The current era on the geologic time scale is the Cenozoic Era. The era began after the K-T extinction resulted in the end of the Mesozoic Era around 65 million years ago.
The extinction of the dinosaurs gave mammals the chance to proliferate and fill most environmental niches. Birds have also thrived in the Cenozoic Era and outnumber mammals two to one. Grass has played an important role in the era and helped to shape the evolution of both mammals and birds. Much of the course of evolution has also been shaped by changing climate. Shifting continents caused changes in ocean currents that resulted in cooling for much of the Cenozoic Era.