Unlike previousmainline Pokemon games, Pokemon Legends: Arceus makes it a requirement to catch all the Pokemon in its region. As with all Pokemon games, there are some Pokemon that are harder to find than others. Although Pokemon Legends: Arceus takes place in the past, the Sinnoh region's Fossil Pokemon, Cranidos and Shieldon, can be caught in Hisui.
There aren't any fossil-reviving mechanisms at the Galaxy Hall or at the Diamond/Pearl Clan's base camps. Unlike Diamond/Pearl, players can directly catch Cranidos and Shieldon out in the overworld. The drawback is their habitat. Cranidos and Shieldon spawn in only one area in all of Hisui, and they won't be available until players are midway through the main story. Whether players want to use the Fossil Pokemon in battle or not, Cranidos and Shieldon, as well as their evolutions, are part of Hisui's 239 Pokemon that need to be «seen» in order to catch Arceus.
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After calming Kleavor in the Obisian Fieldlands, strange phenomenons called space-time distortions will start appearing around Hisui. When a distortion is starting to form, players will receive a notification at the top of their screen. The space-time distortion will then appear on the map.
Players will know they're near one by the distortion's large, dome-like shape. It can take a while for distortions to fully form, and they'll normally last for about five minutes in real-time before disappearing. Once they form, rare Pokemon that can't be found in Hisui will start appearing. Among these Pokemon are Cranidos and Shieldon.
The Fossil Pokemon can't be found in just any space-time distortion, however. Each region of Hisui has different Pokemon in their space-time
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