<P> The Ordovician spanned from approximately 485 million years to approximately 443 million years ago . The Ordovician was a time in Earth's history in which many of the biological classes still prevalent today evolved, such as primitive fish, cephalopods, and coral . The most common forms of life, however, were trilobites, snails and shellfish . More importantly, the first arthropods went ashore to colonize the empty continent of Gondwana . By the end of the Ordovican, Gondwana was at the south pole, early North America had collided with Europe, closing the Atlantic Ocean . Glaciation of Africa resulted in a major drop in sea level, killing off all life that had established along coastal Gondwana . Glaciation may have caused the Ordovician--Silurian extinction events, in which 60% of marine invertebrates and 25% of families became extinct, and is considered the first mass extinction event and the second deadliest . </P> <P> The Silurian spanned from 443 to 416 million years ago . The Silurian saw the rejuvenation of life as the Earth recovered from the previous glaciation . This period saw the mass evolution of fish, as jawless fish became more numerous, jawed fish evolved, and the first freshwater fish evolved, though arthropods, such as sea scorpions, were still apex predators . Fully terrestrial life evolved, including early arachnids, fungi, and centipedes . The evolution of vascular plants (Cooksonia) allowed plants to gain a foothold on land . These early plants are the forerunners of all plant life on land . During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, Siberia), Laurentia (North America), Baltica (Northern Europe), and Avalonia (Western Europe). The recent rise in sea levels allowed many new species to thrive in water . </P> <P> The Devonian spanned from 416 million years to 359 million years . Also known as "The Age of the Fish", the Devonian featured a huge diversification of fish, including armored fish like Dunkleosteus and lobe - finned fish which eventually evolved into the first tetrapods . On land, plant groups diversified incredibly in an event known as the Devonian Explosion when plants made lignin allowing taller growth and vascular tissue: the first trees evolved, as well as seeds . This event also diversified arthropod life, by providing them new habitats . The first amphibians also evolved, and the fish were now at the top of the food chain . Near the end of the Devonian, 70% of all species became extinct in an event known as the Late Devonian extinction and was the second mass extinction event the world has seen . </P> <P> The Carboniferous spanned from 359 million to 299 million years . During this time, average global temperatures were exceedingly high; the early Carboniferous averaged at about 20 degrees Celsius (but cooled to 10 ° C during the Middle Carboniferous). Tropical swamps dominated the Earth, and the lignin stiffened trees grew to greater heights and number . As the bacteria and fungi capable of eating the lignin had not yet evolved, their remains were left buried, which created much of the carbon that became the coal deposits of today (hence the name "Carboniferous"). Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to move farther inland and remain the dominant vertebrates for the duration of this period . Also, the first reptiles and synapsids evolved in the swamps . Throughout the Carboniferous, there was a cooling trend, which led to the Permo - Carboniferous glaciation or the Carboniferous Rainforest Collapse . Gondwana was glaciated as much of it was situated around the south pole . </P>

Why paleozoic era called the age of ancient life