The official Pokémon website has announced that players of Pokémon Sword or Pokémon Shield can encounter Carracosta, Archeops, Tyrantrum, and Aurorus and even Shiny Tyrantrum in Max Raid Battles from March 18th, 2022 until March 27, 2022, at 4:59 p.m. PDT.

Catch Fossil Pokémon and Look for Shiny Tyrantrum in Max Raid Battles

Pokémon Sword and Pokémon Shield players can take on prehistoric Pokémon like Carracosta, Archeops, Tyrantrum, and Aurorus.

Prepare for a blast from the past in your Pokémon Sword or Pokémon Shield game when Pokémon revived from ancient fossils arrive in Max Raid Battles. Until March 27, 2022, at 4:59 p.m. PDT, Carracosta, Archeops, Tyrantrum, and Aurorus will appear more often in Pokémon Dens. Lucky Trainers might even encounter a Shiny Tyrantrum!

For an opportunity to encounter these Pokémon in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet through Y-Comm or by selecting Mystery Gift in the X menu, then selecting Get the Wild Area News. After that, whenever you see a pillar of light shining from a Pokémon Den, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/catch-fossil-pokemon-and-look-for-shiny-tyrantrum-in-max-raid-battles/