Land mammals kept getting larger for 35 million years after the dinosaurs were wiped off the planet, then hit a plateau of 15 tonnes around 30 million years ago.
The first comprehensive study to compare the maximum size of fossils around the world shows how the extinction triggered a growth spurt in the mammals that were left to take over the continents. It reveals that land mammals around the world responded the same way to the death of the dinosaurs 65 million years ago.
There has been a long-running debate over how mammals grew from the tiny shrew-like creatures that hid from dinosaurs to the much larger sizes of recently extinct behemoths such as the woolly mammoth. A decade ago, John Alroy, now at the University of California, Santa Barbara, reported that North American mammals had grown steadily larger for 65 million years after the mass extinction that marked the end of the dinosaurs' reign.
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Now Felisa Smith of the University of New Mexico, Albuquerque, and 19 colleagues have looked at mammalian fossils from Africa, Eurasia and South America to see how maximum body size changed with time. The pattern they found "is replicated across space and time, which is just amazing", says Smith.
Giant herbivore
The fossils show how mammals – which initially weighed in at only 10 to 100 grams – ballooned and eventually reached a maximum of 17 tonnes some 25 million years later. The largest one, Indricotherium transouralicum – which is also the largest mammal to ever walk the earth – was a hornless rhinoceros-like herbivore that stood about 5.5 metres tall at shoulder level.
"Basically, the dinosaurs disappear and all of a sudden there is nobody else eating the vegetation," says co-author Jessica Theodor of the University of Calgary, Alberta, Canada.
All of the largest mammals were plant-eaters. "It's more efficient to be a herbivore when you're big," says Theodor.
Mammalian predators never grew to much more than a tonne, about the size of a modern polar bear. Size can be a problem for predators, says Smith, as it makes it easy for potential prey to spot and elude them.
Smith thinks temperature and energy set the upper limits, because massive mammals have a hard time dissipating body heat in warm climates. Even the largest megafauna were not as big as large dinosaurs, and Smith believes dinosaurs could grow much larger because they generated less internal heat.
Others are not convinced. "I don't think we really know why we have larger animals at given times," says palaeontologist Donald Prothero of Occidental College in Los Angeles. Alroy says that evolutionary trends are better revealed by changes in lineages than by looking only at the largest species.
