Pity the poor Osage orange tree. Researchers believe this tree relied on just a few species to eat its fruit and disperse its seeds, but those animals went extinct thousands of years ago. Now when the tree drops its fruit in the fall, the brain-like orbs are left to rot on the ground.

The knobby Osage orange, about the size of a grapefruit, has the consistency of a raw potato and is mildly poisonous to today's mammals. However, researchers think it may have been a very palatable treat for extinct mammals like mammoths, mastodons, or the giant ground sloths that tromped across North America until around 13,000 years ago.