Prehistoric rhinoceros9/3/2023 Towering over predators is a good way to stay off the menu, and bigger bodies can enable bigger brains, expanded dietary options and many other advantages. And the orthodox explanation for this trend, called Cope’s rule, holds that bigger individuals of a single species have a fitness advantage over smaller ones. This dramatic transformation played out in other mammal lineages too ( SN: 3/31/22). Though the biggest brontotheres (one illustrated at top) weighed several metric tons, the first of these “thunder beasts” (bottom) were no larger than a medium-sized dog. Within about 16 million years, these modest mammals had grown into a family of giants. 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. Brontotheres were among the first mammals to grow to enormous size - their name comes from Greek words for “thunder” and “beast.”īut 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. Previously no larger than cats or coyotes, some mammals rapidly transformed into titans of a metric ton or more. “It’s not this organized, predictable world where progress is a thing in nature and the better adapted always ended up surviving.”Īfter nonavian dinosaurs were wiped out about 66 million years ago, mammals got bigger - and some got really big ( SN: 4/25/22). “It’s a more complex evolutionary world than what pure Darwinism would tell us,” says paleobiologist Juan Cantalapiedra of the University of Alcalá in Madrid.
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