This was the first and longest era of the phanerozoic eon, covering the first half of of the phanerozoic, from about 542 - 251 million years ago. It started with the breakup of Pannotia, a short lived supercontinent that existed after the parts that made up Rodinia broke apart and briefly combined again, and ends with the formation of Pangaea, the last great supercontinent.
This was a very eventful period for life on Earth, beginning with the appearance of most modern phyla, major radiations of various groups of organisms. There were major radiations of several groups of organisms. The first land plants appeared, and eventually became vast forests. There was great diversity and experimentation. And then it all ended abruptly, with the largest mass extinction of life that the Earth has ever known.
The paleozoic is divided into the following periods.
This is the first period of the first era (paleozoic) of the phanerozoic eon. The beginning of the Cambrian is somewhat controversial, and different geologists place it at different points in time. The International Commission on Stratigraphy decided to date it to 542 ± 0.3 million years ago, based on a carbon excursion that can be precisely dated to that time. It ends about 490 million years ago.
As mentioned earlier, multicellular life had started to radiate prior to the Cambrian, in the Ediacaran period of the neoproterozoic. However, this life appears to be somewhat different from the forms found in the early Cambrian. Partly for this reason, it is believed that there was an extinction event at the beginning of the Cambrian. Evidence for this extinction event is also found in the carbon excursion that is used to date the beginning of the Cambrian. It appears that many of the earlier forms of life that appeared in the Ediacaran suddenly disappear at the beginning of the Cambrian, soon to be replaced by other (equally complex) life forms. Even some very ancient life forms (such as stromatolites - colonies of cyanobacteria) which had existed for billions of years, in great profusion, almost vanished in this extinction event, the cause of which is not known.
Trilobites were very common in the Cambrian
The first hard shelled animals appeared during the Cambrian, around 530 million years ago. These are trilobites, a now-extinct class of arthropods, as well as the first crustaceans and mollusks. Note that arthropods may have appeared even earlier. There is some speculation that Parvancorina and Spriggina, elements of the Ediacaran biota about 550 million years old, were also arthropods. Hard shelled organisms fossilize well, hence the profusion of trilobites in the fossil record. The first coral reefs probably appeared in the Cambrian, as well as the first vertebrates, such as Myllokunmingia.
However, there are places where soft-bodied Cambrian fauna have been well-preserved, such as the Burgess Shale, which is about 505 million years old, around the middle Cambrian. Some of the early Burgess Shale finds provoked much controversy, since they seemed to represent animals completely unlike any that are found today, and with very awkward designs. There were examples such as Opabinia, with 5 eyes, a backward facing mouth underneath its head, and a proboscis extending forward from the head to end in a spined claw. Another example was Hallucigenia, which was supposed to have rows of rigid spines on which it walked, with tentacles waving at the top. It was bizarre reconstructions like these which led authors like Stephen Jay Gould to think that the Cambrian was a time of great experimentation due to unusual evolutionary pressure, and that most of these "experiments" failed and did not lead to any modern phyla.
More recent studies have cast doubt, both on the reconstructions, and on the theory. Hallucigenia is not so strange if it's reconstructed upside down - the "tentacles" then become legs for walking, and the rigid spines possibly a defensive mechanism, similar to many later organisms. Opabinia has also been found to be closely related to arthropods, as have many other seemingly bizarre animals from the Cambrian.
So while it remains true that there was a high degree of diversity during the Cambrian, it is less certain if this was anything exceptional, when we find similar degrees of high diversity during the Ediacaran immediately preceding it, as well as during the Ordovician radiation that followed.
Most of Cambrian life evolved and lived solely in the shallow seas that were formed as the supercontinent Pannotia split apart. There is no generally accepted evidence of any life on land at the time. It is possible that some regions of land did have some sort of microbial "scum", consisting of bacteria, algae, or lichens. Such microbial land cover may have evolved even before the Cambrian, though there is little direct evidence of it. Without plants, soils cannot exist. Land would have been either barren rock, with weathered patches of sand. Sand is not capable of holding water. However, films of cyanobacteria have been found even in modern deserts, so it seems that something similar could have existed long ago, before there were any plants. Cyanobacteria, algae and lichens (symbiotes, consisting of a fungus with a photosynthetic partner such as cyanobacteria) may have existed on land in very ancient times.
The end of the Cambrian was marked by another extinction event, which can be dated to approximately 488.3 ± 1.7 million years ago. This marks the beginning of the Ordovician period, which lasted about 44.6 million years, up to about 443.7 ± 1.5 million years ago. The end of the Ordovician is also associated with a major extinction event, which wiped out about 60% of the existing genera at the time.
Most of the Earth's landmass was still in the southern hemisphere at the time, and the climate was warmer than today. There were numerous shallow seas, and a great deal of sedimentary rock dating to this period still exists today. Towards the end of the Ordovician there were some glaciations, so sea levels rose and fell accordingly, but were generally higher than today.
Artist's Impression of Life in Ordovician
Ordovician fauna was very diverse. There was a significant radiation of life during the Ordovician, representing about 12% of all phanerozoic fauna. There were 4x as many marine organisms as during the Cambrian.
Trilobites diversified rapidly during the Ordovician, reflecting the increasing evolutionary pressure of co-evolving organisms. Some trilobites developed ridges and spines as a defensive measure, others started swimming for the first time, instead of just crawling along the sea floor. Cephalopods (from which the modern Octopus is descended) and crinoids developed first during the Ordovician. Other forms of marine life from this period include primitive nautiloids and sharks, the first mosses (bryozoa), and the first jawed fish.
Two important developments of this period include the great profusion of shell secreting organisms (which sequester carbonate in their shells) and the first appearance of land plants. Plant spores dating from the late Ordovician have been found. It is uncertain what the nature of these first plants was. It seems likely that they were some sort of avascular plants, such as moss (bryozoa), though it is possible that marine fungi first colonized the land, in the form of lichens, which are a symbiotic combination of a fungus and some photosynthetic algae.
Towards the end of the Ordovician there was a series of glaciations, which probably led to the end-Ordovician extinctions. These were fairly severe, causing the loss of about 50-60% of all existing genera. It is thought that a series of glaciations raised and lowered sea levels repeatedly, severely affecting the many shallow seas where life flourished. These glaciations are linked with a lowering of atmospheric carbon dioxide. It is uncertain if life had any role in this, though the diversification of carbonate-shell secreting organisms during the Ordovician seems suggestive.
The Silurian began after the end-Ordovician extinction event, about 443.7 ± 1.5 million years ago, and ended about 416 ± 2.8 million years ago. This was a period of recovery after the extinction, during which the glaciers largely retreated, the climate was warm, and there was a minor greenhouse effect going. The climate was relatively stable, unlike previous ages which had been marked by large fluctuations. The warm climate resulted in the melting of glaciers, and therefore sea levels were relatively high.
The split up of Rodinia was far advanced in the Silurian. Land consisted of the supercontinent of Gondwana in the southern hemisphere, surrounded by about 6 smaller continents. The northern hemisphere was mostly a single large ocean. There was no major volcanic activity during the Silurian, although the Caledonian orogeny (which had begun earlier, at the start of the Ordovician), was in full swing.
Silurian biota include the first coral reefs, the first bony fishes, and the first fishes with movable jaws. Arthropods grew to huge sizes, specially the Eurypterids (sea scorpions), which grew to sizes of 6-7 feet, and must have been formidable predators. The earliest common evidence of life on land (arachnids and centipedes) dates from this period, though some arthropods may have colonized land much earlier (in the Cambrian). The earliest recognizable shark scales are from the Silurian. The first leeches appeared also at this time.
On land, the first record of vascular plants dates to this period. There were probably extensive "forests" of mosses, lichens, and the early vascular plants (which show the first signs of xylem and phloem), such as Cooksonia, Baragwanathia, and Psilophyton.
The Silurian ended with a series of minor extinction events, probably due to climate change or impact events.
The Devonian period lasted from about 416 ± 2.8 million to 359.2 ± 2.5 million years ago, a duration of almost 57 million years. This was a relatively warm period, glaciation was minor or non-existent, with consequently high sea levels. A lot of land was submerged beneath the water, forming shallow seas. This was a period of high tectonic activity, as the continents were moving closer together in a process that would eventually lead to the formation of Pangaea. Although the climate was warm, carbon dioxide levels fell through the Devonian, as growing forests of plants on land locked away carbon and became buried (there are oil and gas deposits found today in some Devonian rocks).
Artist's Impression of Devonian Forests
There were significant developments in the Earth's biota during the Devonian. The first vascular plants, which had appeared earlier during the Silurian, now spread across much of the land. In the early Devonian, these plants were quite small, probably no more than a meter in height. In the late Devonian, plants such as lycophytes, sphenophytes, ferns and progymnosperms appeared, which were much larger. Many of these plants had true roots and leaves, which were not a feature of earlier plants. The landscape was probably dominated by huge ferns, and contained many other strange plant-forms, such as the giant fungus Prototaxites (the tall tree-like things in the accompanying artists impression, with tree-like trunks and branches), which formed trunks as wide as a meter and grew to heights of nearly 30 feet.
Towards the end of the Devonian, the seed-bearing plants (spermatophytes) and the first real trees appeared, such as the progymnosperm Archaeopteris, which was probably among the first plants with true wood, thus being a tree. At the end of the Devonian, huge forests existed throughout the land masses, unmolested by land herbivores, which had not yet developed. This enormous diversity of plant life that appeared in the late Devonian is sometimes known as the "Devonian Explosion".
A recent discovery of footprints in what is now southeastern Poland indicates that the colonization of land by animals began fairly early in the Devonian. These footprints have been dated to about 397 million years old, and include a group of animals called tetrapods. Tetrapods developed in the shallow seas long before they walked on land. It is believed that the coasts of such seas, as well as freshwater swamps, formed rich ecosystems, as plants colonized the land. Tetrapods probably evolved in such habitats close to the water's edge, where the profusion of plants made limbs useful in moving around in the underwater clutter of roots, stems, and decaying plant matter. The first tetrapods were probably completely aquatic, and only later did they develop the ability to survive outside the water. These were probably shallow, tidal marine environments, where water surged and retreated with the tides, and it was beneficial to be able to both swim and walk. Some of the tetrapods represented by the footprints from Poland show that they grew up to 10 feet in size.
Devonian fishes, including sharks, ray-finned fishes, placoderms.
It's important to remember that although tetrapods were the first vertebrates to colonize land, arthropods had already done so much earlier. The earliest evidence of life on land goes as far back as the Cambrian. Track ways are found in what must have been wet coastal sand, of a burrowing organism known as Climactichnites, which was possibly an arthropod. During the early Devonian, the early land vegetation (mostly tiny shrubs and plants, many without root systems and leaves, some without vascular systems), probably provided ecosystems for various types of arthropods and the first true insects (which appeared at the beginning of the Devonian, about 426 million years ago) - such as mites, wingless insects, etc. These early land insects are not well known, but they appeared in the early Devonian, whereas the first land vertebrates did not appear until the end of the Devonian. The development of trees with true roots during the late Devonian led to the creation of the first soils, which were probably colonized by burrowing and crawling insects and arthropods. These plants, soils, and insects formed rich ecosystems, which were probably necessary for the beginning of the movement of the first vertebrates to the land, at the end of the Devonian.
A recently discovered fish fossil, showing a fish embryo attached to its mother through an umbilical cord, shows the first evidence of animals giving live birth to their young. This has been dated to about 375 - 380 million years old, the late Devonian. This fish was a placoderm, a kind of armored fish that was very common in the middle paleozoic. This particular species was about 10 inches long, though other placoderms grew up to 20 feet in size. Placoderms became extinct at the end of the Devonian.
The Devonian ended with two extinction events. The earlier event is dated to about 364 million years ago, when most of the fossil agnathan fishes disappeared. A second wave of extinctions followed soon after. These extinctions primarily involved the marine environment, and primarily the warm and shallow seas where life was profuse. Land plants/animals and deep/cool water fishes were much less affected. These extinction events marked the end for many genera, being more severe than the end-Cretaceous extinction that wiped out the dinosaurs. The cause of the extinction remains unknown, though various theories have been proposed (such as asteroid impact, loss of atmospheric carbon dioxide as it was locked up by forests). A cooler climate at the end of the Devonian, with extensive glaciation, is taken to be the probable cause of the extinction.
This period extends from about 359.2 ± 2.5 to about 299 ± 0.8 million years ago, a length of about 60 million years. The period is named after "carbon" or coal, since huge deposits of coal dating back to this period have been found all over the world. This period is typically divided into two epochs, the earlier Mississippian, and the later Pennsylvanian.
The dip in temperatures at the end of the Devonian quickly reversed at the start of the Carboniferous, and for at least the first half of the Carboniferous, the climate was quite warm. However, about 320 million years ago, there was a sudden precipitous drop in temperature, the onset of the Permo-Carboniferous glaciation. This point, which is marked by the division of the Mississippian and Pennsylvanian epochs, is seen in the geological record as a minor extinction event, which hit the crinoids and ammonites specially hard. For the rest of the Carboniferous, the climate was much cooler, and shows a record of repeated glaciations. However, the tropics continued to remain warm and tropical coal-generating forest continued to flourish.
Carboniferous Coal Forests
The extensive formation of coal from this period is somewhat puzzling. Coal forms when hydrocarbon material (dead plants and trees) are buried and not decomposed. Over time, pressure and heat turns them into coal. In modern times, this is not so common, because various organisms quickly decompose dead organic matter.
There were probably many reasons for coal formation during this period. One is the development of trees with bark. Bark is composed of lignin, a chemical compound found in cell walls, which probably first evolved in the Carboniferous. Barked trees were very common in the Carboniferous, and they contained a lot of bark. Bark to wood ratios as high as 8:1 were common in trees of this period, and ratios as high as 20:1 have been found for some trees. This contrasts with ratios of about 1:4 for modern trees, so we can see that trees in this period had up to 80 times more bark per volume of wood than trees today.
Lignin is toxic, and inhibits the decay of organic material. It is likely that microbes that can decompose lignin had not evolved at the time. Even today, few organisms other than Basidiomycitic fungi can digest lignin. It probably evolved to protect the trees from insect herbivores, which were the dominant herbivores at the time. This was also a time before effective insectivore animals had developed, so the populations of such insect herbivores much have been very high.
Another factor may have been the late Carboniferous glaciations, and the consequent fall in sea levels. This produced huge lowland swamps in Europe and North America, where it is especially easy for dead vegetation to get buried quickly.
The burial of vast forests and swamplands produced a surplus of oxygen in the atmosphere, which may have peaked as high as 35% (compared to 21%) today. This led to insect and amphibian gigantism, since insects specially are size limited by atmospheric oxygen concentrations, as their respiratory system does not allow for the diffusion of gases very efficiently at large mass to surface area ratios.
As the continents continued to move closer together, in process of forming Pangaea, the shallow seas that had separated continents began to shrink. Much of the marine environment of previous periods had consisted of such shallow seas and extensive shorelines. This had the result of increasing the amount of dry land available as habitat, at the expense of shrinking marine environments. The trend was further aggravated during the later Cambrian, as glaciation locked way water and decreased sea levels. This may have been the reason for the evolution of land vertebrates, which first became fully terrestrial during the Cambrian.
Land vertebrates such as amphibians were very common in the Carboniferous, and much more diverse than they are today. Some grew to large sizes, as long as 20 feet, though most were smaller. The amphibians were partly aquatic, but many fully terrestrial animals also developed in this period, often with scaly skins to prevent dehydration and protect them. The development that made terrestrialism possible was the evolution of the amniote egg, with its several protective membranes, that allowed the eggs to survive and hatch on land. Amphibians are not amniotes - their eggs require water, so the amphibian lifestyle requires close association with bodies of water. The amniote egg may have developed as early as 340 million years ago, near the beginning of the Carboniferous, as evidenced by fossils of Casineria, a small lizard like creature, about 6 inches long. Casineria may have been the first amniote, and therefore the first known vertebrate to adopt a fully terrestrial lifestyle. It is sometimes placed in the stem group protosauria (or "first lizards"), which includes some amphibians as well as early reptiles.
Early in their history, the amniotes split into two branches: the synapsids (proto mammals) and the sauropsids (proto birds/reptiles). This split happened about 320 million years ago, probably near the beginning of the Permo-Carboniferous glaciations, and the beginning of the Pennsylvanian epoch. Hylonomus, the earliest confirmed reptile, dates back to about 315 million years ago. It was about 6-8 inches in size, and probably ate insects and centipedes. The earliest undisputed synapsid was Archaeothyris, a somewhat larger lizard like creature (about half a meter in length), dated to about 306 million years ago.
By the end of the Carboniferous, both synapsids and sauropsids had diversified into a number of groups and spread across the land, possibly in response to the drier climate of the late Carboniferous.
Insects continued to proliferate during the Carboniferous. The high concentrations of atmospheric oxygen (35%) led to gigantism, as seen in Meganeura, a giant dragonfly-like insect with a wingspan of over 2.5 feet. Other groups to emerge included the ancestors of mayflies, and the ancestors of cockroaches.
The marine environment also greatly diversified during the Cambrian. The foraminifera first became prominent in the marine fauna. Echinoderms, radiolaria, sponges, brachiopods, annelids, gastropods and cephalopods were all numerous among marine invertebrates. Trilobites also existed, but were less common than in earlier periods. Aquatic vertebrates also diversified, in both freshwater and marine environments. Some were very large, such as the rhizodonts, which grew up to 20 - 25 feet, making them the largest freshwater fish ever known.
Sharks had a major evolutionary radiation in the Carboniferous, probably due to the extinction of the placoderms at the end of the Devonian. Sharks probably took over the different niches that had been previously occupied by placoderms.
Among land plants, many earlier Devonian lineages continued, but new plants also appeared. Horsetails and cycads first appeared. Seed ferns and other early gymnosperms continued to flourish, as did various lycophytes. Towards the end of the Carboniferous, the first conifers appeared, usually situated away from the water, in higher, drier ground.
This was the last period of the Paleozoic, and dates from about 299 ± 0.8 to 251 ± 0.5 million years ago, a duration of about 48 million years. It ends with the Permian-Triassic extinction event, probably the worst extinction event in the history of life on Earth.
The Permian saw the completion of the supercontinent Pangaea, which reached its maximum extent about 225 million years ago, though it lasted in some attenuated form for another 100 million years. During the Permian, most of the Earth's land was assembled in a giant C-shaped continent straddling the equator. This assembly of landmasses into one large group drastically reduced the coast lines, decreasing the available shallow marine habitat.
The middle of the "C" shape was the Tethys Sea, and the remaining globe was covered by a single large ocean, known as the Panthalassic Ocean. The large continental landmass meant that the interiors of continents were very arid, and probably large deserts existed. Temperature fluctuations in the interiors of continents must also have been extreme. Monsoon conditions (with highly seasonal rainfall) probably prevailed over much of the land.
The Permian started with the ice age at the end of the Carboniferous. The ice age ended rapidly, and for much of the rest of the Permian, the climate was warm and dry, with alternating warming and cooling periods. Towards the end, the Permian was probably about 60% hotter than today, due to volcanic activity producing greenhouse gases.
The equatorial regions, specially near the Tethys Sea still contained large amounts of swamp land. This area is now the southern region of China, where large Permian deposits have been found. However, further away, conditions were more extreme, and favored the evolution of conifers. This happened somewhere around the middle of the Permian, and conifers quickly spread over much of the inland areas. Many modern trees, such as gingkoes and cycads originated in this period.
Pangaea, about 225 million years ago.
Roaches prospered greatly in the Permian. They were well adapted to the conditions, with an omnivorous digestive system, a gizzard, and sophisticated mouth parts. About 90% of Permian insects were roach-like. Flying insects were dominated by huge predatory dragonflies. Gigantism continued, with species of dragonflies achieving wing spans of over 2 feet. Other important new insect groups that evolved during this period were the beetles and flies.
Land vertebrates included both synapsids and sauropsids. The early Permian was dominated by amphibians and pelycosaurs, which were a class of synapsids. The pelycosaurs varied in size from a few inches up to ten feet and more, eventually giving rise to the therapsids in the middle Permian. Towards the end of the Permian, a branch of the therapsids known as the cynodonts evolved, which would later give rise to mammals in the Triassic. Initially, the sauropsids were not as successful as the synapsids, but towards the end of the Permian, they gave rise to diapsids, such as archosaurs. The coming end-Permian extinction was to change matters drastically, with the synapsids losing their dominance, and the descendents of the archosaurs (dinosaurs, crocodiles, other reptilians) becoming the dominant vertebrates for the next hundred million years, and more.
The Permian ended with the most catastrophic extinction that life on Earth has ever seen - the Permian-Triassic extinction event, which happened about 251 million years ago. About 90-95% of all marine species and about 70% of all land species became extinct. It's thought that about 99.5% of all living organisms died during this event, leaving behind only about 0.5% to populate the earth in the Triassic. Many ancient lineages, which had survived for hundreds of millions of years, such as trilobites, disappeared after this extinction. Indeed, so many species died out that instead of enumerating what perished, it will be easier to list what remained, in the next section on the Triassic.
The cause of the Permian-Triassic extinction is not known. Some of the more popular hypotheses are:
Volcanism in Siberia
Massive flood basalt eruptions in Siberia which continued for nearly a million years, and formed what is today known as the Siberian Traps, may have been the cause. The eruptions may have caused a nuclear winter scenario which lasted for several years, coinciding with major eruptions. Some of these eruptions produced up to 2000 cubic kilometers of lava, and there were many of them, over the course of hundreds of thousands of years. By contrast, the largest eruption in recorded history, the eruption of Mount Tambora in 1812, only produced about 160 cubic kilometers of ejecta. The eruption of Vesuvius in 79 AD, which wiped out Pompeii and Herculaneum only produced 4 cubic kilometers of ejecta. The eruption of Thera in the second millennium BC (which ended the Minoan Civilization) produced about 60 cubic kilometers of ejecta. The aftermath of the Siberian volcanism may have elevated global temperatures by as much as 5 °C due to greenhouse gases, which is an extreme amount if it happens over a short period.
Deep Sea Methane
This is actually an extension of the Siberian volcanism theory. The rise in temperature caused by the volcanism (about 5 °C) might not be enough to cause such a severe extinction as this, but it might be enough to warm the oceans sufficiently to melt the methane hydrate reservoirs at the bottom of the oceans. Methane is one of the most powerful greenhouse gases, and a massive release of methane would lead to a severe greenhouse effect. This theory is supported by the finding of increased carbon-12 levels in the middle layers of deposits from the end-Permian event, and the particular sequence of extinctions (land based extinctions, followed by marine extinctions, followed by more land based extinctions).
Ocean Venting of Hydrogen Sulfide
The deep sea is a relatively anoxic zone, and periodically, it loses almost all of its oxygen. At such times, bacteria produce large amounts of hydrogen sulfide, which accumulates in the depths. This hydrogen sulfide can be released suddenly into the atmosphere. This can cause extinctions in several ways. Hydrogen sulfide itself is toxic to aerobic organisms. Once in the atmosphere, it is rapidly oxidized, depleting oxygen in the process, leading to lowered levels of oxygen in the atmosphere. Finally, it can destroy the ozone layers, exposing the Earth to the Sun's ultraviolet radiation.
The Wilkes Land crater in the Antarctica has been dated to roughly 100 - 500 million years old. Since this is roughly (very roughly!) the period of the end-Permian extinction, some people have wondered if there might be a relationship. This theory seems somewhat weak, because the amount of iridium and fractured quartz at the boundary is significantly smaller than the amount found at the extinction event 65 million years ago (when the dinosaurs died). Since the end-Permian extinction was by far the larger extinction event, one would expect more, not less signs of the impact event compared to the later extinction event. However, there can be doubts, since the end-Permian is much older, and the signs of impact are probably more obliterated by time. The crater is certainly larger (about 500 km) than the one associated with the more recent extinction (Chicxulub crater, 180 km), which is expected. But there are other problems with the theory as well, such as fossils in Greenland, which show that the extinction event lasted nearly 80,000 years, far too long to be caused by a single catastrophic event such as an asteroid.
Most scientists believe that the extinction was caused by a combination of some of the theories listed above, together with ongoing effects such as the shrinking of coastlines due to the formation of Pangaea, the change in climate, etc. There are other, more speculative theories as well, such as a nearby supernova in the Milky Way.