Geological Time Scales 3

	The Cenozoic Era began about 65 million years ago. It also is called the "Age of Mammals" because mammals are the dominant land animals. We will examine this era in the most detail.
	During the Mesozoic Era, reptiles were the dominant vertebrates. The Mesozoic included the reign of the dinosaurs. Mammals appeared during this era, about 220 million years ago, but they were very small--no larger than present-day rodents--and neither numerous nor diverse. Around 200 million years ago, the first birds evolved from dinosaurs.
	During the Paleozoic Era, a dramatic explosion of living creatures occurred. Early on, there was a great profusion of marine invertebrates, like worms, trilobites, gastropods, corals, and cephalopods. The first vertebrates appeared around 500 million years ago, and fish evolved from these and became the dominant vertebrate. From the bony fishes arose the amphibians around 410 million years ago. Amphibians were among the life forms that first colonized the land. Reptiles arose from this amphibian stock around 360 million years ago, and gradually began displacing the amphibians.