Dinosaurs lived on Earth during the Mesozoic Era, which lasted from 248 to 65 million years ago. Geologists divide the Mesozoic Era into three periods: from longest ago to most recent, the Triassic, Jurassic and Cretaceous. Dinosaurs became more diverse as time went by, and at the same time, other life forms both developed and became extinct.
If the Mesozoic Era was the age of dinosaurs, the next era -- the Cenozoic Era -- was the age of mammals. In fact, a past theory about the extinction of the dinosaurs was that newly evolving mammals ate all their eggs. This is partly because most early mammals were small, so dinosaur eggs may have made a tasty and convenient meal for them. The first mammals were monotremes, or mammals that reproduce by laying eggs. Mammals are common today, but only three monotreme species still exist. These are the duck-billed platypus and a couple of spiny anteaters, or echidnas.
Many reptile species died during the K-T event, but snakes, lizards and the crocodilians persevered. Crocodilians have been on the planet for about 240 million years. There are 23 crocodilian species today, including alligators, crocodiles and caimans. Mesozoic crocodilians were generally larger than living species. Another prehistoric order of reptiles is the sphenodontians. Today, there is one living sphenodontian -- the tuatara, which lives in New Zealand.
The most likely evolutionary origin for these reptiles is that in the distant past they arose from amphibians. Very large amphibian species lived before and during the Mesozoic. These are gone today, but in their place three primary types of amphibians survived:
Unlike monotremes and sphenodontians, not every family that lived during the Mesozoic has only a few descendents left today. All modern insect groups either existed before or arose during the Mesozoic Era. Perhaps most notable of these is the eusocial bee -- bees that live in colonies. Most likely, these evolved along with flowering plants, or angiosperms, which started to develop in the Cretaceous Period. Without this co-development, neither humans nor bears would have easy access to honey today.
The oceans were particularly devastated by the K-T event. We'll look at the oceanic survivors that are part of Earth's modern seas on the next page.