatra and North America.
A more advanced phase is represented in the European Lower Oligocene by the Pseudosciuridae, with the genera Pseudosciurus, Sciuroides, Trechomys, Theridomys, &c., in which part of the masseter passes through the broad infra-orbital canal, and the premolars are; the molars being low-crowned, many-rooted and either cusped or ridged.
In the highly specialized mastoid region of the skull, the North American Oligocene Protoptychus approaches to Dipopodomys, while the contemporary Gymnoptychus and Entoptychus likewise appear referable to the Geomyidae.
The Upper Oligocene Cricetodon in Europe and Eumys in America are the earliest known forerunners of the cricetine Muridae; while at the same time primitive beavers appear in the form of Steneofiber, to be succeeded in the European Pleistocene by the gigantic Trogontherium.
In contradistinction to Titanomys, in which the cheek-teeth are rooted, is the North American Upper Oligocene Palaeolagus, where they are rootless.