For a limited time, 7 km eggs will exclusively hatch into fossil Pokemon such as Omanyte, Kabuto, Aerodactyl, Lileep, Anorith, Cranidos, Shieldon, Tirtouga, and Archen. These Pokemon will no longer hatch from other eggs, so the only way to obtain them will be from 7 km ones. Additionally, Riolu–the pre-evolved form of Lucario–will now hatch from 10 km eggs.
— Pokémon GO (@PokemonGoApp) February 18, 2020
Niantic has not yet revealed how long this fossil Pokemon event will run, but you can read more about it on the official Pokemon Go website.