Lasting from 354 million to 290 million years ago, the Carboniferous was a unique time period for Earth. Not only was it mostly covered by swamps, but the oxygen percentage was about thirty percent higher than it is today. It also saw the evolution of a very important group of creatures-the reptiles. Despite their recent evolution, however, they were plagues by an old enemy, one that had not plagued them since the Silurian-new species of giant, killer arthropods.