Giant Mammals

here were no big mammals while the dinosaurs lived but many mammals of elephant or rhinoceros size evolved during the Ceno-

The best known of the giant extinct mammals were the mastodons and mammoths. Mastodons are primitive elephants, distinguished by their simple teeth. Mammoths are much more like modern elephants. Both survived to overlap in time with humans, and many cave paintings of mammoths have been found.

The biggest mammoth species [Mammuthus imperator of North America) stood 4 meters tall at the shoulder. Very large (6 tonne) African elephants are only 3.3 meters tall. The mammouth, 1.2 times as tall, must have been about 1.23 times as heavy, about 10 tonnes. Mastodons were less tall but had relatively longer bodies.

Modern elephants live in hot places where they seem to have trouble keeping cool, but many mammoths lived in temperate or even cold places. Cave paintings in France and Spain show mammoths with long hair, which they may have needed for warmth during the Ice Ages. Frozen carcases of Mammuthus primigenius have been found embedded in ice in Siberia: the mammoths seem to have fallen into crevasses, died and frozen, and to have remained frozen until they were found. They have long black hair, just as shown in the cave paintings, and they also have an 8-centimeter layer of fat under the skin. Both the hair and the fat may have been useful as heat insulation. Modern elephants can maintain their body temperatures in the warm climates of Africa and India, and also in zoos in temperate countries, but mammoths inhabited much colder environments and probably needed extra insulation.

Most extinct mammals, including the mammoths, seem unspectacular in comparison with dinosaurs. The saber-tooths [Smilodon], which zoic era.

preyed on mammoths, were impressive, but they were only about the size of modern lions. The giant Irish deer [Megaloceros] was smaller in the body than a moose though its antlers grew to a span of 3.5 meters. The giant ground sloth [Megatherium) of South America was enormously larger than any modern sloth but its estimated mass (3 tonnes) is no more than that of the biggest modern rhinoceros.

There are various other extinct mammals of modern rhinoceros size, and just one that is enormously larger. It is Indricotherium (formerly called Baluchitherium), a hornless rhinoceros from Mongolia. It is hard to be sure of its maximum size because no complete skeleton has been found, only odd bones from specimens of various sizes. Only two neck vertebrae and a foot bone (a metacarpal) seem to come from the biggest size of skeleton. These three bones have been drawn to scale in figure 12.1, and the rest have been scaled up from smaller skeletons. The sizes of some ribs and vertebrae have had to be guessed because no specimens were found of those particular bones. Thus the evidence for the size of the animal in figure 12.1 is shaky: a small error of judgment could have made it badly wrong. Since no better evidence is available, I will assume that the figure is accurate. It shows an animal 5.3 meters tall, much taller than any elephant.

Indricotherium Skeleton Reconstruction
FIGURE 12.1. A reconstruction of the largest Indricotherium. From Granger and Gregory (1935).

The scientists responsible for the picture estimated the mass of the animal to be 20 tonnes, but I think it may have been even more. The head and body (excluding the tail) are 9.2 meters long, measured along the curve of the back. The same measurement in 0.75-tonne African buffalo is 2.6 meters. The Indricotherium has a body of roughly buffalo-like shape, so if it ws (9.2/2.6) times as long as the buffalo it was (9.2/2.6)1 times as heavy: about 34 tonnes. I have tried calculating its mass in other ways and obtained even larger estimates. If the restoration is accurate, Indricotherium had about the same mass as Apa-tosaurus (figure 1.7).

When we discussed the heat balance of large dinosaurs (chapter 7) we were uncertain whether they had reptile-like or mammal-like metabolism. We concluded that a large sauropod with mammal-like metabolism would have to evaporate a lot of water to avoid overheating in hot climates. Indricotherium is obviously a mammal and presumably had mammal-like metabolism, but the climate in Mongolia must have been reasonably cool in its time, as it is now. The continents had by then reached their present positions on the earth's surface.

Indricotherium is the only land-living mammal known to have grown to the size of large sauropods, but there are bigger mammals in the sea. The Blue whale [Balaeonoptera musculus) grows to an average adult mass of about 100 tonnes and is the biggest animal known to have lived, at any time. Its numbers have been very seriously reduced by whaling but, happily, it survives, so it needs no further discussion in this book on extinct giants.

Principal Sources

Savage and Long (1986) describe the extinct groups of mammals. Economos (1981) discusses Indricotherium. Granger and Gregory (1936) is the source for figure 12.1.

Economos, A. C. 1981. The largest land mammal, journal of Theoretical Biology 89:211-215.

Granger, W. and W. K. Gregory. 1935. A revised restoration of the skeleton of Balu-chitherium, gigantic fossil rhinoceros of Central Asis. American Museum Novi-tates 787:1-3.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide.

London: British Museum (Natural History).

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