The key problem is marine creatures and pterosaurs would require special enclosures and additional mechanics for constructing those, and I imagine Frontier didn't focus on that level of detail at lunch for a handful of species each. It would also be difficult to make a lot of variation on tanks and aviaries - I imagine it didn't seem worth the level of resources they're currently using. Still, I'm sure they recognize in full that fans will want these creatures down the line and hopefully have something planned.
For downloads though, it would be good to release the tools to build such enclosures and a package of three or so creatures for each - one for marine life (Mosasaurus, Plesiosaurus, Ichthyosaurus?) and one for pterosaurs (Pteranodon, Dimorphodon, Cearadactylus?) would be good choices of downloadable content.