'' (far left) and other animals from the
Turtle Cove Formation The early Eocene climate was very warm, with
crocodilians and temperate plants thriving north of the
Arctic Circle. The cooling trend that began in the middle Eocene continued into the Oligocene, bringing both poles well below freezing for the first time in the
Phanerozoic. The cooling climate, together with the opening of some land bridges and the closing of others, led to a profound reorganization of the biosphere and loss of taxonomic diversity. Land animals and marine organisms reached a Phanerozoic low in diversity by the late Oligocene, and the temperate forests and jungles of the Eocene were replaced by forest and scrubland. The closing of the Tethys Seaway destroyed its tropical biota.
Flora The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving
Nothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions.
Angiosperms continued their expansion throughout the world as tropical and sub-
tropical forests were replaced by
temperate deciduous forests. Open
plains and
deserts became more common and
grasses expanded from their water-bank habitat in the Eocene moving out into open tracts. The decline in pCO2 favored
C4 photosynthesis, which is found only in angiosperms and is particularly characteristic of grasses. However, even at the end of the period, grass was not quite common enough for modern
savannas. In North America, much of the dense forest was replaced by patchy scrubland with riparian forests. and
lychee trees present, and temperate woody plants such as
roses,
beeches, and
pines were common. The
legumes spread, while
sedges and
ferns continued their ascent. In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity. In
Pakistan, the flora consisted mainly of dry but dense forests. In northern China, there was a progressive ascendance of open, grassy environments. The Ha Long megafossil flora from the Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is now
Vietnam was very similar to its present flora. Kelps make their first appearance in the fossil record during the earliest Oligocene.
Fauna restored next to Hyaenodon'' Most extant mammal families had appeared by the end of the Oligocene. These included primitive three-toed horses, rhinoceroses, camels, deer, and peccaries. Carnivores such as
dogs,
nimravids, bears, weasels, and raccoons began to replace the
creodonts that had dominated the Paleocene in the Old World. Rodents and rabbits underwent tremendous diversification due to the increase in suitable habitats for ground-dwelling seed eaters, as habitats for squirrel-like nut- and fruit-eaters diminished. The primates, once present in Eurasia, were reduced in range to Africa and South America. Many groups, such as
equids,
entelodonts,
rhinos,
merycoidodonts, and
camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded.
Brontotheres died out in the Earliest Oligocene, and
creodonts died out outside
Africa and the
Middle East at the end of the period.
Multituberculates, an ancient lineage of primitive mammals that originated back in the
Jurassic, also became extinct in the Oligocene, aside from the
gondwanatheres. The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure. The lowering of sea levels closed the Turgai Strait across the Obik Sea, which had previously separated Asia from Europe. This allowed Asian mammals, such as
rhinoceroses and
ruminants, to enter Europe and drive endemic species to extinction. Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene. There was significant diversification of mammals in Eurasia, including the giant
indricotheres, that grew up to at the shoulder and weighed up to 20 tons.
Paraceratherium was one of the largest land mammals ever to walk the Earth. However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts. The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia. The first
felid,
Proailurus, originated in Asia during the late Oligocene and spread to Europe. '' There was only limited migration between Asia and North America. The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover of
gastropods,
amphibians, and
reptiles. Mammals were much less affected. Merycoidodonts, an endemic American group, were very diverse during this time. '' is an early fossil
catarrhine that predates the divergence between
hominoids (
apes) and
Old World monkeys Australia and South America became geographically isolated and developed their own distinctive endemic fauna. These included the New World and Old World monkeys. The South American continent was home to animals such as
pyrotheres and
astrapotheres, as well as
litopterns and
notoungulates.
Sebecosuchians,
terror birds, and carnivorous
metatheres, like the
borhyaenids remained the dominant predators. Africa was also relatively isolated and retained its endemic fauna. These included
mastodonts, hyraxes, arsinoitheres, and other archaic forms.
Egypt in the Oligocene was an environment of lush forested deltas. Nevertheless, the Early Oligocene saw a major reduction in the diversity of many Afro-Arabian mammal clades, including hyaenodonts, primates, and hystricognath and anomaluroid rodents. During the Oligocene, the Tethyan marine biodiversity hotspot collapsed as the Tethys Ocean contracted. The seas around Southeast Asia and Australia became the new dominant hotspot of marine biodiversity. At sea, 97% of marine snail species, 89% of clams, and 50% of echinoderms of the Gulf Coast did not survive past the earliest Oligocene. New species evolved, but the overall diversity diminished. Cold-water mollusks migrated around the Pacific Rim from Alaska and Siberia. The marine animals of Oligocene oceans resembled today's fauna, such as the
bivalves. Calcareous
cirratulids appeared in the Oligocene. The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into the
Indo-Pacific. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The
baleen whales and
toothed whales had just appeared, and their ancestors, the
archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the
requiem sharks, which also appeared in this epoch. Early
desmostylians, like
Behemotops, are known from the Oligocene.
Pinnipeds appeared near the end of the epoch from an
otter-like ancestor. File:NMNH-USNMV15917Poebrotherium.jpg|
Poebrotherium File:Merycoidodon Skull Oligocene Left Side.jpg|
Merycoidodon File:Hoplophoneus primaevus (fossil false sabertooth cat) (Middle Oligocene; Nebraska, USA) 3 (32791323412).jpg|
Hoplophoneus File:Mesohippus barbouri Harvard.jpg|
Mesohippus File:Paraceratherium transouralicum skull.jpg|
Paraceratherium File:Paleoparadoxia Natural History Museum of Los Angeles County 20110330.jpg|
Paleoparadoxia File:Protoceras skeleton.jpg|
Protoceras File:Archaeotherium mortoni.JPG|
Archaeotherium File:Janjucetus hunderi skull.jpg|
Janjucetus ==Oceans==