Both mammals and dinosaurs are represented. There is a huge (almost endless) variety of species that can evolve via DNA chains, and the random tile map board setup is constantly changing due to certain random events (e.g., Erosion, Greenhouse and icehouse Earth, and Milankovich cycles), Catastrophes (e.g., an Impact event or a solar flare), and the addition of new Biome tiles during play.
The presence of mammals and dinosaurs, a vast array of species that can evolve via DNA chains, and the constantly changing tile map board setup due to random events like erosion, greenhouse/icehouse Earth, Milankovich cycles, catastrophes (impact events or solar flares), and the addition of new biome tiles during play.