The Paleozoic Era, which ran from about 542 million years ago to 251 million years ago, was a time of great change on Earth. The era began with the breakup of one super continent and the formation of another. Plants became widespread and the first vertebrate animals colonized land.
The Paleozoic began with the Cambrian Period, 53 million years ago, best known for ushering in an explosion of life on earth. This Cambrian explosion included the evolution of arthropods, (ancestors of today’s insects and crustaceans) and chordates (animals with rudimentary spinal cords).
In the Paleozoic Era, life flourished in the seas. After the Cambrian Period came the 45 million year Ordovician Period, which is marked in the fossil record by an abundance of marine invertebrates. Perhaps the most famous of these invertebrates was the trilobite, an armored arthropod that scuttled around the seafloor for about 270 million years before going extinct.
This was the time when plants evolved, though they most likely did not yet have leaves or the vascular tissue that allows modern plants to siphon up water and nutrients. Those developments would appear in the Devonian Period, the next geological period of the Paleozoic. Ferns appeared, as did the first trees. At the same time, the first vertebrates were colonizing the land. These vertebrates were called tetrapods, and they were widely diverse. Their appearance ranged from lizard-like to snake-like, and their size ranged from 10 cm to 5 m long, according to a study released in 2009 in the journal of anatomy.
As the tetrapods took over, they had company. The Devonian Period saw the rise of the first land living arthropods, including the earliest ancestors of spiders!
Life continued its march in the late Paleozoic. The Carboniferous Period lasted from about 359 to 299 million years ago. Long before birds evolved, tetrapods began laying eggs on land for the first time during this period, allowing them to break away from the amphibious lifestyle.
Trilobites were fading as fish became more diverse. The ancestors of conifers appeared, and dragonflies ruled the skies.
The last period of the Paleozoic was the Permian Period, which began 299 million years ago and wrapped up 251 million years ago. This period would end with the largest mass extinction ever: the Permian Extinction. Before the Permian mass extinction, though, the warm seas teemed with life. Coral reefs flourished, providing shelter for fish and shelled creatures, such as nautiloids and ammonoids.