Interval 3

Too Much of a Mouthful

The success of the grasses lies partly in the their morphology and growth processes, and partly in the diversity of ways that they generate nutrients through photosynthesis. Grasses can be divided into two physiological groups, using two different photosynthetic pathways for carbon fixation, known as C3 and C4. The C4 grasses have a specialized leaf anatomy that particularly adapts them to hot climates and an atmosphere low in carbon dioxide. This is because under conditions of drought, high temperatures and nitrogen or carbon dioxide limitation, 97% of the water taken up by C3 plants is lost through the leaves. In C4 plants this proportion is much lower, giving them an evolutionary edge in a dry environment. The first C4 plants evolved around 25 to 32 million years ago during the Oligocene, probably in response to the development of more open habitat with higher levels of sunlight. By 6 to 7 million years ago, the increasing aridity of the Miocene meant that C4 grasses, with their increased resistance to water stress, were at an advantage compared with C3 grasses. But C4 grasses were also much richer in Silica, making them tougher to eat and digest.

A Different View of Horse Evolution

Traditional views of horse evolution have tended to concentrate on a linear process, from the small, four-toed Hyracotherium to the large, one-toed Equus. However, there is an equally compelling argument that the process involved a series of adaptive radiations of species, with the crucial stage being driven by climate and vegetation during the Miocene. At the end of the Oligocene, horses were still primarily browsers. They had low crowned teeth, suitable for eating leaves and fruit, but there is evidence from microwear studies that they may have started to include some grass in their diet as the open woodland of the Oligocene was gradually replaced by true grassland. Fast-forward to 18 million years ago, and two very different radiations of horses are present. There are still browsing horses, represented in North America by Hypohippus and Archaeohippus. But now there are also true grazing horses, such as Merychippus, exploiting the expanding grasslands. Merychippus has wide molars with flat grinding surfaces and a tough layer of cementum to help cope with a diet of abrasive grasses. By 10 million years ago the browsing horses are all extinct in North America. Now there is a radiation of 8 genera of specialist grazers. But another wave of extinctions towards the end of the Miocene, 6 million years ago, perhaps associated with the spread of C4 left only a single surviving lineage, the ancestor of our modern horses and zebras.

Coping with Grass

Grass is so abundant today, and supports such large communities of herbivores, that it’s easy to forget what a challenging diet it represents. It’s tough and abrasive – difficult to chew and difficult to digest effectively. Herbivorous mammals cope with grass in a variety of ways. The first is to develop molars and premolars that are capable of withstanding the continuous wear caused by chewing grass. These molars tend to be tall, with enamel that extends well past the gum line, and with hard ridges of enamel exposed on their flat grinding surfaces. These are known as hypsodont teeth and they can be found in almost all mammals that feed on grass. The second is to modify the digestive system. All mammals that eat plants have a flora of microbes that live in their guts – these bacteria release enzymes called cellulases, which break down cellulose and release energy. Plants are rich in cellulose and without the presence of these microbes animals would be unable to digest them effectively. Digesting large volumes of plant material requires large quantities of microbes; herbivorous mammals have evolved to house these microbes by turning their guts into giant fermentation vats. Grass is particularly challenging. One group of mammals, the artiodactyls, have evolved to successfully exploit this resource by developing a multi-chambered stomach, through which food can be passed more than once. This allows for more efficient digestion and a greater range of body sizes. Artiodactyls that do this are called ruminants. By contrast, perissodactyls never evolved this ability and are still dependent on relatively inefficient fermentation in the gut. This may be one reason why today artiodactyls are far more diverse and abundant than perissodactyls (224 species vs. 16 species).

Teleoceras and Aphelops: Two Very Different Rhinos

Rhinos in the modern African savannah exist as a browser-grazer species pair; the white rhinos of the genus Ceratotherium are wide-mouthed grazers, whereas the black rhino Diceros is a browser that uses its beak-shaped lip to pluck leaves and twigs. In Miocene North America there was a similar species-pairing, although the two rhinos concerned were only distant relatives of the modern African species. The aceratherine rhino Aphelops was the browser. The greatly-retracted nasal incision in its skull is evidence for the large muscles needed to operate a prehensile upper lip, and instead of upper incisors it has a horny nipping pad, similar to that of modern rhinos, to help pluck leaves and shoots. By contrast, Teleoceras, a short-legged, hippo-like animal, has high-crowned teeth suitable for grazing and a huge, barrel-shaped body, presumably to support a super-sized gut for digesting tough grasses. Both types of rhino were abundant in the Miocene, but by the early Pliocene both were extinct and rhinos had vanished from North America.

 

Left: Skeleton of the Miocene browsing rhino Aphelops. AMNH. Right: Mounted skeleton of the Miocene rhino Teleoceras. AMNH.