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American Association for the Advancement of Science

Evolution of the gigantic mammals

The largest land mammals that ever lived, Indricotherium and Deinotherium, would have towered over the living African elephant. Indricotherium, which lived from about 37 to 23 million years ago, weighed in at 15,000 kg, and Deinotherium, which lived 8.5 to 2.7 million years ago, reached as much as 17,000 kg.
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New research helps explain how mammals around the world evolved to huge sizes after the dinosaurs went extinct.

For the first 140 million years of their evolutionary history, mammals were small, from around 3 grams to 15 kilograms. That's roughly larger than a baby mouse and smaller than a medium-size dog.

After the end-Cretaceous extinction approximately 65 million years ago, when the dinosaurs disappeared, mammals exploded in both size and ecological range. For example, the prehistoric elephant ancestors known as Deinotherium were as much as 17,000 kilograms (almost 40,000 pounds)!

Felisa Smith of the University of New Mexico and colleagues compiled fossil data indicating the body size of land mammals belonging to each taxonomic order, on each continent, throughout their evolutionary history.

Their results show that overall mammal size increased rapidly then leveled off after about 25 million years. This pattern occurred on most of the continents, although data are sparse for South America.

The authors also tested several different hypotheses for the evolution of huge body size. The most likely explanation seems to be that the larger mammals had an advantage because they filled in the ecological niches left empty by the dinosaurs. An ecological niche refers to the various ways that one group of organisms relates to all the other organisms it lives with, for example, what it eats and what eats it.

The limits that stopped these huge mammals from growing even larger seem to have been relatively warm temperatures and a lack of available land.

This research appears in the 26 November issue of the journal Science.