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 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 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 mammals. 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.
8.6.1: 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. This resulted in much of Laurentia (considered mainly synonymous with North America) being inundated with water over the stable platforms surrounding the craton. While sea level fluctuated during transgressions and regressions after the Ordovician, many of the Paleozoic rocks found in the interior of the United States are marine in origin, due to overall relative high sea level throughout the Paleozoic.
In eastern North America, the assembly of Pangea (sometimes spelled Pangaea) started as early as the Cambrian with a series of events including subduction with island arcs and continental collisions and eventually ocean-basin closures known as the Taconic, Acadian, Caledonian, and Alleghanian (also known as Appalachian) orogenies [66; 68]. The name Pangea, originally coined by Alfred Wegener, means “all land.” Colliding lithospheric plates formed the supercontinent, creating a series of mountain ranges and a broad fold-thrust belt, leaving a large global ocean basin known as the Panthalassa Ocean, with the Tethys Sea being the name of the large “bay” that formed between Laurasia (the northern continents of Laurentia and Eurasia) and Gondwana (the southern continents of India, Australia, Antarctica, and Africa). The eroded remains of the collisional mountains formed on Pangea are still in existence today as the Appalachian, Alleghanian, Scandinavian, Marathon, and Ouachita Mountain ranges. Stress from the Alleghanian orogeny reactivated faulting, produced uplifts, and deformation/folding as far west as the Pennsylvanian-aged Ancestral Rocky Mountains of Colorado.
Animation of plate movement in the last 3.3 billion years. Pangea occurs at the 4:40 mark.
Tectonics in western North America during the early part of the Paleozoic was mostly mild, as a long-lived passive margin developed. After the start of the Devonian, the Antler orogeny finally caused faulting and basin development, mostly seen across Nevada today. The Antler belt is most likely a result of an island arc crashing into western North America .
8.6.2: 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, 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 . Regardless, during the Cambrian period, 541-485 million years ago, a large majority of the phyla of modern marine animals appeared . 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 this time 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 ). These astonishing creatures offer a glimpse at evolutionary creativity. At the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms and trilobites had evolved and shared the seafloor.
After the Cambrian Explosion, a similar event occurred which abandoned some of the Cambrian evolutionary animal lines and proliferated others. Known as the Ordovician Radiation or Great Ordovician Biodiversification Event, many common forms and ecosystems recognizable today became common. This includes invertebrates such as mollusks (clams and their relatives), corals, arthropods (insects and their relatives), and vertebrates became more diverse and complex and dominated the oceans .
The most important of these advancements may have been reef-building organisms. Mostly colonial coral, they took advantage of better ocean chemistry for calcite and built large structures , resembling modern reefs like the Great Barrier Reef off of Australia. Many of the organisms of this time swam around, hid in, or crawled over the reefs. Reefs are so important because of their preservation potential, size (some reef fossils are the size of mountains), and the ability to create an in-place ecosystem in and around them. Few other fossil assemblages in the geologic record can offer more diversity and complexity than reefs. Warm temperatures and high sea levels in the Ordovician most likely helped spur this diversification.
A small ice age based on evidence of glacial deposits and associated falling sea levels led to the dramatic mass extinction by the end of the Ordovician, the first one documented in the fossil record. Mass extinction is when an unusually large number of species abruptly vanish and go extinct, and this can be observed in the fossil record (see video below). Life bounced back in the Silurian . The major evolutionary event was the development of the forward pair of gill arches into jaws, allowing fish new feeding strategies and opening up new ecological niches.
3-minute video describing mass extinctions and how they are defined.
The Silurian provides the first evidence of land plants and animals [80; 81]. This includes the first-ever vascular plant, Cooksonia, with woody tissues, veins for transporting water and food, seeds, and roots. The first bony fish and shark are also Silurian, which includes the first primitive jaws. This also saw the start of armored fish, known as the placoderms. Insects, spiders, scorpions, and crustaceans began to inhabit dry-land and freshwater habitats.
The Devonian, called the age of fishes, saw a rise in plated fish and jawed fish , 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 creatures that went on to dominate the land. The first evidence of walking fish, named Tiktaalik (about 375 million years ago), gave rise to amphibians. . Most amphibians live on land but lay soft eggs in water. They would later evolve into reptiles that lay hard-shelled eggs on land. Land plants had also evolved into the first trees and forests . 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 .
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, clubmosses) 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 plants 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 .
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, the second group of synapsids, called the therapsids (or mammal-like reptiles) evolve, and become the ancestors to mammals.
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 . The eruption size is estimated at over 3 million cubic kilometers  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 gasses into the atmosphere. Further, some evidence suggests that volcanism burned vast coal deposits releasing methane (a greenhouse gas) into the atmosphere. As discussed in Chapter 15, 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].
101. Reichow MK, Pringle MS, Al’Mukhamedov AI, et al (2009) The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis. Earth Planet Sci Lett 277:9–20