During the Eocene, plants and marine faunas became quite modern. Many modern
bird orders first appeared in the Eocene. The Eocene oceans were warm and teeming with
fish and other sea life.
Vertebrates Mammals The oldest known
fossils of most of the modern mammal orders appear within a
brief period during the early Eocene. At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like
artiodactyls,
perissodactyls, and
primates, had features like long, thin
legs, feet, and
hands capable of grasping, as well as differentiated
teeth adapted for chewing.
Dwarf forms reigned. All the members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat. Rodents were widespread. East Asian rodent faunas declined in diversity when they shifted from ctenodactyloid-dominant to cricetid–dipodid-dominant after the MECO. Both groups of modern
ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like
Mesonyx. Early forms of many other modern mammalian orders appeared, including
horses (most notably the
Eohippus),
bats,
proboscidians (elephants), primates, and
rodents. Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe,
Patagonia,
Egypt, and
southeast Asia. Marine fauna are best known from
South Asia and the
southeast United States. After the Paleocene–Eocene Thermal Maximum, members of the
Equoidea arose in North America and Europe, giving rise to some of the earliest
equids such as
Sifrhippus and basal European equoids such as the
palaeothere Hyracotherium. Some of the later equoids were especially species-rich;
Palaeotherium, ranging from small to very large in size, is known from as many as 16 species. Established large-sized mammals of the Eocene include the
Uintatherium,
Arsinoitherium, and
brontotheres, in which the former two, unlike the latter, did not belong to ungulates but groups that became extinct shortly after their establishments. Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like
Hyaenodon and
Daphoenus (the earliest lineage of a once-successful predatory family known as
bear dogs).
Entelodonts meanwhile established themselves as some of the largest omnivores. The first
nimravids, including
Dinictis, established themselves as amongst the first
feliforms to appear. Their groups became highly successful and continued to live past the Eocene.
Basilosaurus is a well-known Eocene
whale, but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in
cetaceans occurred. The first
sirenians were evolving at this time, and would eventually evolve into the extant
manatees and
dugongs. File:Dinoceras mirabile Marsh MNHN.jpg|Cast of skull of
Uintatherium anceps, a dinoceratan File:Andrewsarchus09DB.jpg|Reconstruction of
Andrewsarchus, an artiodactyl File:Basilosaurus isis fossil, Nantes History Museum 03.jpg|
Basilosaurus, a whale File:Pakicetus Canada.jpg|
Pakicetus, an amphibious cetacean File:Moeritherium lyonsi (fossil mammal) (Eocene) (32167459460).jpg|
Moeritherium, a basal proboscidean File:Brontotherium skull IMG 4441.jpg|
Megacerops, a brontotheriid File:Hyracodon nebraskensis.jpg|
Hyracodon, a perissodactyl File:Leptictidium auderiense skeleton.JPG|
Leptictidium, a small mammal, likely bipedal File:Peratherium skull.jpg|
Peratherium, a herpetotheriid File:Hesperocyon skull Smithsonian.jpg|
Hesperocyon, a canid
Birds Eocene birds include some enigmatic groups with resemblances to modern forms, some of which continued from the Paleocene. Bird taxa of the Eocene include carnivorous
psittaciforms, such as
Messelasturidae,
Halcyornithidae, large flightless forms such as
Gastornis and
Eleutherornis, long legged falcon
Masillaraptor, ancient
galliforms such as
Gallinuloides,
putative rail relatives of the family
Songziidae,
various pseudotooth birds such as
Gigantornis, the
ibis relative
Rhynchaeites, primitive swifts of the genus
Aegialornis, and primitive penguins such as
Archaeospheniscus and
Inkayacu. Many Eocene birds in
Central Europe evolved tuberculate vertebrae as an adaptation against predation, with flightless birds facing low predation pressure during this time as a result. File:Primobucco mcgrewi USNM PAL 336284 img1.jpg|
Primobucco, an early relative of the
roller File:Pseudocrypturus Smithsonian fossil.jpg|
Pseudocrypturus, Early Eocene, Wyoming File:Diatrymaskeleton.JPG|
Gastornis, a flightless bird
Fishes Fishes, both
Chondrichthyes such as sharks and rays, and
Osteichthyes (bony fishes), are abundant in the
London Clay. File:Carcharocles sokolowi teeth.png|Teeth of
Otodus sokolovi, an otodontid shark File:Xiphodolamia.jpg|Teeth of
Xiphodolamia. a mackerel shark,
London Clay File:Heliobatis radians with two Knightia eocaena (3bce1f6e-f693-43d1-aa0f-67ba7fc50895).tif|
Heliobatis and
Knightia File:FOS655.jpg|Eel, cf.
Echelus branchialis, collected by
Louis Agassiz File:FOS794.jpg|Vertebrae of a
lamniform shark, London Clay,
Isle of Sheppey Reptiles Reptile fossils from this time, such as fossils of
pythons and
turtles, are abundant. File:Turtle with crocodile bite marks, Eocene, Green River Formation, Kemmerer, Lincoln County, Wyoming, USA - Houston Museum of Natural Science - DSC02006.JPG|
Hummelichelys with crocodile bite marks File:Borealosuchus wilsoni (15529256785).jpg|
Borealosuchus, a crocodyliform
Molluscs File:Cerithium giganteum, snails, Middle Eocene, Verona, Italy - Houston Museum of Natural Science - DSC01968.jpg|
Cerithium shells
Arthropods Several rich fossil insect faunas are known from the Eocene, notably the
Baltic amber found mainly along the south coast of the
Baltic Sea, amber from the
Paris Basin, France, the
Fur Formation,
Denmark, and the
Bembridge Marls from the
Isle of Wight, England. Insects found in Eocene deposits mostly belong to genera that exist today, though their range has often shifted since the Eocene. For instance the
bibionid genus
Plecia is common in fossil faunas from presently temperate areas, but only lives in the tropics and subtropics today. Platypleurin cicadas diversified during the Eocene. Ostracods flourished in the oceans. File:Harpactocarinus punctulatus, crab, Eocene, Rialo Formation, Monte Baldo Quarry, Verona, Italy - Houston Museum of Natural Science - DSC01954.JPG|
Harpactocarcinus, a crab File:Palaeoncoderes eocenicus L. PITON et N. THEOBALD 1937 Holotype.jpg|
Palaeoncoderes, a
beetle Other phyla File:Echinolampas ovalis M Eocene Civrac-en-Médoc France.JPG|
Echinolampas, a
sea urchin Microbes Calcareous nannoplankton were a prominent feature of Eocene marine ecosystems. == See also ==