The large predatory birds continued their domination into the Eocene (‘dawn of new’ life). Mammalian predators were represented by the creodont hyaenodontids (‘hyena teeth’). Considerably more successful than the related oxyaenids, the hyaenodontids survived into the Miocene. They were mostly medium dog-sized but Hyaenodon gigas stood about 1.4 metres (4.6 ft) at the shoulder.
In the Late Eocene the wolf-like mesonychids produced animals such as Andrewsarchus (‘Andrews + ruler’), a massive predator or scavenger. Only its skull has been found so far and from this it has been estimated that it stood 1.8 metres (6 ft) at the shoulder and was 3.7 metres (12 ft) long.
Herbivores were mostly small creatures during the Palaeocene but during the Eocene they increased in size and variety. The pantodonts (‘all teeth’) began as cat-sized in the Palaeocene but by Early Eocene they had become cow-sized herbivores and the largest land mammals of their time. Coryphodon (‘pointed teeth’) stood about one metre (3 ft) at the shoulder and was up to three metres (9 ft) long. The pantodonts survived until the Middle Eocene.
The Eocene saw the introduction of most of the modern mammalian orders, including bats, marsupials, primates, proboscideans (‘trunk feeders’), rodents and ungulates. Ungulates walk on the terminal bones of the toes and their hoofs are actually enlarged toenails. There are several groups of hoofed animals but the two main orders are artiodactyls (‘even toed’) and perissodactyls (‘odd toed’). In the former the animal’s weight is taken on the two middle toes, and the order includes camels, cows, deer, goats, hippopotamuses, pigs and sheep. With smaller forms such as sheep and goats the hoof can simply be cloven. With perissodactyls the animal’s weight is taken on the middle toe, and the order is divided into three families: horses, rhinoceroses and tapirs.
The earliest ancestor of modern horses, Hyracotherium (‘Hyrax-like beast’), associated with Eohippus (‘dawn horse’), was about the size of a terrier, had four hoofed toes on each front foot and three at the rear. It is believed to have been a forest browsing herbivore that ate primarily leaves but also fruits and nuts. The extra toe on the front feet disappeared during the Late Eocene with Mesohippus (‘middle horse’).
Living whales (cetaceans) are divided into two groups: odontocetes (‘toothed whales’), sperm whales, killer whales, dolphins and porpoises; and mysticetes (‘moustached whales’), which comprise the baleen whales. The mysticetes have rows of baleen plates in the upper jaw, looking like combs of thick hair, that trap krill and other food.
The earliest known proto-whale is Pakicetus (‘Pakistan whales’), from Early Eocene rocks in Pakistan. With their hoofed feet and long thick tails they must have looked somewhat like dogs. Their link to whales is based on the similarity in the structure of their ears. Their teeth resemble those of mesonychids but their skeletons relate to those artiodactyls that took to the water after splitting from the mesonychids. The implication is that the proto-whales were early artiodactyls that retained aspects of their mesonychid ancestry (such as triangular teeth) that modern artiodactyls have since lost.
Sirenia (siren) is an order of aquatic herbivorous placental mammals that include modern sea cows (manatees and dugongs) and their extinct relatives. The earliest known sea cows are found in the Early Eocene. By the close of the Eocene the sirenians had acquired their familiar streamlined body with the forelimbs modified as paddles, no hind limbs, and a horizontally flattened tail. After its discovery in 1741, Steller’s sea cow Hydrodamalis gigas (‘giant sea calf’) was hunted to extinction.
Hyraxes (‘shrewmice’) first appeared during the Eocene and for millions of years they were the primary terrestrial herbivore in Africa. There were many different species and the descendents of giant hyracoids evolved in a number of different ways. Some became smaller and gave rise to the modern hyrax family. Others appear to have taken to the water and ultimately gave rise to the elephant family and also perhaps to the sirenians. DNA evidence supports this hypothesis. The forelimbs of bats are developed as wings, making them the only mammals capable of sustained flight. Other mammals such as flying squirrels glide over short distances. Since bats are terrestrial and light-boned, there are few fossilised remains. Onychonycteris (‘clawed bat’) was unique among the bats in that it had claws on all five fingers (two or three is normal). Onychonycteris occurred alongside Icaronycteris (‘Icarus + nocturnal’), which was previously thought to be the most primitive known bat species.
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