I realised the old rules were just written out wrong. At some point in bringing them over to the website I’d lost how they should be typed.

Out with the old

When a tile is stacked, migrating critters to the tile of your choosing within the connected biome. The player whose critter you moved may not move the critter until the Critter movement step of their turn.

In with the new

Placing a tile

Newly bought tiles must stack and connect to at least one edge of an existing tile. Rotate the new tile to match the connecting biomes on at least one edge.

Note: the more connections across your biomes at the end of the game will increase your score and give places for your critters to evacuate from incoming lava and other player’s stacks!

When stacking, if the are remaining tiles to a connected biome, the critter moves to one of the available tiles. If no biome is available, remove any critter(s), friend or foe, from the lower tile and return it to the player’s hand.