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EARLY LIFE AND BIOGEOGRAPHY OF THE PALEOZOIC ERA Precambrian Supereon fte planet Earth formed about 4.6 billion years ago (BYA), but life didn’t appear for approximately another 600 million years, late in the Hadean Eon. Early single-celled organisms were chemosynthetic, oxidizing inorganic materials for energy. fte last universal common ancestor appeared in the early Archean Eon. Fig. 7.1 shows a phylogenetic tree linking major taxa to the last universal common ancestor, based on completely sequenced genomes. By 3.5 BYA, prokaryotes, bacteria and archaea had appeared, and some bacteria were photosynthetic, although they did not produce oxygen as a byproduct. Photosynthetic oxygen-producing cyanobacteria appeared about 3 BYA, eventually paving the way for aerobic organisms. Single- celled eukaryotes first came on the scene in the Proterozoic Eon, about 2 BYA. Complex multicellular organisms had appeared by the late Proterozoic. Figure 7.1: Phylogenic tree of life. Eukaryotes are colored red, archaea green and bacteria blue. The central node represents the last universal common ancestor. (Wikipedia ‘Phylogenetic tree;’ Attribution: User A1) Cambrian period fte Phanerozoic Eon opened with the Cambrian Period of the Paleozoic Era, ca. 540 million years ago (MYA). See Fig. 5.12 for timeline of geological periods. fte Cambrian Period opened with the Cambrian explosion, a relatively brief (20 to 25 million year) evolutionary event in which major diversification occurred in the oceans, leading to the appearance of most modern animal phyla. Included among these were the vertebrates (Phylum Chordata) and the Phylum Arthropoda, such as Opabinia (Fig. 7.2), a close relative of the ancestral arthropods. Climatically, the early Cambrian was cold, but would warm near the end of the period. Most land mass was concentrated in the Southern Hemisphere early on, but over time separation occurred with the land masses gradually moving north. Plants had yet to invade land in the Cambrian, but fungi and microbes were beginning to contribute to soil formation that would pave the way for terrestrial plants. fte dominant animal life in the oceans consisted of arthropods such as trilobites (Fig. 7.3), although the trilobites are probably overrepresented in the fossil record because of their heavy exoskeletons. Chordates and various shelled animals appeared in the oceans during the Cambrian, and the first vertebrates arrived on the scene in the form of the jawless, armored ostracoderms. Several extinction events caused a dramatic decline in marine diversity in the late Cambrian. Figure 7.2: Artist’s impression of Opabinia, a close relative of the ancestral arthropods, at the sea floor. (Wikipedia ‘Opabinia;’ Attribution: Nobu Tamura) Figure 7.3: Olenoides superbus, a trilobite from the Cambrian period. (Wikipedia ‘Olenoides;’ Attribution: Daderot) Ordovician period fte Ordovician Period began ca. 485 MYA. By this time, southern continents were clustered into a single landmass, Gondwana, which began the period near the equator, but gradually drifted southward. fte more northern landmasses, Laurentia (which would eventually become present day North America), Siberia, and Baltica (eventually to be northern Europe) were still separated. High levels of CO2 and the accompanying greenhouse effect produced generally high temperatures during the early Ordovician, with ocean temperatures probably well over 40°C. But by the mid-Ordovician the climate had cooled considerably, and the ensuing glacial advances probably contributed to the large-scale extinction events during the latter part of the period. Green algae were abundant in the Ordovician, and gave rise to the early non-vascular plants that colonized shoreline areas in the late Ordovician-early Silurian. Early mycorrhizal fungi helped facilitate this invasion through symbiotic relationships with plant roots, helping the plants obtain nutrients. In the animal world, significant adaptive radiation took place in the oceans during the period, with various filter feeding groups such as crinoids, brachiopods, and bryozoans becoming abundant, as well as early cephalopods and corals. Shelled molluscs such as bivalves, gastropods, and nautiloid cephalopods diversified as well. Trilobites remained abundant, especially in the shallow continental seas, with many forms evolving elaborate defenses, such as spines and stalked eyes, in response to the growing number of predators. fte first jawed fish appeared in the late Ordovician. fte end of the Ordovician is marked by the Ordovician extinction, the second largest mass extinction in the Earth’s history. Probable causes of these extinction pulses were glaciation associated with cooling temperatures. ftese cooling temperatures were likely the result of multiple factors. ftese include an increase in volcanic activity and an associated decrease in CO2 (a greenhouse gas) in the atmosphere, and the position of Gondwana over the South Pole, which resulted in ice caps that increased the Earth’s albedo. fte high volcanic activity deposited large amounts of silica rock, which draws CO2 from the atmosphere during the erosion process. ftis decrease in CO2 in turn reduced the greenhouse effect. One effect of glaciation was a decrease in ocean levels, which in turned caused a decline in the shallow continental seas that supported so much of Ordovician biodiversity. Silurian period fte Silurian Period began ca. 444 MYA. During this period, Gondwana remained intact, covering much of the equatorial and southern hemisphere regions. Further north, a second supercontinent, Euramerica (also known as Laurussia, not to be confused with the later supercontinent Laurasia), was forming through the collision of several tectonic plates. fte climate during much of the Silurian was generally warm and stable, and melting ice caps increased ocean levels. Later in the period, cooling resulted in lowering of ocean levels. Late in the Silurian, vascular plants (land plants that have specialized vascular tissues that conduct water and nutrients) appeared and diversified. fte first recognizable fossils of land animals were arthropods that also appeared in the late Silurian. fte arthropod exoskeleton provided effective pre-adaptations for support, resistance to desiccation, and efficient locomotion on land. ftese are important features for organisms surrounded by dry air and at the mercy of gravity without the buoyancy that water provides. Sea scorpions (eurypterids) were abundant predators in the oceans, particularly shallow sea habitats. Some of these arthropods reached over 2 m in length. Also in the oceans, suspension feeders such as brachiopods, bryozoans, and crinoids remained abundant and diverse, as well as trilobites and molluscs. fte first bony fish appeared in the form of the Acanthodii, shark-like fish with skeletons that had cartilaginous as well as bony components. Devonian period fte Devonian Period began roughly 420 MYA, at a time when Euramerica and Gondwana were moving together (Fig. 7.4) to eventually form the giant supercontinent Pangaea. ftis period was relatively warm and glacier-free, with high ocean levels. In the marine environment, jawed fish and armored placoderms were abundant, although the placoderms would disappear by the end of the period. fte Devonian is often referred to as the “age of fishes” because of the great diversity of this group during the period. Among the marine invertebrates, brachiopods, bryozoans, crinoids, trilobites, and corals continued to be abundant, and ammonites (primarily spiral-shelled cephalopods) first appeared. Figure 7.4: Positions of the landmasses and Paleotethys Ocean during the middle Devonian (Wikipedia ‘Devonian;’ Attribution: http://www-sst.unil.ch/ research/plate_tecto/alp_tet_main.htm#Introduction) In the terrestrial environment, plant colonization and diversification accelerated. Rooted plants were contributing to further soil development. By the mid-Devonian, shrubby forests consisting of ferns, horsetails, lycophytes, and progymnosperms had appeared. In the late Devonian, the first true trees would appear in the form of certain progymnosperms that would eventually give rise to the true gymnosperms. By the end of the period, the first seed plants (land plants that produce embryos with a protective outer covering) had appeared. fte well-developed soils harbored mites, myriapods, and other arthropods. fte first probable insects appeared in the early Devonian; this would lead to the long coevolutionary relationship between insects and plants. fte first tetrapods, evolving from lobe-finned fish in shallow coastal waters, had appeared by the mid-Devonian as well. Carboniferous period fte Carboniferous Period began ca. 359 MYA. fte period is named for the abundance of coal deposits that formed as a result of burial of lowland forests under soil. ftis eventually resulted in trapped carbon in buried peat bogs. High pressures and high temperatures converted this dead vegetation to coal. During the Carboniferous, the single supercontinent Pangaea was forming. fte early Carboniferous was relatively warm, with high ocean levels, but a cooling trend occurred in the mid-Carboniferous. Lowering ocean levels led to a large marine extinction. fte cooler, drier climate also led to the Carboniferous rainforest collapse, in which the extensive rainforests of the Euramerican equatorial belt were fragmented into smaller and smaller islands. Lycophytes disappeared and tree ferns (not to be confused with Glossopteris and other seed ferns) became dominant. Among the tropical animals, amphibians declined while reptiles increased in abundance and diversity. Plant life of the early Carboniferous was dominated by horsetails, club mosses, and ferns, with cycads, conifers, and the tree-like lycopodiophyte Sigillaria (Fig. 7.5) appearing later in the period. fte Division Lycopodiophyta is the oldest extant vascular plant division. Figure 7.5: Ancient in situ Lycopodiophyta, probably Sigillaria, from the Joggins Formation (Pennsylvanian), Cumberland Basin, Nova Scotia. (Wikipedia ‘Carboniferous;’ Attribution: Michael C. Rygel via Wikimedia Commons) Most of the dominant Devonian marine invertebrates, such as brachiopods, bryozoans, and crinoids, continued to be abundant in the Carboniferous, and bivalve molluscs increased in importance. However, by this time the trilobites were declining toward their eventual extinction at the end of the Permian. Bivalve molluscs were becoming abundant in freshwater environments as well, and amphibious sea scorpions (eurypterids) were also common in these habitats. Shark diversity increased dramatically in the Carboniferous oceans, possibly as a result of open ecological niches left by the extinction of the placoderms. On land, insects and other arthropods were abundant, and some of these reached unusually large sizes. ftese included millipede-like arthropods over 2 m in length, and some of the dragonfly-like griffinflies, which had wingspans of well over a half meter. Explanations for these unusually large insects are still under debate. One possibility is the very high oxygen content of the atmosphere at this time. Insects obtain oxygen primarily by diffusion through their tracheal system, a network of tubes that runs throughout the body. fte rate of oxygen diffusion places an upper limit on insect body size, a limit that would have been relaxed in the high oxygen environment of the Carboniferous. It has also been suggested that the absence of flying vertebrate predators played a role in allowing these flying insects to evolve such large body sizes. Among the vertebrates, amphibians were abundant and diverse in the early Carboniferous, but declined as a result of the aforementioned rainforest collapse. In contrast, a major evolutionary innovation allowed reptiles to survive and flourish in drier environments. ftis was the amniote egg, with a sturdy outer shell and sac-like allantois that prevented desiccation while still allowing gas exchange and waste storage. fte pelycosaurs, an early synapsid (tetrapods with one or more fenestrae, or holes, in the temporal bone of the skull) amniote group, appeared in the late Carboniferous. fte synapsids would give rise to the therapsids, a lineage that would lead to the mammals. Permian period fte Permian period, the last period of the Paleozoic Era, began ca. 299 MYA. During the Permian, the landmasses were formed into one supercontinent, Pangaea (Fig. 7.6). fte existence of a single landmass contributed to a continental climate with extremes of heat and cold, and increased aridity in the interior regions. ftese conditions in turn played a great role in the evolutionary patterns shown by many plant and animal groups. At the beginning of the period, the cool temperatures and extensive glaciation from the end of the Carboniferous continued; later in the Permian, dry conditions and cycles of warm and cold temperatures predominated. Figure 7.6: Early formation of Pangaea, about 290 million years ago in the early Permian. (Wikipedia ‘Paleo-Tethys Ocean;’ Attribution: http://www-sst.unil.ch/ research/plate_tecto/alp_tet.htm) In the marine environment, many of the groups that were dominant in the Carboniferous continued to be predominant. Corals, echinoderms, and molluscs were abundant; these animals, as well as brachiopods, sponges, foraminiferans, and bryozoans formed rich shallow-water reef communities. Bony fishes continued to diversify. On land, the dry continental conditions favored the diversification of seed plants such as conifers, seed ferns, and cycads. Insects were abundant, particularly various cockroach-like groups. Early in the period, amphibians were abundant, but increased aridity favored reptiles. fte synapsids gave rise to the therapsids; these animals had more complex skull, jaw, and tooth structure than the earlier pelycosaurs. fte therapsid lineage would eventually give rise to the mammals. fte Permian period ended with the Permian-Triassic mass extinction, the largest mass extinction in the Earth’s history. ftis mass extinction probably had multiple distinct pulses or phases, causes by different factors, including global warming caused by methane (a greenhouse gas) released from permafrost and ocean sediments, volcanism, and one or more meteor impact events. ftis event saw the extinction of over 90% of marine species and 70% of terrestrial species; it would take many millions of years for the Earth to recover these losses in biodiversity.

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