During the early Paleozoic Era, life existed in the oceans.

All plants and animals evolved from marine microorganisms and had not yet made the transition to life on land.

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During the Devonian Period, the first vertebrates ventured onto land.

Some fish developed the ability to scoot across land using their fins to propel them. They moved over short distances at first, probably just using this ability to move from one pool of water to another or to get across a small strip of land separating two parts of a body of water. This ability greatly helped some fish to survive, and the ability to move on land developed further in them. These fish would evolve into the first amphibians.

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Two significant occurrences during the Mesozoic Era were the development of flowering plants and the rule of dinosaurs and other reptiles.

The first flowering plants arose during the Cretaceous Period, the last period of the Mesozoic Era. They are superior at protecting their seeds and dispersing them over ferns and other plants with spores and also over cone-bearing plants because they encase their seeds in fruits. The dominance of reptiles during this era has led to it being dubbed “The Age of Reptiles” by some. The dinosaurs in particular became very diverse and were major parts of most ecosystems.

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A paleontologist investigating fossils from the Cenozoic Era would find fossils of many different kinds of mammals- grazing animals, dogs, big cats, whales, bats, and many many others- as well as birds, fish, arthropods (like insects and crustaceans), and much, much more.

The Cenozoic Era has many creatures that are similar to those of earlier eras- lizards, turtles, sharks, birds, fish, arthropods, and others- but there are also many new types of mammals that would also have been present. Some of these might have included mammoths, sabre toothed tigers, dire wolves, primitive horses, giant sloths, whales, and a multitude of more primitive mammals from the earlier portions of the Cenozoic Era when mammals were just beginning to diversify and take on more important roles in many ecosystems.

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