100 million AD, a warm global climate has caused the polar icecaps to melt sea levels to rise by around 330 feet (100 meters). Lower-lying parts of the continents are flooded and the oceans have spread southwards from the Arctic and eastwards from the Atlantic. Vast tracts of Russia are now almost entirely underwater. The Shallow Seas, which stretch across northern Europe and Asia, are punctuated by rocky islands (the peaks of mountains not yet covered by water).
The sun-filled, nutrient-rich, waters of the Shallow Seas provide ideal conditions for the formation of reefs. Reefs are essentially calcium deposits, built up by generations of reef-building marine organisms. These organisms contract calcium dissolved in seawater and use it to lay down protective shells. Over successive generations, the shells and skeletons of the reef-building organisms accumulate to create a great edifice (a solid foundation upon which reef-builders live and photosynthesize). This edifice is a reef.
The Shallow Seas and their colorful, complex reefs have presented a stable environment for a long time. They have persisted for so many millions of years that effective living systems, established early on in the history of the habitat, have been able to survive without serious challenges. As a result, creatures' body shapes, feeding methods and symbiotic relationships have modified only slightly over time. However, these warm seas have allowed one thing to change a lot, and that is a creatures' size.