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The Precambrian Era
● For most of this era, life was microscopic, prokaryote cells living in oceans.
● Eukaryotes evolved about 2/3 through the Precambrian.
● By the late Precambrian (630–542 mya), many kinds of multicellular soft-bodied animals had evolved.
● Some were very different from any animals living today, and may have no living descendants.
● Beginning of the Paleozoic Era
● O2 concentration was approaching modern levels.
● Continents formed large land masses, the largest called Gondwana and rapid diversification of life took place—called the Cambrian explosion.
● Most of the major groups of animals living today appeared in the Cambrian.
● A great radiation of marine organisms occurred, especially among the brachiopods and mollusks.
● At the end of the period, massive glaciers formed over Gondwana, sea levels were lowered, and a mass extinction occurred.
● Marine life rebounded from the late Ordovician extinction.
● The first vascular plants appeared in the late Silurian, as well as some terrestrial arthropods—scorpions and millipedes.
● The northern landmass (Laurasia) and southern landmass (Gondwana) moved towards each other.
● There were evolutionary radiations of corals and squid-like cephalopods. Jawed fishes replaced jawless forms.
●Club mosses, horsetails, and tree ferns became common in terrestrial habitats whose roots accelerated weathering of rocks and soil formation.
● Ancestors of gymnosperms appeared. Fish-like amphibians began to occupy land.
● The mass extinction at the end of the Devonian resulted in loss of 75% of marine animals. Two meteorite impacts may have contributed to this extinction. The craters are in Nevada and western Australia.
● Large glaciers formed over high latitudes but great swamp forests of horsetails and tree ferns grew on the tropical continents. These swamp plants became fossilized as coal.
● Diversity of terrestrial animals increased. Snails, centipedes, scorpions, and insects were abundant.
● Insects evolved wings. Flight gave them access to tall plants.
● Plant fossils show evidence of chewing by insect herbivores.
● Amphibians became larger; their lineage split from the amniotes—vertebrates with well-protected eggs that can be laid in dry places.
● Continents came together to form the supercontinent Pangaea.
● By the end of the period, the reptiles split from a second amniote lineage (which would lead to the mammals).
● Ray-finned fishes became common in fresh waters.
● At end of the Permian, massive volcanic eruptions, ash blocked sunlight, caused climate cooling, resulting in the largest glaciers in Earth’s history.
● O2 dropped to ~12%, most animals unable to survive >500 m ASL. Result: greatest mass extinction in Earth’s history (96% of all spp extinct).
● Surviving organisms inhabited a relatively empty world. Continents began to drift apart, sea levels rose, and flooded the continents forming large shallow seas.
● Three groups of phytoplankton became ecologically important: dinoflagellates, coccolithophores, and diatoms. New seed plants replaced the trees of the Permian forests.
● Earth’s biota became increasingly provincialized -- distinct biota’s evolved on each continent.
● Pangaea began to break apart.
● On land, conifers became dominant. A great radiation of reptiles began, which gave rise to crocodilians, dinosaurs, and birds.
● A mass extinction at the end may have been caused by a meteorite impact in present-day Quebec. Loss of 65% of all spp.
● Land once again in two continents, Laurasia and Gondwana.
● Ray-finned fishes began a great radiation. First lizards and flying reptiles (pterosaurs) appeared; most large terrestrial animals were dinosaurs.
● Several groups of mammals appeared. Flowering plants appeared.
● A continuous sea encircled the tropics. Earth was warm and humid.
● Dinosaurs diversified. Flowering plants began radiation that led to current dominance. Many mammal groups had evolved.
● Another mass extinction at end of Cretaceous caused by meteorite. On land, all animals larger than about 25 kg became extinct. Many insects went extinct, perhaps because of lack of plant food.
●The positions of the continents resembled those of today. Characterized by an extensive radiation of mammals. Flowering plants came to dominate forests except in cool regions.
● Mutations in one group of plants allowed them to form symbiotic associations with N-fixing bacteria. This dramatically increased N available for terrestrial plants.
● Many flowering plants evolved herbaceous forms. Grasslands spread. Snakes, lizards, birds, and mammals underwent extensive radiations.
● Rodents, marsupials, primates, and hoofed mammals appeared in North America for the first time.
(Divided into Pleistocene and Holocene epochs.)
● Pleistocene: drastic cooling and climate fluctuation; 4 major and 20 minor “ice ages,” glaciers spread, shifting ranges of plants and animals towards the equator. The last glaciers retreated from temperate latitudes ~15,000 yrs ago
● Pleistocene: hominid evolution and radiation. Many large mammal species became extinct in Australia and the Americas when Homo sapiens arrived—possibly due to hunting pressure.
Three great evolutionary radiations occurred resulting in major new faunas. The Cambrian explosion, Paleozoic fauna, and Modern fauna after the Permian extinction (in the Triassic).