38 TO 34 MILLION YEARS AGO

Thunder Beasts

As the climate cooled on Earth and dense forests gave way to more open woodland, one group of plant-eating mammals evolved into massive creatures.

This massive skeleton belongs to *Megacerops* (meg-ah-SAIR-ops), a member of an extinct group of plant-eating mammals that O.C. Marsh called brontotheres (BRONT-oh-theers), or “thunder beasts,” after his understanding of the local Lakota name for the fossils. These relatives of horses started out as pig-sized animals living in dense forests but evolved to reach huge sizes once their environment became more open.

Brontothere
Stocktrek Images, Inc./Alamy Stock Photo