Paleozoic Era

 The Paleozoic Era is the earliest era of the Phanerozoic Eon. It is the longest of the Pahnerozoic eras, lasting from 541 to 251.902 million years ago. (I am left wondering why such an odd date for an ending? Why not 252 million years ago? I hope they offer an explanation.)

The Paleozoic was a dramatic time, incorporating geological, climatic and evolutionary changes. There was an explosion of variety in lifeforms, in which almost all modern families appeared. This began in the ocean, but eventually transitioned onto land. Great forests of primitive plants covered the continents, and towards the end of the Paleozoic, the first modern plants (conifers) appeared.

The Paleozoic Era also saw the largest extinction event in the history of Earth. This catastrophe was so devastating that it took life on land 30 million years into the next era to recover. Life in the sea may have recovered much faster.

During the early part of this era, the climate was probably moderate, becoming warmer as the second-greatest sea level rise of the era occurred, where the sea level was 200 meters above today's levels. Gondwana moved south until West Gondwana (Africa and South America) lay directly over the South Pole, while most of the parts that now reside in the northern hemisphere remained in the tropical zone, and China and Australia lay in a temperate zone. This warm period ended rather abruptly with a short but severe ice age that caused the second-greatest mass extinction of the Phanerozoic time. This ice age was only 30 million years long, and occurred 445 million years ago.

Sea levels dropped, of course, during the ice age, but slowly recovered over the middle of the Paleozoic. Bits and pieces of Gondwana moved northward, which created numerous new regions of warm, shallow sea floor. As plants took hold on the continental edges, oxygen level increased and carbon dioxide dropped. The far southern parts of Antarctica and West Gondwana became less barren.

Then a spike in atmospheric oxygen (while carbon dioxide plummeted) destabilized the climate and led to one or perhaps two ice ages. These were even more severe than the brief one already mentioned, but the effects on the world biota were mostly inconsequential. The oxygen and carbon dioxide level returned to more normal levels, but the assembly of Pangaea created huge inland areas that were subject to temperature extremes. The end of the era saw a huge mass extinction event.

While macroscopic plant life possibly appeared before this era began, plants mostly remained aquatic until about 420 million years ago, when they began to explore dry land. They reached a point where towering lycopsid (a type of plant that includes clubmosses, firmosses and quillworts) rainforests dominated the tropical belt of Euramerica. Climate change caused this rainforest to collapse, fragmenting this habitat and diminishing the diversity of plant life.

Nearly all of the invertebrate animal phyla appeared in great abundance at the beginning of this era. The first vertebrates were primitive fish, which lost no time in diversifying. Some fish had lung and powerful bony fins that allowed them to crawl onto land about 367.5 million years ago. Their fins evolved into legs about 390 million years ago. Amphibians were dominant for a time, until the climate change that reduced the rainforests also greatly reduced the amphibian diversity. Then reptiles prospered and increased in number and variety by the end of the era.

I'm sure we'll get even more details when we study each period of this era.

 

https://en.wikipedia.org/wiki/Paleozoic