The Paleozoic, Mesozoic, and Cenozoic are the three main eras of complex multicellular life on Earth. The Paleozoic is the age of invertebrates, terrestrial plants, amphibians, and synapsids. The Mesozoic is the age of reptiles, and the Cenozoic is the age of mammals. The Ediacaran fauna existed around 60 million years before the start of the Paleozoic. The Paleozoic began about 542 million years ago when the earth was devoid of life. The Mesozoic was dominated by various types of reptiles, including dinosaurs. The Cenozoic is characterized by mammals, birds, fish, and whales.
The Paleozoic, Mesozoic and Cenozoic refer to the three main eras of complex multicellular life on Earth: in Greek, the words mean “ancient life”, “middle life” and “new life”. The Paleozoic spans from 542 to 251 million years ago, the Mesozoic from 251 to 65.5 million years ago, and the Cenozoic from 65.5 million years ago to the present.
Extremely broadly speaking, the Paleozoic could be considered the age of invertebrates, terrestrial plants, amphibians and synapsids (the ancestors of mammals), the Mesozoic was the age of reptiles (especially dinosaurs) and the Cenozoic is the age of mammals . Since the terms were coined, it has been discovered that complex multicellular life actually existed around 60 million years before the start of the Paleozoic Era. This life is referred to as the Ediacaran fauna.
The Paleozoic began about 542 million years ago when the earth was devoid of life and all living things were aquatic. The oceans were filled with small, simple organisms such as trilobites, cnidarians (relatives of jellyfish), and frond-like tufted “mattresses” and “bags” that were the Ediacaran fauna. Quickly, the Ediacaran fauna died out and was replaced by a rapidly diversifying set of organisms that branched off into the early ancestors of nearly all modern animal phyla. This abrupt episode of adaptive radiation is now known as the Cambrian explosion.
Some of the most significant evolutionary milestones occurred in the rest of the Paleozoic. Animals ranged from averaging just a couple of inches long to forms much larger than humans, including the predatory fish Dunkleosteus, which grew to 6 m (20 ft) long and was the first vertebrate apex predator. Dunkleosteus lived in the shallow seas of the Devonian period, about 370-360 million years ago. Life went from being entirely marine to colonizing land, a process that began with the first plants 475 million years ago or more, and wasn’t completed until the first known trees began forming forests about 380 millions of years ago. The Paleozoic ended with the largest mass extinction in history, the Permian-Triassic extinction, which wiped out about 90% of all animal species on the planet.
The Mesozoic was dominated by various types of reptiles, including pterosaurs, which occupied the air and were the first flying vertebrates; the ichthyosaurs and plesiosaurs, marine reptiles that occupied the oceans; the sauropods, huge four-legged dinosaurs that were dominant herbivores; the theropods, carnivorous dinosaurs that walked on two legs and included the Tyrannosaurus rex, and numerous other dinosaurs, which used armor and other adaptations to avoid being eaten. The flora and fauna of the Mesozoic era have been well publicized by various dinosaur-focused documentaries and films.
The Cenozoic, the most recent era, is characterized by mammals, survivors of the mass extinction that killed off the dinosaurs and went on to dominate the planet. Birds, which evolved from a group of theropod dinosaurs, evolved to conquer the skies, while fish and whales took over the seas. Mammals of all sizes evolved, even though they were smaller than dinosaurs on balance. Several lineages of predatory and herbivorous mammals evolved, culminating in the evolution of humans, the most intelligent species the planet had ever seen.
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