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From Wikipedia, the free encyclopedia

Paleozoic Era
541 - 251.902 million years ago
Key events in the Paleozoic
-550 —
-500 —
-450 —
-400 —
-350 —
-300 —
-250 —
An approximate timescale of key Paleozoic events.
Axis scale: millions of years ago.

The Paleozoic (or Palaeozoic) Era ( /ˌpliəˈzɪk, ˌpæ-/;[1][2] from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life"[3][4]) is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 251.902 million years ago, and is subdivided into six geologic periods (from oldest to youngest): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic comes after the Neoproterozoic Era of the Proterozoic Eon and is followed by the Mesozoic Era.

The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, synapsids and diapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms. Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated diapsids and synapsids were dominant and the first modern plants (conifers) appeared.

The Paleozoic Era ended with the largest extinction event in the history of Earth, the Permian–Triassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic Era to recover.[5] Recovery of life in the sea may have been much faster.[6]

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Transcription

Welcome back to our mini-series on the history of life on Earth! So far, we’ve covered the Archean and Proterozoic eons -- the first 3 billion years of life, which was mostly single-celled that whole time. Next comes the Phanerozoic eon, and here, we’re going to zoom in a bit: to the first chunk of the Phanerozoic eon, the Paleozoic era, which lasted from 542 to 252 million years ago. Right at the beginning of the Paleozoic, there was a huge explosion of more complex life. And that’s when things started to get really interesting. The Paleozoic era is divided up into 6 periods. And by the first one, the Cambrian, multicellular life -- including animals -- already existed. But it hadn’t … done much. Animals were simple things like sponges. They didn’t have complex organs, and really, they weren’t much more than lumps, eating bacteria they strained out of the water. All that changed about 542 million years ago, with the Cambrian Explosion. There was an increase in oxygen levels just before the beginning of the Cambrian, caused by a boom in life that produced oxygen using photosynthesis. Scientists still debate how big this oxygen event was and when exactly it happened, but it might have made predation -- the predator-prey relationship -- possible for the first time. And the filter-feeding lumps got gobbled up. Predation involves chasing after the things you want to eat, and that takes more energy than sitting on the ocean floor waiting for food to come to you. To maintain that high-energy lifestyle, predators needed a whole lot of oxygen. Once predators evolved, prey started to evolve better defenses and ways to run away -- which led to predators getting faster and better at capturing their prey. It was basically an evolutionary explosion. Hard body tissues like shells and skeletons begin to show up in the fossil record just before the Cambrian. It took a lot of energy to produce that tissue, but it was worth it since these animals were less likely to be eaten. Officially, the beginning of the Cambrian was when animals started to burrow under the thick mat of bacteria on the ocean floor to escape predators. These predator-prey relationships combined with other factors, like changes in the minerals in the oceans and flooding that opened up shallow habitats. This led to such an enormous boom in diversity that almost every major animal group that exists today evolved during the Cambrian -- including arthropods, molluscs, and the chordates that eventually gave rise to vertebrates. The second period of the Paleozoic era was the Ordovician, which started 485 million years ago. The name comes from a Celtic tribe, since many of the best-studied rocks from the Paleozoic come from Britain. The Ordovician was when vertebrates first appeared. They were fish without jaws. Then came the Silurian period, named for another Celtic tribe, which started 443 million years ago. Sometime during the Ordovician and into the Silurian, life made the jump to land. Moving to land wasn’t as easy as washing up onshore and going about your business. Life began in water, and living in water has some advantages over living in air. Water holds your body up. It helps you with gas exchange. And if you release your sperm or eggs into the Big Blue, it’s much more likely that they’ll meet up with other gametes to reproduce. None of that is true for air. So the earliest land organisms had to evolve support structures, new respiratory systems, ways to avoid drying out, and methods of reproduction that were a little more controlled. Extremely simple plants might go back as far as the early Ordovician or even the Cambrian. They were spore-forming and had very little in the way of internal support. The oldest fossil we have that isn’t a plant spore or a single-celled organism is a fungus called Tortotubus. It’s about 440 million years old, from around the Ordovician-Silurian cutoff. This fungus was a total game-changer. Tortotubus probably helped pave the way for more complex plants, and for animals too. It lived as an underground network of filaments, much like modern fungi. It might have formed mushrooms to disperse its spores, but we don’t know for sure. It fed by rotting the few other organisms on land, like early plants and microbes. By breaking down nutrients, it helped develop the soil on Earth’s surface. That helped complex plants grow and develop soil even further. Now, there are Dr. Who creatures called Silurians. But they’re pretty badly named, because there were no land vertebrates during the Silurian period, let alone intelligent humanoid reptiles. But! There were insects and other arthropods, earthworms, and other terrestrial invertebrates colonizing the land. They formed the first simple land ecosystems, along with plants and fungi. The fourth period of the Paleozoic, called the Devonian, started 419 million years ago and ended 359 million years ago. It’s sometimes called the Age of Fishes, because it’s when the first fish with jaws appeared. And it was followed by a LOT of fish with jaws. These fish, called placoderms, had tough, bony armor surrounding their skulls. They were the earliest vertebrates with jaws, and jaws were pretty useful for eating stuff, so they were a big success, in an evolutionary sense. Placoderms showed up in the middle Devonian, before the Devonian was over, the first tetrapods, or four-footed creatures with backbones, had already evolved. In fact, there’s evidence that tetrapods may go back 395 million years or more -- smack in the middle of the Devonian. Which means that those fish -- the ancestors of birds, mammals, reptiles, and amphibians -- got around to having legs and crawling out of the ocean almost instantly, on an evolutionary timescale. At the beginning of the Devonian, jawless vertebrates were the most complex life around. By the end of the Devonian: there were early, amphibian-like land dwellers walking around. That is a gigantic leap. Arthropods and land plants had a huge boom too, meaning those simple land-based ecosystems from the end of the Silurian were a lot more complex by the end of the Devonian. Then, 359 million years ago, the 5th period of the Paleozoic began:, the Carboniferous period. You might’ve heard that fossil fuels are made of dinosaurs. But they’re actually much, much older than that. The Carboniferous was when land plants really started to establish themselves. The climate was mild enough for plants to grow year-round, and huge forests grew. The word “carboniferous” means coal-bearing, and for good reason: hundreds of millions of years later, we’re digging up the remains of those forests as coal. The forests pumped oxygen into the atmosphere like crazy -- much more oxygen than there is today -- which led to the development of the first big land animals: arthropods. Bugs, basically. They grew huge in the oxygen-rich atmosphere. That’s right. 350 million years ago, Earth was full of giant bugs. Land vertebrates were still fairly small in the Carboniferous, but they did develop one major evolutionary innovation: the amniotic egg, which is the reason you can store a chicken egg without it drying out. Amniotic eggs don’t need to be kept in water, because they have a tough shell and membranes to manage gas exchange without letting the embryo dry out. The reptiles that laid these eggs were less dependent on water than the first tetrapods, who still had to return to the water to lay their eggs. But the amniotes could spend their entire life cycle on land, and they got better and better at it. And they got bigger. The Permian, the last period of the Paleozoic, began 299 million years ago, arthropods, It was the first age that was dominated by land vertebrates -- including the first big vertebrate land predators, like the fin-backed Dimetrodon. If you had a dinosaur-themed coloring book or toy set that featured Dimetrodon as a kid, you should know two things: First, dinosaurs didn’t evolve until after the Paleozoic era, during the Mesozoic era. Dimetrodon is way older than those guys! Second, Dimetrodon was on the same evolutionary branch as today’s mammals, not today’s reptiles and birds -- so it’s more closely related to you than to any dinosaur. It was a member of the group of so-called “mammal-like reptiles” that came before the dinosaurs. Even though they weren’t technically reptiles, it can be a helpful way to think of them. Not mammals yet, but getting there. Dimetrodon was a carnivore, but there were synapsids that ate plants, as well. Like the similar-looking Edaphosaurus, which Dimetrodon probably ate. Plant-eating was its own kind of evolutionary innovation, because herbivores couldn’t really survive until there were enough plants to sustain the animals that ate them. Plus, herbivorous animals had to evolve digestive systems that could extract nutrients from leaves, which is much harder and less energy-efficient than getting all your calories from meat. So plant-eating was another major evolutionary development that happened during the Permian. At the end of the Permian, 251 million years ago, the Paleozoic era ended. And everything else nearly ended along with it. There was a mass extinction event so unimaginably widespread that it’s sometimes called the Great Dying. Something like 90% -- or more -- of Earth’s marine species went extinct. Most of those big synapsids died out. Marine species were hit even harder. Something so awful happened that life nearly met its match. There were ice ages and smaller extinctions throughout the Paleozoic. But this one was the big one. So what was it? What caused the Great Dying? We don’t know for sure. The prime suspect is a plume of lava in present-day Siberia that was deposited 250 million years ago, just when the Permian extinction took place. This was a volcanic eruption of sorts, but if you’re imagining a Vesuvius or Krakatoa, think bigger. A huge plume of heat welled up under Earth’s crust and melted it for hundreds of square kilometers. The region was flooded by enough lava to cover two thirds of the United States. The reason this volcanic plume is such a likely suspect is because it could have done all sorts of life-ending things. It could have caused rapid cooling by blocking out the sun. It could have also set fire to buried coal, releasing carbon dioxide and causing runaway global warming -- there’s evidence for both kinds of temperature extremes. It could have released chemicals into the atmosphere that led to large-scale acid rain, or changed the chemistry of the oceans. We don’t know exactly what those eruptions did, but we know they did something and it probably wasn’t pretty. Other suspects include methane-producing bacteria warming the planet, a catastrophe that somehow got rid of all the oxygen in the oceans, an asteroid impact. The Great Dying could also have been caused by the formation of the supercontinent Pangea -- continents crashing into each other would have destroyed a lot of continental shelf habitat, killing some of the richest parts of the oceans. Having one big continent in one place would also have rearranged ocean currents and altered the climate. But Pangea formed a little too early to account for such a widespread die-out. And all the other hypotheses have their strengths and weaknesses, too -- none of them can explain everything. So, some scientists have suggested what’s known as the Murder on the Orient Express Hypothesis: #spolier like in Agatha Christie’s classic novel, there are multiple culprits. It’s like an exam question where the answer might be “some of the above” or “all of the above.” Whatever the cause, nearly everything died. But a few lucky life forms hung on, clearing the way for the archosaurs, the group that includes the dinosaurs. They were the dominant vertebrates during the Mesozoic era, which we’ll talk about next time. Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!

Contents

Geology

The Paleozoic era began and ended with supercontinents and in between were the rise of mountains along the continental margins, and flooding and draining of shallow seas between.[clarification needed] At its start, the supercontinent Pannotia broke up. Paleoclimatic studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwana (510 million years ago) formed or was forming. By mid-Paleozoic, the collision of North America and Europe produced the Acadian-Caledonian uplifts, and a subduction plate uplifted eastern Australia. By the late Paleozoic, continental collisions formed the supercontinent of Pangaea and resulted in some of the great mountain chains, including the Appalachians, Ural Mountains, and mountains of Tasmania.

Periods of the Paleozoic Era

There are six periods in the Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous (alternatively subdivided into the Mississippian Period and the Pennsylvanian Period), and the Permian.[7]

Cambrian Period

The Cambrian spans from 541 million years to 485 million years and is the first period of the Paleozoic era of the Phanerozoic. The Cambrian marked a boom in evolution in an event known as the Cambrian explosion in which the largest number of creatures evolved in any single period of the history of the Earth. Creatures like algae evolved, but the most ubiquitous of that period were the armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the supercontinent Pannotia begins to break up, most of which later became the supercontinent Gondwana.[8]

Ordovician Period

Cephalaspis (a jawless fish)
Cephalaspis (a jawless fish)

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 Ordovician, 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.[9]

Silurian Period

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.[10]

Devonian Period

Eogyrinus (an amphibian) of the Carboniferous
Eogyrinus (an amphibian) of the Carboniferous

The Devonian spanned from 416 million years to 359 million years ago. 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, which was the Earth's second mass extinction event.[11]

Carboniferous Period

The Carboniferous spanned from 359 million to 299 million years ago. 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).[12] 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.[13]

Permian Period

Synapsid: Dimetrodon
Synapsid: Dimetrodon

The Permian spanned from 299 to 252 million years ago and was the last period of the Paleozoic Era. At the beginning of this period, all continents joined together to form the supercontinent Pangaea, which was encircled by one ocean called Panthalassa. The land mass was very dry during this time, with harsh seasons, as the climate of the interior of Pangaea was not regulated by large bodies of water. Diapsids and synapsids flourished in the new dry climate. Creatures such as Dimetrodon and Edaphosaurus ruled the new continent. The first conifers evolved, and dominated the terrestrial landscape. Near the end of the Permian, however, Pangaea grew drier. The interior was desert, and new species such as Scutosaurus and Gorgonopsids filled it. Eventually they disappeared, along with 95% of all life on Earth, in a cataclysm known as "The Great Dying", the third and most severe mass extinction.[14][15]

Tectonic activity

Geologically, the Paleozoic started shortly after the breakup of the supercontinent Pannotia. Throughout the early Paleozoic, that landmass was broken into a substantial number of continents. Towards the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area.

Climate

The earliest two periods, the Ordovician and Silurian, were warm greenhouse periods, with the highest sea levels of the Paleozoic (200 m above today's); the warm climate was interrupted only by a 30 million year cool period, the Early Palaeozoic Icehouse, culminating in the Huronian glaciation, 445 million years ago at the end of the Ordovician.[16]

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.

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.

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.

Flora

An artist's impression of early land plants
An artist's impression of early land plants

While macroscopic plant life appeared early in the Paleozoic Era 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.[17]

Fauna

A noteworthy feature of Paleozoic life is the sudden appearance of nearly all of the invertebrate animal phyla in great abundance at the beginning of the Cambrian. The first vertebrates appeared in the form of primitive fish, which greatly diversified in the Silurian and Devonian Periods. The first animals to venture onto dry land were the arthropods. Some fish had lungs, and powerful bony fins that in the late Devonian, 367.5 million years ago, allowed them to crawl onto land. The bones in their fins eventually evolved into legs and they became the first tetrapods, 390 million years ago, and began to develop lungs. Amphibians were the dominant tetrapods until the mid-Carboniferous, when climate change greatly reduced their diversity. Later, reptiles prospered and continued to increase in number and variety by the late Permian.[17]

See also

References

  1. ^ "Paleozoic". Dictionary.com Unabridged. Random House.
  2. ^ "Paleozoic". Merriam-Webster Dictionary.
  3. ^ "Paleozoic". Online Etymology Dictionary.
  4. ^ The term "Palaeozoic" was coined by the British geologist Adam Sedgwick (1785–1873) in: Sedgwick, Adam (1838). "A synopsis of the English series of stratified rocks inferior to the Old Red Sandstone – with an attempt to determine the successive natural groups and formations". Proceedings of the Geological Society of London. 2 (58): 675–685. ; see p. 685.
  5. ^ Sahney, S. & Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society B: Biological Sciences. 275 (1636): 759–65. doi:10.1098/rspb.2007.1370. PMC 2596898. PMID 18198148.
  6. ^ http://www.economist.com/node/16524904 The Economist
  7. ^ "The Paleozoic Era". www.ucmp.berkeley.edu. University of California Museum of Paleontology. 2011.
  8. ^ University of California. "Cambrian". University of California.
  9. ^ University of California. "Ordovician". University of California.
  10. ^ University of California. "Silurian". University of California.
  11. ^ University of California. "Devonian". University of California.
  12. ^ Monte Hieb. "Carboniferous Era". unknown.
  13. ^ University of California. "Carboniferous". University of California.
  14. ^ Natural History Museum. "The Great Dying". Natural History Museum.
  15. ^ University of California. "Permian Era". University of California.
  16. ^ Munnecke, A.; Calner, M.; Harper, D. A. T.; Servais, T. (2010). "Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology. 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001.
  17. ^ a b Sahney, S.; Benton, M.J. & Falcon-Lang, H.J. (2010). "Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica" (PDF). Geology. 38 (12): 1079–1082. Bibcode:2010Geo....38.1079S. doi:10.1130/G31182.1.

Further reading

External links

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