The Paleozoic Era is seen in the fossil record as an explosion of life forms. The Cambrian Period marks the beginning of an era with thousands of new life forms in the ancient seas. Prior to the Cambrian period fossils are extremely rare but after about 540 million years ago the fossil record shows many and varied life forms.
Life In The Seas
It is this characteristic, life in the ancient sea, which distinguishes the Paleozoic Era from all others. You see, the autotroph's makeup didn't just add oxygen to the atmosphere. It was an entirely new sort of cell structure: one that had a cell membrane or wall (or both) that enclosed a central nucleus. Today we call this a eukaryotic cell.
These new cells could now join together to do specialized tasks, the kind of specialization needed to make the tissues of plants and animals. Now species could exist that responded to the diversity of their watery environments. Some would live on the bottom sediments, burrowing with tube-shaped bodies or walking with many tiny legs. Others would live in the ocean column, developing fins for effective swimming.
New Phyla
Many phyla were represented in this era. In fact all of the phylum in today's world were represented during the Cambrium Period. Some of those representatives including: arthropods (Trilobites were everywhere!), mollusks, Lophophorata (Brachiopods), appeared almost immediately in the Cambrian while Orthoceras, a straight-shelled mollusk, came in around the Ordovician Period, echinoderms (The flower-like crinoid thrived in seas throughout the Era.) and, eventually, fish in the Devonian Period. All of them had specialized tissues for performing the functions of their bodies: eating, breathing, moving, and reproducing.
Life Moves Onto Land
Plants and animals first moved onto land in the Silurian Period. But they didn't really flourish there until the Carboniferous Period when huge forests covered the land. These forests were so dense and covered so much of the earth that oxygen was 35% of the atmosphere compared to present day levels of 21%.
Giant Insects
It is this high oxygen content that is believed to be the cause of the gigantic arthropods that lived in these forests. A dragonfly called Meganeura had a wingspan 70 cm! The centipede like Arthropleura could reach a length of 1.8 meters and giant scorpions grew over 50 cm long. Of course there were an abundance of insects of sizes we would find unremarkable as well.
Tetrapods
The high oxygen content may have made it easier for the amphibians that also left the water during the carboniferous period. These four legged creatures called tetrapods lived at the waters edge. The tetrapods became very successful evolving many different forms some of which grew quite large over 2 meters in length! They became the dominant life forms in the lakes, rivers and swamps of the Carboniferous Period. The first reptiles showed up late in the Carboniferous Period as well but they did not play an important part during this time.
Life on our planet wasn't exactly like that of today, but the similarities had arrived. Earth's miracle of life had begun and was taking hold in ancient seas of the Paleozoic.
The Permian Extinction: End of An Era
The Paleozoic Era ended with a mass extinction event called the Permian extinction. This event killed off 95% of all life on earth. Even the event that caused the extinction of the dinosaurs 65 million years ago cannot compare to the destruction of life at the Permian Extinction.
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