The following table shows the three eras and eleven geological periods that comprise the Phanerozoic. The Age of Ancient Life Of the three main eras that make up the Phanerozoic, the Paleozoic is the longest and most diverse, spanning the period from very early multicelluar life that only inhabited the oceans to quite advanced amphibians and reptiles and extensive forests on land. Coal forests of giant lycopods, calamites, pteridophytes and ferns cover the tropical landmasses. Reptiles conquer the land. Shallow oceans cover much of the continents, breaking the land into large islands.
Mammals remain small, possibly nocturnal.
Most modern groups of organisms appear. During this period, following the extinction of the dinosaurs, mammals evolved from small shrew-like types into all the diverse types around today, as well as many different prehistoric forms. The modern world. Land masses take their present shape. The climate, originally tropical, becomes increasingly more seasonal as ice age conditions develop, possibly triggered by the rise of the Himalayan mountain uplift.