Paleozoic

• 1. Introduction

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  The Paleozoic Era holds a span of time so vast it defies easy comprehension: roughly 287 million years, from 538.8 million years ago to 251.9 million years ago. In that time, life on Earth went from a scattered assortment of soft-bodied ocean dwellers to a world of forests, fish with bony fins, and reptiles ruling a single enormous landmass. The name itself tells you what the people who coined it believed they were looking at. Adam Sedgwick, working in 1838, drew on the Greek words for "old" and "life" to produce a label that meant, simply, ancient life. John Phillips then expanded the term's reach in 1840 to cover the full sweep from the Cambrian through to the Permian. Six periods, two informal sub-eras, and enough geological drama to fill any documentary many times over. What triggered the sudden explosion of nearly every animal body plan in a geologically brief window? How did creatures that had never left the water learn to breathe air and walk on land? And what ended this long chapter of ancient life in a single catastrophic event so severe it took 30 million years for land ecosystems to recover? Those are the questions the Paleozoic forces us to ask.

• 2. Drawing the Boundaries

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  Fortune Head, on the Burin Peninsula of Newfoundland, is the kind of rocky coastal outcrop that most visitors would walk past without a second glance. To geologists, it is one of the most important sites on Earth. After two decades of deliberation, the International Commission on Stratigraphy chose this spot as the Global Stratotype Section and Point that defines where the Paleozoic begins. The marker is not a trilobite, as Adam Sedgwick originally proposed when he named the era in 1835. It is a trace fossil assemblage, specifically the Treptichnus pedum traces, appearing just above the last known occurrence of the Ediacaran fossils Harlaniella podolica and Palaeopsacichnus. Trace fossils were preferred because, unlike skeletal remains, they are preserved across a wide range of sediment types and environments, making global correlation far more reliable. Ediacaran traces are simple, near-horizontal feeding marks. The appearance of more complex traces signals more complex animal bodies, and that complexity is the true threshold the Paleozoic represents. Radiometric dating pins this lower boundary at 538.8 plus or minus 0.2 million years ago. The upper boundary, where the Paleozoic gives way to the Mesozoic, is anchored at Meishan in Zhejiang Province, southern China, fixed by volcanic clay layers to 251.902 plus or minus 0.024 million years ago. The signal used there is the first worldwide occurrence of the conodont Hindeodus parvus, the earliest biostratigraphic event reliably associated with recovery from the end-Permian extinctions. In non-marine rock sequences, the equivalent marker is the disappearance of Permian Dicynodon tetrapods.

• 3. Continents in Motion

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  At the opening of the Paleozoic, a supercontinent called Pannotia was already breaking apart. Its fragmentation began with the opening of the Iapetus Ocean and coincided with a dramatic rise in sea level. Paleoclimatic evidence, including traces of ancient glaciers, suggests that Central Africa sat close to the polar regions during the early Paleozoic. The pieces of Pannotia eventually coalesced into a new arrangement, dominated by the enormous continent of Gondwana. By the middle of the era, a collision between North America and Europe threw up the Acadian-Caledonian uplifts, while a subducting plate was pushing eastern Australia skyward. The late Paleozoic saw all of these landmasses press together into Pangaea, ringed by a single global ocean called Panthalassa. That assembly produced great mountain chains: the Appalachians, the Caledonides, the Ural Mountains, and the mountains of Tasmania all owe their origins to those collisions. Pangaea's interior, far from any ocean, became extremely arid. Harsh seasonal swings replaced the moderated coastal climates that had nurtured earlier life, and that aridity would prove to be one of the defining pressures on the final period of the Paleozoic.

• 4. A Climate of Extremes

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  The early Carboniferous averaged global temperatures of around 20 degrees Celsius before cooling to roughly 10 degrees Celsius during the middle of the period. That range illustrates how restless Paleozoic climate was across its entire length. The early Paleozoic ran warm, with sea levels during the Ordovician and Silurian reaching as much as 200 metres above present levels, the highest of any point in the entire Paleozoic. Then a brief but severe late Ordovician ice age interrupted that warmth, pulling sea levels sharply down and driving the second-greatest mass extinction of the Phanerozoic Eon. The Mississippian epoch, at the start of the Carboniferous, saw atmospheric oxygen spike while carbon dioxide plummeted to new lows, destabilising climate enough to trigger one or possibly two further ice ages. By the Cisuralian Epoch, both gases had returned toward more typical levels. But the Lopingian Epoch, the final chapter of the Permian, brought falling sea levels, rising carbon dioxide, and a steady climatic deterioration that set the stage for the end-Permian catastrophe. The eruption of the Siberian Traps flood basalts, ocean anoxia, ocean acidification, and the resulting mass extinction are all now dated to the very latest Permian, just below the boundary that the conodont Hindeodus parvus marks.

• 5. Life Moves Ashore

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  Plants stayed in the water for most of the early Paleozoic, then began moving onto dry land around 420 million years ago, during the Silurian. The genus Cooksonia represents some of the earliest known vascular plants, and its appearance opened the way for all land plant life that followed. A pivotal escalation came during the Devonian, when plants evolved lignin, the structural compound that allowed them to grow taller and develop true vascular tissue. That shift triggered the Devonian explosion of plant diversity, producing the first trees and the first seeds. By the Carboniferous, towering lycopsid rainforests dominated the tropical belt of the landmass geologists call Euramerica. Those forests were eventually undone by the Carboniferous Rainforest Collapse, a climate-driven fragmentation that reduced plant diversity through the late Carboniferous and into the Permian. The trees that survived, and the first conifers that emerged in the Permian, were adapted to drier, harsher conditions than the coal-swamp forests they replaced. Many of the coal beds that underlie Europe and eastern North America are the compressed remains of those Carboniferous forests, a physical record of how thoroughly plant life once covered the continents.

• 6. Animals Claim the Land

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  Arthropods were the first animals to walk on dry land, colonising the continent of Gondwana during the Ordovician. The sea scorpions of the Silurian were still apex predators even as early arachnids, fungi, and centipedes began establishing fully terrestrial existences. Fish took a different path to land. Some possessed lungs and powerful bony fins, and by 367.5 million years ago, in the late Devonian, they were crawling out of the water. The bones inside those fins gradually transformed into legs, producing the first tetrapods. Amphibians remained the dominant tetrapods until around the middle of the Carboniferous, when climate change sharply reduced their diversity. The key innovation that broke amphibians' dependence on water was the amniotic egg, which allowed animals to reproduce on dry land and move farther inland. Amniotes split into two groups shortly after their origin: the synapsids, which were the dominant lineage, and the sauropsids. By the late Permian, creatures such as Dimetrodon and Edaphosaurus ruled Pangaea's landscapes, while the pareiasaurs, large herbivorous sauropsids, were among the few of that lineage to reach sizes comparable to the largest synapsids. The Paleozoic marine world, meanwhile, looked strikingly different from the seas familiar today: predators made up roughly 4% of faunal assemblages, compared to 17% in temperate Cenozoic seas and 31% in tropical ones.

• 7. The Great Dying

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  Near the end of the Permian, Pangaea grew steadily drier. The interior became desert, and new groups such as Scutosaurus and the Gorgonopsids filled that harsh terrain. Then came an extinction event so total that 95% of all species on Earth disappeared. Geologists call it the Permian-Triassic extinction, and informally it carries the name "The Great Dying." It was the largest extinction event of the entire Phanerozoic Eon, far more catastrophic than the event that would later end the dinosaurs. Recovery on land took 30 million years into the following Mesozoic Era. Recovery in the sea may have been faster, though the sources of that speed are still a subject of study. The boundary itself, fixed at Meishan in China and dated to 251.902 million years ago, is the sharpest punctuation mark in the entire geological record of the Paleozoic, closing an era that had lasted nearly 287 million years and had witnessed the invention of almost every major animal body plan, the colonisation of the land, and the rise of the forests that today fuel much of human industry.