‘Thunder beast’ fossils show how some mammals might have gotten big
Rather than a straightforward march, evolving to be big sometimes involved getting smaller
By Elise Cutts
For some mammals, the evolutionary path to gigantism after the dinosaurs’ demise wasn’t always a straight road.
Species of extinct, hefty, rhinoceros-like creatures called brontotheres evolved into both bigger and smaller forms over their 22-million-year run, researchers report in the May 12 Science. But big brontotheres seem to have enjoyed relatively uncrowded ecological niches compared with their smaller kin and so were less prone to extinction, explaining the group’s overall trend toward larger bodies over time.