- 600 It is thought that earliest multicellular life on Earth was a sponge-like creature.
Sponges are among simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs.
Sponges ( Porifera) are phylogenetically oldest animal phylum extant today.
In some ways they are closer to being cell colonies than multicellular organisms.
- 580 movement of all animals may have started with cnidarians. Almost all cnidarians possess nerves & muscles and, because they are simplest animals to possess it, their direct ancestors were very likely first animals to use nerves & muscles together. Cnidarians are also first animals with an actual body of definite form & shape. They have radial symmetry.
- 550 Flatworm
Flatworms are earliest animals to have a brain, & simplest animals alive to have bilateral symmetry. They are also simplest animals with organs that form from three germ layers.
- 543 - 248 Paleozoic Era (Ancient Life)  |
- 543 - Present Phanerozoic Era |
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- 540 Acorn worms are considered more highly specialised & advanced than other similarly shaped worm-like creatures. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structure it uses for breathing, a structure similar to that of primitive fish. Acorn worms are thus sometimes said to be a link between vertebrates & invertebrates.
- 530 Pikaia earliest known ancestor of chordates. It is first known animal with a notochord.Pikaia is believed to be ancestor of all chordates & vertebrates.
Lancelet, still living today, retains some characteristics of primitive chordates. It resembles Pikaia
Other earliest known chordate-like fossils is from a conodonts a "eel-shaped animal of 4-20 cm long" with a pair of huge eyes at head end were & a complex basket of teeth.
- 505 Agnatha, first vertebrates appear: ostracoderms, jawless fish related to present-day lampreys & hagfishes. Haikouichthys & Myllokunmingia are examples of these jawless fish, or Agnatha. (See also prehistoric fish). They were jawless & their internal skeletons were cartilaginous. They lacked paired (pectoral & pelvic) fins of more advanced fish. They were Precursors to bony fish. |
Cambrian 543 - 490
Geologic - Two major land masses form: Laurasia (North America, Greenland, Eurasia, & Scandinavia) to north of equator, & Gondwana (South America, Africa, peninsular India, Australia, & Antarctica) to south. Collisions between Laurasia & Gondwana form major mountain ranges. Coal-forming sediments are laid down in vast swamps.
Global climatic changes occur, changing from warm & wet to cooler & drier. result is a long interval of glaciation in southern hemisphere.
Biologic - The first reptiles appeared. Great swamp forests covered land.
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Devonian Period
Geologic - Europe & North America collide, forming northern part of ancestral Appalachian mountain range. Europe & North America straddle equator. Africa & South America are positioned over South Pole.
climate is generally warm & moist.
Biologic - The first amphibians, insects and spiders appeared.
diversification of fish from Devonian Period continues in both marine & freshwater environments, though armored fish become extinct. Benthic (bottom-dwelling) marine communities include a variety of invertebrates: crinoids, blastoids, & brachiopods. ammonites are common in open marine waters.
Insects, such as cockroaches, flourish. first reptiles evolve.
Land environments are dominated by plants, from small, shrubby growths to tall trees. Early club mosses, horsetails, forest trees (Cordaites), & ferns are common.
This period is dominated by various forms of fish—armored fish, lungfish, & sharks.
Ammonites evolve from nautiloids & become one of dominant invertebrate forms.
As ozone layer forms, first air-breathing arthropods—spiders & mites—evolve on land. Amphibians evolve & venture onto land.
Plant life, including lowland forests of giant psilophyta plants, develop & spread over planet.
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Biologic - First land plants &
- Eurypterids, first
giant sea scorpions appeared.
- Life in seas still dominated by invertebrates:
corals, arthropods, & crinoids.
- Rapid evolution occurs among suspension feeders, & pelagic (open ocean) predators, such as nautiloids, become abundant. Fish evolve jaws. Late in Silurian Period, first sharks appear.
earliest land plants are represented by leafless, vascular plants called psilophytes.
Geologic - The North American, European, & Asian land masses are situated on or near equator. Laurentia & Baltica collide. Gondwana sits in south polar region. Shallow flooding of continental areas deposits sediments; later withdrawal of ocean water leaves oxidized "red beds" & extensive salt deposits. |
Geologic - barren continents of Laurentia, Baltica, Siberia, & Gondwana are separated by large oceans. Shallow seas cover much of North America at beginning of period. As seas recede, they leave a thick layer of limestone. Later in period, seas recover North America, depositing quartz, sandstones, & more limestone.
Biologic - first nautiloids appeared. Corals & trilobites were common. Metazoan invertebrates are still dominant form of life on Earth. Corals, crinoids, & clams evolve, as well as first early vertebrates—primitive fish with bony armor plates.
Late in Ordovician Period, mass extinctions of marine life occur, opening niches for benthic (bottom-dwelling) & planktonic (floating, swimming) organisms. |
- 480 Placodermi were prehistoric fish. Placoderms were first of jawed fishes, their jaws evolving from first of their gill arches .Their head & thorax were covered by articulated armoured plates & rest of body was scaled or naked.
- 400 First Coelacanth appears; this order of animals had been thought to have no extant members until living specimens were discovered in 1938. It is often referred to as a living fossil.
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- 375 Tiktaalik is a genus of sarcopterygian (lobe-finned) fishes from late Devonian with many tetrapod-like features.
- 365 Some fresh water lobe-finned fish ( Sarcopterygii) develop legs & give rise to Tetrapoda. first tetrapods evolved in shallow & swampy freshwater habitats.
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth & a short snout, whose upward-facing eyes show that it was a bottom-dweller, & which had already developed adaptations of fins with fleshy bases & bones. "living fossil" coelacanth is a related lobe-finned fish without these shallow-water adaptations. These fishes used their fins as paddles in shallow-water habitats choked with plants & detritus. universal tetrapod characteristics of front limbs that bend backward at elbow & hind limbs that bend forward at knee can plausibly be traced to early tetrapods living in shallow water.
- Panderichthys is a 90-130 cm long fish from Late Devonian period. It have a large tetrapod-like head. Panderichthys exhibits features transitional between lobe-finned fishes & early tetrapods. Lungfishes retain some characteristics of early Tetrapodas. One example is Australian Lungfish.
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Mississipian 354 - 323
Geologic - supercontinent of Pangea begins to breakup as North America separates from Eurasia & Africa. Atlantic Ocean begins to form. Tectonic plate subduction along western North America causes Earth's crust to fold & mountains form in western part of continent.
Biologic - The first birds appeared (e.g. Archaeopteryx).
Dinosaurs included diplodocus, stegasaurus, brachiosaurus.
Reptiles adapt to life in sea, in air, & on land. Dinosaurs are dominant reptile on land.
Archaeopteryx, first bird, evolves.
Early amphibians, extinct by late Triassic, are succeeded by first frogs, toads, & salamanders.
Mammals are small, shrew-like animals.
Plant forms are dominated by
cycads & cycadeoides. Conifers & gingkoes are widespread.
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- 315 Acanthostega Ichthyostega is an extinct amphibian, among first animals to have recognizable limbs. It is a candidate for being one of first vertebrates to be capable of coming onto land. It lacked wrists, & was generally poorly adapted to come onto land. limbs could not support animal's weight. Acanthostega had both lungs & gills, also indicating it was a link between lobe-finned fish & terrestrial vertebrates.
Ichthyostega is an early tetrapod. Being one of first animals with legs, arms, & finger bones, Ichthyostega is seen as a hybrid between a fish & an amphibian. Ichthyostega' had legs but its limbs probably weren't used for walking, they may have spent very brief periods out of water & would have used their legs to paw their way through mud.
Amphibia were first four-legged animals to develop lungs.
Amphibians living today still retain many characteristics of early tetrapods.
Carboniferous Period 354 - 290
Geologic - Sedimentary rocks (sandstone, shale, limestone, conglomerate) form in shallow seas over continents. Rodinia begins to break up into northern & southern portions. Gondwana in south incorporates South America, Africa, Antarctica, & Western Australia as well as peninsular India & parts of Arabia.
global climate is generally mild.
Biologic - The first fish, corals, trilobites and shellfish appeared.
Marine metazoans with mineralized skeletons, such as sponges, bryozoans, corals, brachiopods, molluscs, arthropods, and echinoderms, flourish. One group of arthropods, trilobites, are particularly dominant in shallow-water marine habitats.
Plant life is limited to marine algae. |
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