At the start of the Oligocene (‘little new’ life) glaciers formed in Antarctica for the first time. The world became cooler and more seasonal. Many animals better suited to the more tropical Eocene world became extinct. The subtropical forests gave way to woodlands. Grasses, which had appeared for the first time at water margins in the Eocene, began to occupy open habitats. By the Miocene the subtropical woodlands had been replaced by savannah.
During the Eocene while the perissodactyls diversified and got bigger the artiodactyls had remained small. During the Oligocene the artiodactyls adapted to the change from perennial to seasonal fruit and vegetation and became bigger and more specialised as herbivores.
The first ruminants (deer, goats, sheep) appeared. These animals pass food into a fore-stomach, regurgitate it (‘chew the cud’) and swallow it again. This recycling digestive process extracts all the nourishment from plant food, usually grass. With their superior ability to digest fibrous food, the ruminants spread and diversified to become the dominant large herbivores.
Many animals in the Oligocene belonged to orders that still exist, but of families now extinct. The large browser Arsinoitherium (the discovery was made near the palace of the Ptolemaic queen Arsinoe) superficially resembled a rhinoceros. It stood 1.8 metres (6 ft) at the shoulder and was about three metres (10 ft) long. Actually related to the elephant it seems to have been the broad equivalent of the uintatheres.
Entelodonts (‘perfect teeth’), often called ‘giant pigs’ though they are not closely related, were very abundant in Oligocene times. Daeodon, the largest, stood 2.1 metres (7 ft) at the shoulder and was three metres (10 ft) long. Oreodonts (‘mountain teeth’) were sheep-sized cud-chewing plant eaters.
Titanotheres (‘Titan + beasts’) began as small pig-like creatures in the Eocene. By the Palaeocene they had developed into animals such as Brontotherium (‘thunder beast’), also a rhinoceros-like browser but related to horses. This animal was 2.5 metres (8 ft) at the shoulder.
Rhinoceroses seem to have developed from small tapir-like Eocene animals such as Hyrachyus. During the Oligocene appeared some of the largest land-living animals known, the very tall, hornless rhinoceroses or indricotheres (‘Indrik + beasts’); Indricotherium was five metres (16 ft) tall.
Camels were very common in North America but they were slender and diminutive compared to other ancient and modern camelids. Stenomylus (‘narrow molar’) stood only 61 cm (2 ft) tall on average. It was a slender animal with a long neck, having some resemblance to a modern gazelle.
The huge bear-like and dog-like mesonychids disappeared and were replaced by the recognizable ancestors of modern dogs and cats. The 80 cm (2.6 ft) long Hesperocyon (‘western dog’) looked more like a small raccoon than a canine. All modern carnivores, including cats, evolved from miacoids, small marten-like carnivores from the Palaeocene and Eocene epochs. Proailurus (‘first cat’) was a little larger than the domestic cat and weighed about 9 kg (20 lb). Hoplophoneus (‘armed murderer’) is sometimes referred to as sabre-toothed cat, but it was not a true cat.
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