This and shastasaurus are two really good suggestions. While I think Shasta should be in the center map I would like to make a suggestion for the crab being part of genesis part 1
Could easily split it across the 4 biomes and make whichever variant you get determine its starter shell. So obsidian in the lava biome, crystal in the ocean biome, torpor in the bog and stone in the snow. Could still swap the shells out at any time but that way if there was one you specifically wanted you could just get it from the corresponding biome.
I actually think it'd work really well as it'd be one of the few creatures which could appear in all 4 biomes realistically AND have a gameplay reason for doing so