<P> The early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second - greatest sustained sea level rise in the Phanerozoic got underway . However, as if to offset this trend, Gondwana moved south, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over the South Pole . The early Paleozoic climate was also strongly zonal, with the result that the "climate", in an abstract sense, became warmer, but the living space of most organisms of the time--the continental shelf marine environment--became steadily colder . However, Baltica (Northern Europe and Russia) and Laurentia (eastern North America and Greenland) remained in the tropical zone, while China and Australia lay in waters which were at least temperate . The early Paleozoic ended, rather abruptly, with the short, but apparently severe, late Ordovician ice age . This cold spell caused the second - greatest mass extinction of Phanerozoic time . Over time, the warmer weather moved into the Paleozoic Era . </P> <P> The middle Paleozoic was a time of considerable stability . Sea levels had dropped coincident with the ice age, but slowly recovered over the course of the Silurian and Devonian . The slow merger of Baltica and Laurentia, and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow sea floor . As plants took hold on the continental margins, oxygen levels increased and carbon dioxide dropped, although much less dramatically . The north--south temperature gradient also seems to have moderated, or metazoan life simply became hardier, or both . At any event, the far southern continental margins of Antarctica and West Gondwana became increasingly less barren . The Devonian ended with a series of turnover pulses which killed off much of middle Paleozoic vertebrate life, without noticeably reducing species diversity overall . </P> <P> There are many unanswered questions about the late Paleozoic . The Mississippian (early Carboniferous Period) began with a spike in atmospheric oxygen, while carbon dioxide plummeted to new lows . This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous . These were far more severe than the brief Late Ordovician ice age; but, this time, the effects on world biota were inconsequential . By the Cisuralian Epoch, both oxygen and carbon dioxide had recovered to more normal levels . On the other hand, the assembly of Pangaea created huge arid inland areas subject to temperature extremes . The Lopingian Epoch is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction . </P> <P> While macroscopic plant life appeared early in the Paleozoic Eon and possibly late in the Neoproterozoic Era of the earlier eon . Plants mostly remained aquatic until sometime in the Silurian and Devonian Periods, about 420 million years ago, when they began to transition onto dry land . Terrestrial flora reached its climax in the Carboniferous, when towering lycopsid rainforests dominated the tropical belt of Euramerica . Climate change caused the Carboniferous Rainforest Collapse which fragmented this habitat, diminishing the diversity of plant life in the late Carboniferous and Permian . </P>

What is a description of the paleozoic era