The Archaic period in the Americas saw a changing environment featuring a warmer more arid climate and the disappearance of the last megafauna. The majority of population groups at this time were still highly mobile hunter-gatherers; but now individual groups started to focus on resources available to them locally, thus with the passage of time there is a pattern of increasing regional generalization like, the Southwest, Arctic, Poverty, Dalton and Plano traditions. This regional adaptations would become the norm, with reliance less on hunting and gathering, with a more mixed economy of small game, fish, seasonally wild vegetables and harvested plant foods.
A colder, more arid climate happened in what period?