Cambrian Period (543-490 mya)
Land, which now covers about a third of the planet, remainsdevoid of life during the Cambrian period. Remnants of Rodinia, the dominantlandmass during the late Proterozoic era, drift further apart. Where movingtectonic plates collide, mountain ranges form, especially on Gondwana, the largestof the fragments.
Atmospheric and marine oxygen levels reach new heights in thegenerally warm and stable climate. These environmental factors may help triggeran "explosion" in animal diversity. In the seas, some three dozen animal phylafirst appear, each with a distinctive body plan. While several of these groupsdisappear over time -- beginning with a series of extinction episodes near the endof the Cambrian -- most persist (though greatly modified) to the present day.
543 mya: Hard-shelled animals
Beginning in the early Cambrian, many animals evolve hard external skeletons of all shapes and sizes that shield and support their bodies. Shells, tubes, and spines, made primarily of calcium phosphate or calcium carbonate, comprise a major part of the fossil record.
540 mya: Oldest arthropod fossils
Arthropods, which include the insects and crustaceans, will become the most diverse phylum both on land and in the oceans. They are known for their segmented bodies, jointed legs, well-defined head area, and hard outer coating, or cuticle. While the oldest definitive arthropod fossils, which were left in abundance by communities of bottom-dwelling sea creatures called trilobites, date to the early Cambrian, trace fossils suggest they may have appeared even earlier, during the Vendian period.
535 mya: Chordates
Members of the phylum Chordata, which include humans and all other vertebrates, have a stiff yet flexible rod that runs down the middle of the back, called a notochord. Made of cartilage-like material, not bone, the notochord provides support for the nerve cord. Other features characteristic of the chordate body plan are a tail and gill-like openings between the throat and the neck. In many chordates like the vertebrates, some of these features are apparent only in the early stages of embryonic development.
530 mya: The Cambrian explosion
The basic body plans of the major animal phyla are establishedover a relatively short period of roughly 10 million years. All the major animalphyla that exist today -- about three dozen -- evolve from these Cambrian faunas.
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| Read more The Cambrian explosion (530-520 mya) While scientists now know that animal life existed prior tothe Cambrian explosion, the diversity of life that evolves during its 10 millionyears remains significant. While the soft-bodied Ediacaran animals had no protectivecoverings, many Cambrian animals evolved skeletons, such as shells or other brittlecoatings. Among the more familiar groups to appear include sponges, brachiopods (lamp shells), spiny-skinned echinoderms,early gastropods (snails), cone-shelled cephalopods, andprimitive arthropods called trilobites. Several other creatures that areunrelated to any currently living form also appear, but they die off after a shorttime. A combination of environmental factors probably contributes tothis evolutionary burst. Oxygen, which is plentiful in both the atmosphere and inthe oceans, allows physically larger animals to evolve. Warm, free-flowing oceancurrents probably carry these animals to new marine niches, where they adapt to newsettings and evolve different characteristics. Environmental factors alone, however, cannot explain why majoranimal phyla have not evolved in the 500 million years or so since theCambrian explosion. Studies comparing fossilized embryos and a wide range ofcontemporary specimens suggest that homeobox genes -- genes that control whethercertain cells specialize to form muscles, nerves, or glands, for example -- areremarkably similar in all species. The mutations that give rise to these controlgenes may be advantageous only in the earliest, simplest animals. | |||||
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530 mya: Marine reefs
Marine reefs are important ecosystems that support a wide variety oforganisms. They form in shallow, tropical waters and are highly sensitive to changes inocean conditions, like fluctuations in temperature and sea level. The archaeocyathids,spongelike animals that build the first reefs, go extinct only 10-15 million years afterthey first appear. Among the major reef-building groups that succeed the archaeocyathidsinclude stromatoporoids and, much later, corals.
500 mya: Gondwana forms
A remnant of the Rodinia supercontinent, Gondwana is the southernhemisphere's primary landmass throughout the Paleozoic era. The present-day continentsAfrica, Antarctica, Australia, and South America, as well as the subcontinent India,were originally parts of Gondwana.
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