In survival mode, the future main mode of the game, players will start as a juvenile of whichever creature they choose and grow into the adult form over a period of time. Lower tier creatures existed primarily as "filler" and as such, most will not be included in the new game mode. Currently, survival has only a few dinosaurs, but others are planned to be added in the future.
Later down the development line, you will only be able to play a previously designated starter dinosaur. Growing up to adult on this(and any other) dinosaur will grant you a token which will then be able to be used to unlock another dinosaur on the server.