They're fossils. They appear when your zombies mine the minerals in your main mine. Collect these fossils to fill up the gauge at the top of the screen and receive valuable rewards. All events feature similar fossil items. 
 
Please note: fossils can only appear while you’re actively playing the game. Only up to four fossils may appear on a level at a time, so don’t forget to claim them quickly!

You can increase this limit by upgrading the “Limit Increase” epic card.