8.4: Paleozoic Era

The Phanerozoic Eon is the most recent eon and represents time in which fossils are common, 541 million years ago to today. The word Phanerozoic means “visible life.” Older rocks, collectively known as the Precambrian (sometimes referred to as the Cryptozoic, meaning “invisible life”), are less common and have only rare fossils, and the fossils that exist represent soft-bodied life forms. The invention of hard parts like claws, scales, shells, and bones made fossils more easily preserved, and thus, easier to find. Since the younger rocks of the Phanerozoic are more common and contain the majority of fossils, the study of this eon yields much greater detail. It is further subdivided into three eras: Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”). The next three sections are on these important eras.

Figure \(\PageIndex{1}\): The trilobites had a hard exoskeleton and were an early arthropod, the same group that includes modern insects, crustaceans, and arachnids.

The Paleozoic era was dominated by marine organisms, but by the middle of the era, plants and animals had evolved to live and reproduce on land, including amphibians and reptiles. Fish evolved jaws and fins evolved into jointed limbs. Lungs evolved and life emerged from the sea onto land to become the first four-legged tetrapods, amphibians. Amphibians eventually evolved into reptiles once they developed hard-shelled eggs. From reptiles evolved an early ancestor to birds and mammals, and their scales became feathers and fur. The Carboniferous Period near the end of the Paleozoic had some of the most productive forests in the history of Earth and produced the coal that powered the Industrial Revolution in Europe and the United States. Tectonically, during the early Paleozoic, North America was separated from the other continents until the supercontinent Pangea formed towards the end of the era.

Figure \(\PageIndex{2}\): Trilobites, by Heinrich Harder, 1916.

Paleozoic Tectonics and Paleogeography

After the breakup of Rodinia toward the end of the Proterozoic, sea level remained high relative to land in the early Paleozoic. During the Paleozoic Era, sea levels rose and fell four times. With each sea-level rise, the majority of North America was covered by a shallow tropical ocean. Evidence of these submersions are the abundant marine sedimentary rocks such as limestone with fossils corals and ooids. Extensive sea-level falls are documented by widespread unconformities. Today, the midcontinent has extensive marine sedimentary rocks from the Paleozoic, and western North America has thick layers of marine limestone on block faulted mountain ranges such as Mt. Timpanogos near Provo, Utah.

Figure \(\PageIndex{3}\): Laurentia, which makes up the North American craton.

The assembly of Pangea (sometimes spelled Pangaea) was completed by the late Paleozoic Era. The name Pangea, originally coined by Alfred Wegener, means “all land.” Pangea formed a supercontinent by a series of tectonic events including subduction with island arc accretion, continental collisions and eventually ocean-basin closures. In North America, the tectonic events that occurred on the east coast and are known as the Taconic, Acadian, Caledonian, and Alleghanian orogenies [66; 68]. The Appalachian Mountains are the erosional remnants of these mountain building events. Surrounding Pangea was a global ocean basin known as the Panthalassa Ocean. Continued plate movement extended the ocean into Pangea, forming a large bay called the Tethys Sea that formed between Laurasia (the northern continents of Laurentia and Eurasia) and Gondwana (the southern continents of South America, India, Australia, Antarctica, and Africa).

Figure \(\PageIndex{4}\): A reconstruction of Pangea, showing approximate positions of modern continents.

While the east coast of North America was tectonically active during the Paleozoic Era, the west coast remained mostly inactive as a passive margin during the early Paleozoic. The western edge of North American continent was near the present-day Nevada-Utah border and was an expansive shallow continental shelf near the paleoequator. However, by the Devonian Period, the Antler orogeny started on the west coast and lasted until the Pennsylvanian Period. The Antler orogeny was a volcanic island arc that was accreted onto western North America with the subduction direction away from North America [71]. This created a mountain range on the west coast of North American called the Antler highlands and was the first part of building the land in the west that would eventually make most of California, Oregon, and Washington states. By the late Paleozoic, the Sonoma orogeny began on the west coast and was another collision of an island arc. The Sonoma orogeny marks the change in subduction direction to be toward North America with a volcanic arc along the entire west coast of North America by late Paleozoic to early Mesozoic Eras.

Animation of plate movement in the last 3.3 billion years. Pangea occurs at the 4:40 mark. (https://youtu.be/ovT90wYrVk4)

Paleozoic Evolution

The earliest Paleozoic had a significant biological explosion and contains evidence of a wide variety of evolutionary paths, including the evolutionary invention of hard parts like shells, spikes, teeth, and scales. Paleontologists refer to this event as the Cambrian Explosion, named after the first period in the Paleozoic. Scientists debate whether this was a manifestation of a true evolutionary pattern of diversification as a result of warmer climate following the late Proterozoic glacial environments, better preservation from easier to fossilize creatures, or simply an artifact of a more complete recent rock record. Ediacaran fauna, which lacked easily-fossilized hard parts, may have already been diverse and set the stage for the Cambrian Explosion [72]. Regardless, during the Cambrian period, 541-485 million years ago, a large majority of the phyla of modern marine animals appeared [73]. These new organisms had simple cone- or tube-shaped shells that quickly became more complex. Some of these life forms have survived to today, and some were “experimental” whose lineage did not continue past the Cambrian period.

Fossil evidence of the Cambrian Explosion was first discovered by Charles Walcott in a rock layer called the Burgess Shale in western Canada in 1909. The Burgess Shale is a lagerstätte, or fossil site of exceptional preservation, including impressions of soft body parts. This allowed scientists to learn immense details of the animals that existed at the time, in addition to their tough shells, spikes, and claws. Other lagerstätte sites of similar age in China and Utah have allowed the forming of a fairly detailed picture of what the biodiversity was like in the Cambrian. The biggest mystery is the animals that do not fit existing lineages and are unique to that time. This includes famous fossil creatures like the first compound-eyed trilobites, and many other strange ones, including Wiwaxia, a spiked shell creature; Hallucigenia, a walking worm with spikes; Opabinia, a 5-eyed lobed arthropod with a trunk and a grappling claw at the end; and the related Anomalocaris, the alpha predator of the time, complete with grabbing arms and a deadly circular mouth full of teeth. Most notably at this time, an important ancestor to humans evolved. Pikaia, a segmented worm, is thought to be the earliest ancestor of the Chordata phylum (including vertebrates; animals with backbones [76]). These astonishing creatures offer a glimpse at evolutionary creativity.

Figure \(\PageIndex{5}\): Left: Original drawing from Walcott’s 1912 description of Opabinia, with labels: fp = frontal appendage, e = eye, ths = thoracic somites, i = intestine, ab = abdominal segment. Top: Fossilized side view of Opabinia collected by Walcott from the Burgess Shale. (By Han Zeng, public domain via Wikimedia Commons.) Bottom: Animated illustration of Opabinia. (By Junnn11; CC BY-SA 4.0 via Wikimedia Commons.)

By the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms, and trilobites covered the sea floor. Although most animal phyla appeared by the Cambrian, the biodiversity at the family, genus, and species level was low until the Ordovician Period. During the Great Ordovician Biodiversification Event, vertebrates and invertebrates (animals without backbone) became more diverse and complex at family, genus, and species level. The cause of the rapid speciation event is still debated but some likely causes are a combination of warm temperatures, expansive continental shelves near the equator, and more volcanism along the mid-ocean ridges. Some have shown evidence that an asteroid breakup event and consequent heavy meteorite impacts correlate with this diversification event. The additional volcanism added nutrients to ocean water helping support a robust ecosystem. Many life forms and ecosystems that would be recognizable in current times appeared at this time. Mollusks, corals, and arthropods in particular multiplied to dominate the oceans. [77].

Figure \(\PageIndex{6}\): A modern coral reef.

One important evolutionary advancement during the Ordovician Period was reef-building organisms, mostly colonial coral. Corals took advantage of the ocean chemistry, using calcite to build large structures [78] that resembled modern reefs like the Great Barrier Reef off the coast of Australia. These reefs housed thriving ecosystems of organisms that swam around, hid in, and crawled over them. Reefs are important to paleontologists because of their preservation potential, massive size, and in-place ecosystems. Few other fossils offer more diversity and complexity than reef assemblages. Warm temperatures and high sea levels in the Ordovician most likely helped spur this diversification.

Figure \(\PageIndex{7}\): Guadalupe National Park is made of a giant fossil reef complex.

According to evidence from glacial deposits, a small ice age caused sea levels to drop and led to a major mass extinction by the end of the Ordovician. Mass extinction is when an unusually large number of species abruptly vanish and go extinct. This is the earliest of five mass extinction events documented in the fossil record. During this mass extinction, an unusually large number of species abruptly disappear in the fossil record (see video below).

3-minute video describing mass extinctions and how they are defined. (https://youtu.be/aO9mOAKXvJs?list=PLElB7nLNHZvhSor-RW0mv1FE_IDi9ZuiA)

Life bounced back in the Silurian [78]. The period’s major evolutionary event was the development of jaws from the forward pair of gill arches in bony fishes and sharks. Hinged jaws allowed fish to exploit new food sources and ecological niches. This period also included the start of armored fishes, known as the placoderms. In addition to fish and jaws, Silurian rocks provide the first evidence of terrestrial or land-dwelling plants and animals [80; 81]. The first vascular plant, Cooksonia, had woody tissues, pores for gas exchange, and veins for water and food transport. Insects, spiders, scorpions, and crustaceans began to inhabit moist, freshwater terrestrial environments.

Figure \(\PageIndex{8}\): The armor-plated fish (placoderm) Bothriolepis panderi from the Devonian of Russia.

The Devonian Period, called the Age of Fishes, saw a rise in plated fish and jawed fish [85], along with the lobe-finned fish. The lobe-finned fish (relatives of the modern lungfish and coelacanth) are important for their eventual evolution into tetrapods, the four-limbed vertebrate animals that can walk on land. The first evidence of land-walking fish, named Tiktaalik (about 385 million years ago), gave rise to early tetrapods. [87]. Though Tiktaalik was clearly a fish, it had some tetrapod structures as well. Several fossils from the Devonian are more tetrapod-like than fish-like but these weren’t fully terrestrial. The first fully terrestrial tetrapod appeared in the Mississippian (early Carboniferous) period. By the Mississippian (early Carboniferous) period, tetrapods had evolved into two main groups, amphibians and amniotes, from a common tetrapod ancestor. The amphibians were able to breathe air and live on land but still needed water to nurture their soft eggs. The first reptile (an amniote) could live and reproduce entirely on land with hard-shelled eggs that wouldn’t dry out.

Land plants had also evolved into the first trees and forests [88]. Toward the end of the Devonian, another mass extinction event occurred. This extinction, while severe, is the least temporally defined, with wide variations in the timing of the event or events. Reef-building organisms were the hardest hit, leading to dramatic changes in marine ecosystems [89].

Figure \(\PageIndex{9}\): Several different types of fish and amphibians that led to walking on 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 [90]. 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 [92]. 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 [93]. This glacial event lasted into the early Permian [94].

Figure \(\PageIndex{10}\): A reconstruction of the giant arthropod (insects and their relatives) Arthropleura.

By the Permian, with Pangea assembled, the supercontinent led to a drier climate and even more diversification and domination by the reptiles [95]. 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, the second group of synapsids, called the therapsids (or mammal-like reptiles) evolve, and become the ancestors to mammals.

Figure \(\PageIndex{11}\): Reconstruction of Dimetrodon.

Permian Mass Extinction

The end of the Paleozoic era is marked by the largest mass extinction in Earth history. The Paleozoic era had two smaller mass extinctions, but these were not as large as the Permian Mass Extinction, also known as the Permian-Triassic Extinction Event. It is estimated that up to 96% of marine species and 70% of land-dwelling (terrestrial) vertebrates went extinct. Many famous organisms, like sea scorpions and trilobites, were never seen again in the fossil record. What caused such a widespread extinction event? The exact cause is still debated, though the leading idea relates to extensive volcanism associated with the Siberian Traps, which are one of the largest deposits of flood basalts known on Earth, dating to the time of the extinction event [100]. The eruption size is estimated at over 3 million cubic kilometers [101] that is approximately 4,000,000 times larger than the famous 1980 Mt. St. Helens eruption in Washington. The unusually large volcanic eruption would have contributed a large amount of toxic gases, aerosols, and greenhouse gases into the atmosphere. Further, some evidence suggests that volcanism burned vast coal deposits releasing methane (a greenhouse gas) into the atmosphere. Greenhouse gases cause the climate to warm. This extensive addition of greenhouse gases from the Siberian Traps may have caused a runaway greenhouse effect that rapidly changed the climate, acidified the oceans, disrupted food chains, disrupted carbon cycling, and caused the largest mass extinction.

Figure \(\PageIndex{12}\): Map of global flood basalts. Note the largest is the Siberian Traps.

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