This means that Omanyte, Kabuto, Aerodactyl, Lileep, Anorith, Cranidos, Shieldon, Tirtouga, and Archen are the only Pokémon coming out of eggs you get from friends’ gifts. With this shift, none of these will hatch out of any other eggs and Riolu will now hatch out of 10 km eggs.
There isn’t a set date for this fossil bonanza to end, but you can use this opportunity to grab a bunch of Cranidos, if you’re lucky. Its evolution, Rampardos, is one of the best rock-type attackers in the game. Bastiodon, the evolved form of Shieldon, is also a pretty solid Great League PvP Pokémon.
Tirtouga and Archen are still fairly new additions to this game, so this is also a good chance to grab them and enough candy to evolve them. Of course, you could be unlucky and end up with a bunch of Omanyte or Lileep that you don’t need anymore.
Note that this change does not affect any 7 km eggs you may have gotten before today. Those will still have older Pokémon in them, but any egg you get from friends starting now will hatch fossil Pokémon.