Big ol’ brontotheres.

Science News reports on new discoveries shedding light on how prehistoric mammals grew so dang big, evolutionarily, once the dinosaurs cleared out:

Brontotheres were among the first mammals to grow to enormous size — their name comes from Greek words for “thunder” and “beast.”

But while more than half of the nearly 60 known brontothere species tipped the scale at over a metric ton, the first footfalls of these “thunder beasts” weren’t thunderous at all. When the earliest brontotheres appeared in the lush forests of ancient North America and Asia in the early Eocene Epoch, roughly 56 million years ago, they were about the size of a border collie — hardly the stuff of thundering herds.

Within about 16 million years, these modest mammals had grown into a family of giants.

[T]he orthodox explanation for this trend, called Cope’s rule, holds that bigger individuals of a single species have a fitness advantage over smaller ones.

[University of Alcalá paleobiologist Juan] Cantalapiedra and colleagues tested how well three different evolutionary scenarios — including Cope’s rule — could explain the fossil record of brontotheres. They also used the fossil data to look for links between brontotheres’ size, ecology and the likelihood of going extinct or splitting into new species.

The new analysis suggests that brontothere species usually stayed about the same size until splitting off into new species, which could be either bigger or smaller than their forebears. But because smaller species branched into new species and went extinct more often, brontotheres overall tended to get bigger with time.

Comparing the family tree of brontotheres with a bonsai, Cantalapiedra says that this mechanism is a bit like letting the bonsai tree’s branches split normally while “cutting branches away from one side.” Cope’s rule, on the other hand, would be like using wires to continuously guide the tree in one direction.

You can read more of Cantalapiedra’s research here, in Science.