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The Phanerozoic and the history of Complex Life

Introduction

The Phanerozoic is a geologic time period that spans the past 541 million years and is characterized by the evolution and diversification of life on Earth. During this time, the Earth has undergone numerous changes, including periods of mass extinctions, continental drift, and climate change.

Almost the entire Phanerozoic or eon of complex life can fit in the approximate 500 million year interval - evolution from trilobites to dinosaurs to the modern world. Complex animal life appeared slightly earlier, during the Ediacaran period, which was actually the very latest subdivision of the Precambrian, although it was only during the following, Cambrian period that it became diverse.

This time period includes the formation and breakup of the supercontinent Pangea, leading ultimately to the current continents.

For the equivalent time span projected into the future, see the near term very far distant future. Without megascale engineering by a very technologically advanced civilisation, this would mark the absolute limit of life on Earth, due to the sun heating as it grows older.

A short history of the Phanerozoic

The geological time scale provides a framework for understanding the evolution of life and the history of the Earth. This constitutes a time span of about 541 million years and represent a rich history of life on Earth.

Cambrian period: The Cambrian period, which began about 541 million years ago, saw a major evolutionary explosion of life known as the “Cambrian explosion”. A variety of complex organisms emerged during this time, including the first animals with hard skeletons, such as trilobites and brachiopods.

Ordovician period: The Ordovician period, which lasted from about 485 to 443 million years ago, was marked by a continuation of a further diversification of life, including many new invertebrate types such as molluscs and articulate brachiopods. The end of the Ordovician featured an ice age and a mass extinction event.

Silurian period: The Silurian period, which lasted from about 443 to 419 million years ago, saw the emergence of the first land-dwelling animals, including arthropods, the development of the coral reefs, and the colonization of the land by plants. The period is also notable for the evolution radiation of fish, which were mostly armoured, jawless forms.

Devonian period: The Devonian period, which lasted from about 419 to 358 million years ago, is often referred to as the “Age of Fishes” due to the abundance of fish species that evolved during this time. The period also saw the emergence of the first tetrapods, the evolution of forests and the first land-dwelling insects.

Carboniferous period: The Carboniferous period, which lasted from about 358 to 298 million years ago, saw the evolution of the first reptiles and the development of coal-forming swamp forests. Sharks, echinoderms, and amphibians flourished, and terrestrial arthropods grew to large size.

Permian period: The Permian period, which lasted from about 298 to 252 million years ago, was marked by several evolutionary radiations of mammal-like reptiles; synapsids, represented by the pelycosaurs such as the sailback Dimetrodon (early Permian), dinocephalians (middle) and advanced therapsids (late Permian). It ended with the largest mass extinction event in the history of life on Earth.

Triassic period: The Triassic period, which lasted from about 252 to 201 million years ago, was characterised by a diverse assemblage of archosaurs, which replaced the Perian synapsids, as well as many types of marine reptiles. It also saw the evolution of the first mammals and the development of the first dinosaurs and flying reptiles

Jurassic period: The Jurassic period, which lasted from about 201 to 145 million years ago, was characterized by new types of marine reptiles, ammonites, the evolution of giant dinosaurs (the long-necked sauropods), the first birds, and the development of teleost fish.

Cretaceous period: The Cretaceous period, which lasted from about 145 to 65 million years ago, was marked by the evolution of flowering plants and the development of the first modern birds. It included several dynasties of dinosaurs, including, at the very end Cretaceous, the iconic Tyrannosaurus and Triceratops. The period is also notable for the extinction of the dinosaurs, which was likely caused by an asteroid impact.

During the Paleogene Period, which lasted from 66 to 23 million years ago, is divided into the Paleocene, Eocene, and Oligocene, the mammals continued to diversify and evolve into a wide range of forms, including early primates and ungulates. The evolution of mammals was accompanied by major changes in the Earth's climate and geography, including the Paleocene thermal maximum, and later transition from the tropic Paleocene and Eocene to the cooler and drier Oligocene.

This period saw the evolution of modern birds, the formation of the Rocky Mountains, and the first stages of the development of the Gulf Stream, which had a significant impact on global climate.

The Neogene Period from 23 to 2.58 million years ago, saw the rise of primates, including the evolution of early human ancestors such as Australopithecus. During this time, the Earth's climate became more seasonal and the planet saw the formation of the large ice sheets that covered much of the northern hemisphere. The Neogene also saw the evolution of modern species of mammals, including elephants, horses, and whales.

The Quaternary period, which began 2.58 million years ago to present day, and is the shortest geological period in the Phanerozoic Eon. It is known for its Pleistocene megafauna (mammoths, cave lions, woolly rhinos, reindeer, horses, ground sloths, diprotodons) alongside modern animals, the evolution of various hominin (human and human-related) species, and dramatic changes in the climate. It is divided into two epochs: the Pleistocene and the Holocene.

Each of these geological periods represents a unique and important chapter in the history of life on Earth, and collectively they provide us with a comprehensive understanding of the evolution of life and the history of our planet.

Timeline

This timeline of the last 535 million years is mostly from Wikipedia. Hopefully, these pages will be upgraded at some time. For an earlier and much more complete coverage, see Palaeos

• 535 Mya. Major diversification of living things in the oceans: arthropods (e.g. trilobites, crustaceans), chordates, echinoderms, molluscs, brachiopods, foraminifers and radiolarians, etc.
• 530 Mya. The first known (arthropod) footprints on land date to 530 Mya.
• 520 Mya. Earliest graptolites.
• 511 Mya. Earliest crustaceans.
• 505 Mya. Fossilization of the Burgess Shale
• 500 Mya. Jellyfish have existed since at least this time.
• 485 Mya. First vertebrates with true bones (jawless fishes).
• 450 Mya. First complete conodonts and echinoids appear.
• 440 Mya. First agnathan fishes: Heterostraci, Galeaspida, and Pituriaspida.
• 420 Mya. Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions.
• 410 Mya. First signs of teeth in fish. Earliest Nautilida, lycophytes, and trimerophytes.
• 488–400 Mya. First cephalopods (nautiloids) and chitons.
• 395 Mya. First lichens, stoneworts. Earliest harvestmen, mites, hexapods (springtails) and ammonoids. The earliest known tracks on land named the Zachelmie trackways which are possibly related to icthyostegalians.
• 375 Mya. Tiktaalik, a lobe-finned fish with some anatomical features similar to early tetrapods. It has been suggested to be a transitional species between fish and tetrapods.
• 365 Mya. Acanthostega is one of the earliest vertebrates capable of walking.
• 363 Mya. By the start of the Carboniferous Period, the Earth begins to resemble its present state. Insects roamed the land and would soon take to the skies; sharks swam the oceans as top predators, and vegetation covered the land, with seed-bearing plants and forests soon to flourish. Four-limbed tetrapods gradually gain adaptations which will help them occupy a terrestrial life-habit.
• 360 Mya. First crabs and ferns. Land flora dominated by seed ferns. The Xinhang forest grows around this time.
• 350 Mya. First large sharks, ratfishes, and hagfish; first crown tetrapods (with five digits and no fins and scales).
• 350 Mya. Diversification of amphibians.
• 325-335 Mya. First Reptiliomorpha.
• 330-320 Mya. First amniote vertebrates (Hylonomus, Paleothyris).
• 320 Mya. Synapsids (precursors to mammals) separate from sauropsids (reptiles) in late Carboniferous.
• 305 Mya. The Carboniferous rainforest collapse occurs, causing a minor extinction event, as well as paving the way for amniotes to become dominant over amphibians and seed plants over ferns and lycophytes. First diapsid reptiles (e.g. Petrolacosaurus).
• 280 Mya. Earliest beetles, seed plants and conifers diversify while lepidodendrids and sphenopsids decrease. Terrestrial temnospondyl amphibians and pelycosaurs (e.g. Dimetrodon) diversify in species.
• 275 Mya. Therapsid synapsids separate from pelycosaur synapsids.
• 265 Mya. Gorgonopsians appear in the fossil record.
• 226 Mya. One galactic year ago (time it takes the sun and solar system to orbit the center of our Galaxy, at a speed of about 230 km/s).
• 251.9–251.4 Mya. The Permian–Triassic extinction event eliminates over 90-95% of marine species. Terrestrial organisms were not as seriously affected as the marine biota. This "clearing of the slate" may have led to an ensuing diversification, but life on land took 30 million years to completely recover.

• 250 Mya. Mesozoic marine revolution begins: increasingly well adapted and diverse predators stress sessile marine groups; the "balance of power" in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
• 250 Mya. Triadobatrachus massinoti is the earliest known frog.
• 248 Mya. Sturgeon and paddlefish (Acipenseridae) first appear.
• 245 Mya. Earliest ichthyosaurs
• 240 Mya. Increase in diversity of cynodonts and rhynchosaurs
• 225 Mya. Earliest dinosaurs (prosauropods), first cardiid bivalves, diversity in cycads, bennettitaleans, and conifers. First teleost fishes. First mammals (Adelobasileus).
• 220 Mya. Seed-producing Gymnosperm forests dominate the land; herbivores grow to huge sizes to accommodate the large guts necessary to digest the nutrient-poor plants.[citation needed] First flies and turtles (Odontochelys). First coelophysoid dinosaurs. First mammals from small-sized cynodonts, which transitioned towards a nocturnal, insectivorous, and endothermic lifestyle. First pterosaurs (see above image)
• 205 Mya. Massive Triassic/Jurassic extinction. It wipes out all pseudosuchians except crocodylomorphs, who transitioned to an aquatic habitat, while dinosaurs took over the land and pterosaurs filled the air.
• 200 Ma First accepted evidence for viruses infecting eukaryotic cells (the group Geminiviridae). However, viruses are still poorly understood and may have arisen before "life" itself, or may be a more recent phenomenon.
• Major extinctions in terrestrial vertebrates and large amphibians. Earliest examples of armoured dinosaurs.
• 195 Mya. First pterosaurs with specialized feeding (Dorygnathus). First sauropod dinosaurs. Diversification in small, ornithischian dinosaurs: heterodontosaurids, fabrosaurids, and scelidosaurids.
• 190 Mya. Pliosauroids appear in the fossil record. First lepidopteran insects (Archaeolepis), hermit crabs, modern starfish, irregular echinoids, corbulid bivalves, and tubulipore bryozoans. Extensive development of sponge reefs.
• 176 Mya. First stegosaurian dinosaurs.
• 170 Mya. Earliest salamanders, newts, cryptoclidid and elasmosaurid plesiosaurs, and cladotherian mammals. Sauropod dinosaurs diversify.
• 165 Mya. First rays and glycymeridid bivalves. First vampire squids.
• 163 Mya. Pterodactyloid pterosaurs first appear.
• 161 Mya. Ceratopsian dinosaurs appear in the fossil record (Yinlong) and the oldest known eutherian mammal: Juramaia.
• 160 Mya. Multituberculate mammals (genus Rugosodon) appear in eastern China.
• 155 Mya. First blood-sucking insects (ceratopogonids), rudist bivalves, and cheilostome bryozoans. Archaeopteryx, the earliest known bird, appears in the fossil record, along with triconodontid and symmetrodont mammals. Diversity in stegosaurian and theropod dinosaurs.
• 131 Mya. First pine trees.
• 140 Mya. Orb-weaver spiders appear.
• 135 Mya. Rise of the angiosperms. Some of these flowering plants bear structures that attract insects and other animals to spread pollen; other angiosperms are pollinated by wind or water. This innovation causes a major burst of animal coevolution. First freshwater pelomedusid turtles. Earliest krill.
• 120 Mya. Oldest fossils of heterokonts, including both marine diatoms and silicoflagellates.
• 115 Mya. First monotreme mammals.
• 114 Mya. Earliest bees.
• 112 Mya. Xiphactinus, a large predatory fish, appears in the fossil record.
• 110 Mya. First hesperornithes, toothed diving birds. Earliest limopsid, verticordiid, and thyasirid bivalves.
• 100 Mya. First ants.
• 100–95 Mya. Spinosaurus, the largest theropod dinosaur, a specialised fish-eater.
• 95 Mya. First crocodilians evolve.
• 90 Mya. Extinction of ichthyosaurs. Earliest snakes and nuculanid bivalves. Large diversification in angiosperms: magnoliids, rosids, hamamelidids, monocots, and ginger. Earliest examples of ticks. Probable origins of placental mammals (earliest undisputed fossil evidence is 66 Ma).
• 86–76 Mya. Diversification of therian mammals.
• 70 Mya. Multituberculate mammals increase in diversity. First yoldiid bivalves. First possible ungulates (Protungulatum).
• 68–66 Mya. Tyrannosaurus, Triceratops, and Edmontosaurus make up the classic megafauna of western North America (Laramidia).
• 66 Mya. The Cretaceous–Paleogene extinction event eradicates about half of all animal species, including mosasaurs, pterosaurs, plesiosaurs, ammonites, belemnites, rudist and inoceramid bivalves, most planktic foraminifera, and all of the dinosaurs excluding the birds.

• 65 Mya. Rapid dominance of conifers and ginkgos in high latitudes, along with mammals becoming the dominant species. First psammobiid bivalves. Earliest rodents. Rapid diversification in ants.
• 63 Mya. Evolution of the creodonts, an important group of meat-eating (carnivorous) mammals.
• 62 Mya. Evolution of the first penguins.
• 60 Mya. Diversification of large, flightless birds. Earliest true primates, along with the first semelid bivalves, edentate, carnivoran and lipotyphlan mammals, and owls. The ancestors of the carnivorous mammals (miacids) .
• 59 Mya. Earliest sailfish appear.
• 56 Mya. Gastornis, a large flightless bird, appears in the fossil record.
• 55 Mya. Modern bird groups diversify (first song birds, parrots, loons, swifts, woodpeckers), first whale (Himalayacetus), earliest lagomorphs, armadillos, appearance of sirenian, proboscidean mammals in the fossil record. Flowering plants continue to diversify. The ancestor (according to theory) of the species in the genus Carcharodon, the early mako shark Isurus hastalis, is alive. Ungulates split into artiodactyla and perissodactyla, with some members of the former returning to the sea.
• 52 Mya. First bats appear (Onychonycteris).

Page by M Alan Kazlev, 2023