In order to unlock the dig sites for Stegosaurus, you will need 5 expedition buildings which won't happen until after you have unlocked the last island. (NOTE: you can only have one expedition per island park)
Then you continue sending out expeditions until you have 50% of the genome. However, in order to unlock the last island, you will need to get 3 stars on Isla Pena which unlocks this last island and gives you this achievement: