By Robert Hazen, George Mason University
Earth’s history is divided into several long time periods, based on the distinctive characteristic of rocks and fossils from those periods. The Paleozoic Era stretched from 570 million years ago to about 245 million years ago. This saw diversification in life, and the introduction of many of the familiar groups of animals and plants that we now know.
Tens of thousands of feet of Paleozoic sediments have been found in the central United States alone, layer upon layer upon rock. We can look at these from the lowest level of rock, from nearly 570 million years ago, right up through the whole Paleozoic; and see the succession of changing life as we go through these rock cuts.
Many times, when we drive across the center of the United States, we’ll see these rock cuts in Kansas, and in Nebraska, and in other Midwest states. We’ll see these beautiful rock cuts, layer upon layer of rock, and they contain an abundance of life.
The Paleozoic Era also saw the closing of a precursor Atlantic Ocean. Later on, it began opening up again, and that’s the modern Atlantic Ocean. We see all these continuous plate tectonic processes also bringing continents together, closing them up, and these are evidenced also from fossil rocks.
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First Appearance of Animals
The “Cambrian explosion” 570 million years ago was an explosion of life, characterized by the first appearance of animals with calcium carbonate shells; the first appearance of hard parts in the fossil record.
We saw the first ancestors of snails. We saw the first arthropods, especially the beautiful trilobites, which can occur in specimens up to three feet across. Many other kinds of shell organisms—brachiopods and so forth. This period extended to about 520 million years ago.
First Corals and Fish
The next period is called the Ordovician Period, 520 to 450 million years old. This was a time of extensive limestone-reef formation, reefs with many kinds of primitive organisms, corals. The first primitive fish occurred around that time; those were the first vertebrates with bony skeletons.
Also, an abundance of shellfish called brachiopods preserved around the world as fossils. When you find one brachiopod, you’re likely to find hundreds of them, because these were very abundant organisms.
First Land Plants, Insects, and Reptiles
The next period is the Silurian, from about 450 to 420 million years old. It was marked by the first land plants, the first insects, and also an explosion in the variety of fishes.
We then had the Devonian Period, 420 to 375 million years old; this is when seed plants first developed, as did amphibians, which were the first of the land vertebrates.
The Carboniferous Period comes next, 375 to 285 million years. This is when the first winged insects appeared, things like dragonflies; the first reptiles, diverse land plants that formed thick coal deposits.
We can also find larger structures, which indicate there may have been trees and other large types of plants that grew much, much taller than in previous times.
The last part of the Paleozoic was the Permian Period, from 285 to 240 million years ago. It was marked by the appearance of the first mammal-like vertebrates, and an increasing diversity of land plants and land animals.
The Permian Period also saw the final assembly of Pangaea, which was a supercontinent with land masses from what are now Europe, North America, Africa, and South America, all joined together.
First Beetles and Dinosaurs
Next we come to the Mesozoic Era, the age of reptiles, from 240 to 65 million years ago. This began with a shocking, and still unexplained, extinction of more than 90 percent of all the species on Earth. All the trilobites disappeared, most of the brachiopods, and many other large groups of organisms just vanished completely from the fossil record. This mass extinction provided opportunities for whole new groups of plants and animals to fill the vacated ecological niches.
We have the Triassic Period, that’s 240 to 195 million years ago, which saw the appearance of the first true mammals, the first beetles, the first dinosaurs. These dinosaurs rapidly diversified and filled many of these ecological niches. They became dominant on land, on sea, in the air.
We also had the first ammonites in this period. Ammonites were an amazing, successful group of marine organisms, absolutely exquisite and beautiful. Some of these organisms grew up to six feet across.
First Birds and Flowering Plants
Next we come to the Jurassic Period, 195 to 135 million years ago; that was the height of the age of dinosaurs. The era also saw the first birds.
The first flowering plants, then, evolved during the next period, the Cretaceous Period, from 135 to 65 million years ago.
If you visited that early Earth at the end of the Mesozoic, the world’s forests and meadows and grasslands would have a familiar feel to them, though the dominance of all the large lizards around would probably seem somewhat unsettling.
Common Questions about Fossil Records of the Origin of Plants and Animals
The Cambrian explosion 570 million years ago was an explosion of life, characterized by the first appearance of animals with calcium carbonate shells; the first appearance of hard parts in the fossil record. We saw the first ancestors of snails. We saw the first arthropods, especially the beautiful trilobites, and shell organisms, like brachiopods.
The Jurassic Period, 195 to 135 million years ago, was the height of the age of dinosaurs. The era also saw the first birds.
The Triassic Period, that’s 240 to 195 million years ago, saw the appearance of the first true mammals, the first beetles, the first dinosaurs. These dinosaurs rapidly diversified and filled many of these ecological niches. They became dominant on land, on sea, in the air.