12.7: Late Paleozoic Life - The Carboniferous and Permian

The Carboniferous and Permian: Tetrapods and Plants Take Over the Land

The next time period, called the Carboniferous (North American geologists have subdivided this into the Mississippian and Pennsylvanian periods), saw the highest levels of oxygen ever known, with forests (e.g., ferns, club mosses) and swamps dominating the landscape. This helped cause the largest arthropods ever, like the millipede Arthropleura, at 2.5 meters (6.4 feet) long! It also saw the rise of a new group of animals, the reptiles. The evolutionary advantage that reptiles have over amphibians is the amniote egg (egg with a protective shell), which allows them to rely on non-aquatic environments for reproduction. This widened the terrestrial reach of reptiles compared to amphibians. This booming life, especially plant life, created cooling temperatures as carbon dioxide was removed from the atmosphere. By the middle Carboniferous, these cooler temperatures led to an ice age (called the Karoo Glaciation) and less-productive forests. The reptiles fared much better than the amphibians, leading to their diversification. This glacial event lasted into the early Permian.

Figure \(\PageIndex{1}\): A reconstruction of the giant arthropod (insects and their relatives) Arthropleura.

By the Permian, with Pangea assembled, the supercontinent led to a dryer climate, and even more diversification and domination by the reptiles. The groups that developed in this warm climate eventually radiated into dinosaurs. Another group, known as the synapsids, eventually evolved into mammals. Synapsids, including the famous sail-backed Dimetrodon are commonly confused with dinosaurs. Pelycosaurs (of the Pennsylvanian to early Permian like Dimetrodon) are the first group of synapsids that exhibit the beginnings of mammalian characteristics such as well-differentiated dentition: incisors, highly developed canines in lower and upper jaws and cheek teeth, premolars and molars. Starting in the late Permian, a second group of synapsids, called the therapsids (or mammal-like reptiles) evolve, and become the ancestors to mammals.

Figure \(\PageIndex{2}\): Reconstruction of Dimetrodon.

Invertebrates Learn to Fly

Coevolution among insects and with other animals, coevolution with plants, adaptation to freshwater, and variable predation in larval versus adult forms are just some of the means by which insects have spread across the terrestrial realm. Perhaps most importantly though was the adaptation to the sky: arthropods are the only phyla of invertebrates to develop wings and flight. The first flying insects show up in the fossil record of the Carboniferous. While there is still debate in scientific circles, it is generally assumed wings represent another modification of appendages that were given a molecular and mechanical boost from high oxygen levels in muscles achieved via tracheae. The development of flight also corresponds to increases in atmospheric oxygen on a global level during the Carboniferous as vascular plants diversified, grew into large populations, and pumped out oxygen as a byproduct of photosynthesis. This rapid diversification of insects into all available ecological niches is known as an adaptive radiation, and this pattern is repeated by other terrestrial animal groups later in geologic time. Today, there are about one million described insect species, representing almost 75% of all known animals.

Figure \(\PageIndex{3}\): Large fossilized Meganeura dragonfly. (Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.)

Tetrapod Evolution: Introducing Amphibians

The first fully terrestrial tetrapods transitioned to land in the Mississippian (early Carboniferous) period and were the ancestors of today’s amphibians (although modern amphibian groups generally evolved during the Triassic). The amphibians were able to breathe air and live on land but still needed water to nurture their soft eggs. The eggs of these animals must be laid in water because they do not have any sort of protective outer coating to protect the egg from drying out. Additionally, early life stages after hatching (for example, a tadpole stage in frogs) require water until the organism develops enough to survive on land.

Figure \(\PageIndex{4}\): Skeleton of Eryops, an early large carnivorous amphibian, from the Smithsonian’s National Natural History Museum. (Image by Daderot via Wikimedia.)

Other structural changes to the vertebrate body plan were also required for success on land. The term "tetrapod" refers to animals able to walk on four feet. Without the buoyancy provided by water, the bones of tetrapods would need to be more robust to provide more structural support to the body and allow for larger muscles to develop to aid in locomotion. Bony, lobe-finned fish (those in the sarcopterygian class) possess the basic vertebrate body plan in their appendages, and this same musculoskeletal template in their ancestors would have allowed for the evolution of muscular arms and legs. Specifically, sarcopterygian fins exhibit the same sequence of bones as all other tetrapod limbs: one bone at the point of body attachment, followed by a joint, two bones, another joint, then lots of little bones.

Figure \(\PageIndex{5}\): Comparison between the fins of A., Crossopterygiian fishes and the legs of B., tetrapods. Bones which correspond to each other have the same color. (Public domain image.)

Other changes are not particularly well-fossilized but also would have been key to success on land: 1) development of a three-chambered heart and more complex lungs to extract oxygen from air, 2) continual modification to auditory systems to allow for better hearing in air, and 3) development of more waterproof skin to retain tissue moisture. By the Mississippian (early Carboniferous) period, tetrapods had evolved into two main groups, amphibians and amniotes, from a common tetrapod ancestor.

Rise of the Amniotes: Origination of Reptiles

Towards the end of the Paleozoic, as Pangea assembled and the climate dried out, amphibians were at a disadvantage. A new evolutionary leap occurred that allowed some animals to lay waterproof eggs on land, something their amphibian relatives were unable to do. This new group was the amniotes: a clade that includes reptiles (including dinosaurs and birds) and mammals. Exactly which animal qualifies as the first amniote is a subject of debate; Casineria, first described in 1999 from fossils from Scotland is a good candidate (although its exact classification is controversial). If this animal scurried in front of you right now, you would probably identify it as a reptile, even though it would not be considered a true reptile.

Figure \(\PageIndex{6}\): Casineria reconstruction (by ДиБгд via Wikimedia.)

The amniotic egg was an important evolutionary innovation that allowed tetrapods to expand out of terrestrial aquatic environments. An amniotic egg protects a developing embryo and its required nourishment (yolk sac) within several protective layers. These layers provide protection from environmental elements, and also allow for the transmission of gases needed for respiration and a place to store metabolic wastes. Whether the outermost shell layer is leathery or more hardened, this change to eggs allowed tetrapods to spread out into different inland environments, further away from permanent water sources.

Figure \(\PageIndex{7}\): Modern chicken egg showing structures of amniotic egg. (Creative Commons Attribution-Share Alike 4.0 International license.)

This ability to exploit more terrestrial environments allowed amniote groups to diversify rapidly and persist for millions of years, with natural selection aided by the constant shift of tectonic plates and resulting climate patterns. During the Late Carboniferous, two distinct lineages of tetrapods arose: the sauropsids and synapsids. Within the sauropsid clade, an enormous diversity of body forms and life habits arose, particularly after the End-Permian Mass Extinction cleared out a variety of ecological niches and allowed reptiles to undergo an adaptive radiation. This lineage leads to reptilian groups like crocodilians, turtles, snakes, dinosaurs, and dinosaurs’ cousins, birds.

Amniotes Branch Off: Synapsids and Mammals

The Late Carboniferous synapsids began with a reptilian-like, low-slung body plan, and a distinctive single opening in the skull behind the eye socket. Many of the larger early members of the synapsid group such as Dimetrodon and Cynognathus were greatly affected by the end-Permian and end-Triassic mass extinctions. Eventually, changes within this clade led to the various groups of mammals we see today, including monotremes (egg-laying mammals, like platypus), marsupials (pouched mammals, like koalas and kangaroos) and placental mammals (most mammals, including such diverse groups as cows, whales, cats, and humans).

Figure \(\PageIndex{8}\): Cynognathus, a cynodont from the Triassic of South Africa, pencil drawing by N. Tamura. (Creative Commons Attribution-Share Alike 3.0 Unported license.)

Synapsid means fused arch, referring to the hole near the base of the skull. This hole in the synapsid skull is correlated to the temple in your skull. The first group of synapsids to diversify and specialize were the pelycosaurs. The pelycosaurs were the dominant and largest group of tetrapods in the Permian, and were formerly known as the mammal-like reptiles. However, since they are not true mammals or reptiles, this name has fallen out of favor.

Figure \(\PageIndex{9}\): A typical synapsid skull, this of Dimetrodon. Note the hole toward the base of the skull, here seen as the furthest left hole above the jaw line. (Photo from the University of Michigan Natural History Museum by Daderot via Wikimedia.)

The most famous member of this group is Dimetrodon. Its name means ‘two-measure tooth’ because it had different teeth for different uses. This was one of the earliest animals found in the fossil record that have differentiated teeth for different purposes, which is a hallmark of mammals. It is more closely related to humans than dinosaurs. Though it is often portrayed with dinosaurs in kids’ books, it lived 40 million years before them. It lived in the Permian, and the first dinosaurs were Triassic. From track sites, it has been hypothesized that Dimetrodon could do a ‘high walk’ similar to what crocodiles can do, which is more efficient.

Figure \(\PageIndex{10}\): Dimetrodon reconstruction. The posture is controversial, but here shown upright, based on possible Dimetrodon trackways. (Reconstruction by Max Bellomio via Wikimedia.)

One of the branches of the pelycosaurs evolved into a new form called therapsids. These replaced pelycosaurs as the dominant land animals toward the end of the Permian. The therapsids had several advantages over their less derived pelycosaur relatives, features more akin to their modern mammal descendants. They had a fully-upright stance, with their limbs always beneath the body. There is also circumstantial evidence that they were endothermic (commonly known as warm-blooded) and had at least some hair. Scientists have found whisker pits in the skull, which is good evidence of hair. Examples of therapsids include the ferocious Gorgonops, one of the first saber-toothed animals.

Figure \(\PageIndex{11}\): Gorgonops, a large Permian predatory therapsid. (Reconstruction by Dmitry Bogdanov via Wikimedia.)

End-Permian Mass Extinction

The end-Permian mass extinction was the most extreme of any in Earth history. It’s sometimes dubbed “The Great Dying,” with 62% of marine genera going extinct, as well as severe impacts among terrestrial biota. Perhaps only 17% of species on Earth survived it. It’s the sharp cliff at the end of the “Paleozoic Plateau.” It’s the closest our biosphere has ever come to being deleted. Writer Peter Brannen astutely called it “the worst thing that ever happened.”

By the end of the Paleozoic, life had rebounded from the late Devonian extinction. In the sea, crinoids and their cousins the blastoids were having a heyday. Tabulate and rugose corals had made great progress in building up reefs in the late Paleozoic ocean, reefs that almost certainly served as shelter and nurseries for many other species of marine invertebrates. Giant single-celled fusilinid foraminiferids were rolling around the seafloor like fat grains of rice.

Figure \(\PageIndex{12}\): Fusulinid foram fossil from Permian limestones in west Texas. (Callan Bentley photo.)

On land, the Glossopteris seed fern reigned supreme, at least over the southern supercontinent Gondwana. In its shade, the waterproof amphibian-descendants called reptiles had diversified and spread into a great number of ecological roles. Dominant among them were the synapsids, those that used to be dubbed “mammal-like reptiles.” In some cases, these animals could grow to large sizes. You would not have wanted to meet Dimetrodon in a dark alley. The sail-like “fin” rising from its back is its most eye-catching feature, but its mouth was full of sharp teeth.

Figure \(\PageIndex{13}\): Glossopteris sp. Late Permian, Australia. At the Royal Tyrrell Museum of Palaeontology. (Bloobityboop via Wikimedia, licensed under CC BY-SA 4.0.)

Gorgonops and Dimetrodon went extinct at the end of the Permian, along with 2/3 of therapsids. Some cynodonts survived, and these diversified during the Mesozoic that followed, some of them ultimately leading to you and me. One that practically overran the world in the early Triassic was Lystrosaurus, whose populations rose unchecked after the the demise of all its predators. But in the Mesozoic, it was the sauropsids that diversified most successfully, and led to dinosaurs, plesiosaurs, ichthyosaurs, pterosaurs, and mosasaurs. They were the big winners from the end-Permian extinction, but there were so many losers.

Figure \(\PageIndex{14}\): Lystrosaurus murrayi, about the size of a small dog. (CC-BY; Sketch by Dmitry Bogdanov, via Wikimedia.)

There were two pulses to the end-Permian extinction. Because they were separated by ~8 million years, it has become commonplace in recent years to hear them discussed independently: the Capitanian (or ‘end-Guadalupian’) at 260 Ma and the true end-Permian event at 252 Ma. Both appear to have the same general cause: global warming.

Figure \(\PageIndex{15}\): El Capitan and the Guadalupe Mountains of west Texas, a Permian reef complex and the namesake of the Guadalupian epoch of the Permian period. (Public domain; NPS photo.)

The careful study of variations in oxygen isotopes in strata spanning the Permian-Triassic boundary shows an lightening of the oxygen content of the oceans. This correlates with a big boost in heat energy, making \(\ce{^{18}O}\)-based water molecules as likely to evaporate as \(\ce{^{16}O}\)-based water molecules. Using the ratio of \(\ce{^{16}O}\) to \(\ce{^{18}O}\), it has been calculated that the global ocean warmed by about 6 \(^{\circ}\)C. As with the discussion of ‘cooling by 5 \(^{\circ}\)C’ in the end-Ordovician extinction above, please realize that it takes a truly enormous amount of energy to shift ocean temperatures by six whole degrees Celsius. That extra energy would have been radiated to space, had it not been trapped by extra \(\ce{CO2}\) in the late Permian atmosphere.

Now let’s consider where that extra \(\ce{CO2}\) came from, and then examine the havoc it caused in the ocean. There were two major large igneous provinces that erupted during the late Permian. The first was in China: the Emeishan Traps, where “Traps” derives from the Swedish word trappa, meaning “stairsteps.” This refers to the step-like landscape where the many layers of a vast flood basalt province get etched out. (The Deccan Traps in India have a similar morphology, as do the lava layers of the Columbia Plateau in eastern Washington state.) The Emeishan basalts erupted from 265 to 259 Ma.

The second, much larger, was in Siberia. Today we call it the Siberian Traps. The Siberian Traps represents one of the biggest eruptive episodes in Earth history. Presumably the result of a mantle plume slamming into the bottom of the north Asian continent, the volcanic rocks of the Siberian Traps have been shown to span the Permian-Triassic boundary, including the mass extinction at 252 Ma.

Figure \(\PageIndex{16}\): Putorana Plateau, Russia, part of the Siberian Traps flood basalt province. (CC-BY; Photo by Александр Лещёнок, via Wikimedia.)

Size estimates of the Siberian Traps boggle the mind: A volume of more than 1.5 million cubic kilometers of lava was erupted, smothering roughly 2 million square kilometers of northern Siberia. That is enough basaltic lava to bury the entire contiguous United States to a depth of half a mile. It’s enough basalt to make three towers, each with a footprint of 1 square kilometer, that reach to the Moon. So much lava being erupted carries with it a commensurate amount of volcanic gas. Most of that gas is water vapor, which is rapidly removed from the atmosphere via precipitation. But the second most common volcanic gas is \(\ce{CO2}\), which is a potent greenhouse gas.

In addition, one of the layers that the Siberian Traps basalt passed through on its way from the mantle to the surface was a vast Carboniferous-aged coal field (the world’s largest) in the Tunguska Basin, which caught fire and burned, adding even more \(\ce{CO2}\) to the total that was released! Estimates are that the level of \(\ce{CO2}\) in the atmosphere reached to at least 3000 ppm, and maybe as high as 30,000 ppm. Average estimates for the end-Permian are ~8000 ppm. (For reference, in 2026, the world was at about 430 ppm, up from ~280 ppm in pre-Industrial times.) So the end of the Permian had almost twenty times as much \(\bf{\ce{CO2}}\) in its atmosphere as we do today, an insane level that would have warmed the planet to an astonishing degree.

A warm climate doesn’t necessarily spell the end of the world, but in this case, the oceans played a key role. In the modern world, with cold polar regions and warm tropics, our ocean vigorously circulates in a vast, floppy loop. The energy that drives this flow of water comes from the difference in temperature (and thus density) of the water. When warm water grows cold, it gets more dense, and it sinks. Sinking water pulls other water after it, and a flow is induced. But in the Permian, as the polar oceans warmed up to roughly the same temperatures as the tropics, this flow was reduced, and — apparently — stopped completely. In short, the oceans became stagnant.

As with the late Devonian, this produced anoxia in the oceans, which is recorded as usual by jet black sedimentary deposits. The anoxia was at first most pronounced in the deep ocean, but it appears to have eventually invaded the coastal shallows. This is where most marine life makes its home, and that was of course quite disastrous.

Figure \(\PageIndex{17}\): Outcrops in Inuyama, Japan, show a Triassic sequence of sedimentary strata that record changes in deep ocean oxygen levels after the end-Permian mass extinction event. The strata have been tectonically rotated due to accretion with the Japanese mainland. (Photo by Yukio Isozaki; reproduced with permission.)

Worse, though, is what happened when the Sun’s rays hit the anoxic shallow water. This is the rare combination of factors that promotes the growth of sulfur-reducing bacteria, which release hydrogen sulfide gas (\(\ce{H2S}\)) as a waste product. You know \(\ce{H2S}\): it’s the stink of rotten eggs — or the nauseating stench of really gross farts. In low levels, it’s revolting and annoying, but in high levels, it’s deadly. The appearance in seafloor sediments of little raspberry-shaped clusters of pyrite (\(\ce{FeS2}\)) suggest that the ocean at the end of the Permian was quite rich in sulfur. The inference is that these distinctive purple bacteria must have bloomed in profusion without any of their usual microbial competitors.

Figure \(\PageIndex{18}\): Purple sulfur bacteria from a modern euxinic lake in British Columbia. (J. Glass photo; permission requested.)

Thus the oceans were not only anoxic, but also probably also euxinic — the word that describes being thoroughly infused with \(\ce{H2S}\). In 2005, Lee Kump and colleagues suggested something bold – that the oceans were so euxinic during the end-Permian that \(\ce{H2S}\) began to exsolve out of the sea and into the air, where it acted like an agent of chemical warfare, gassing terrestrial animals to death.

In the aftermath, the world lay obliterated, an apocalyptical denuding of the biosphere that probably reeked of rotten eggs. On land, the formerly terrifying forms of Dimetrodon and Gorgonops lay still; corpses rotting away in the foul air. The forests of sphenopsids and lycopsids were done forever. From the rotting foliage, a few therapsids nosed out and sniffed the air. In the sea, there were no more blastoids, no more fusulinid forams, no more trilobites or rugose corals. No more eurypterids; no more goniatitic ammonoids. Brachiopods were greatly reduced in number and diversity (a 96% decline). They would be replaced by the bivalves in the years to come. Still depleted in oxygen and utterly emptied out, the early Triassic seas were overtaken by one genus of clam that was adapted to low-oxygen conditions: Claraia. Spread far and wide, unimpeded by any competition, Claraia is so omnipresent and numerous in early Triassic strata that it makes an excellent index fossil for the first part of the Triassic period: a bizarro “Clamworld.”

Figure \(\PageIndex{19}\): A layer of limestone made of nothing but shells of the clam Claraia. (Modified from a photo by Toedi3614, via Wikimedia.)

This was the biggest reset of Earth’s biosphere. Phyla that had dominated the oceans for 200 million years were gone, and oceans would never look the same. The survivors went on to diversify into the Modern Fauna; mollusks would now reign supreme, scleractinian corals would build reefs, and echinoids, sea stars, and sea cucumbers would now represent the echinoderms (replacing crinoids and blastoids).

Key Terms

• amniotes - a group of tetrapod vertebrates comprising reptiles, birds, and mammals defined by their ability to lay eggs on land or retain fertilized eggs within the mother
• synapsids - a lineage of amniotes that includes mammals and their extinct "mammal-like reptile" ancestors, characterized by a specific skull structure with a single opening behind each eye
• therapsids - a major clade of tetrapods that gave rise to mammals