The Cambrian Period was named in 1835 by the geologist Adam Sedqwick after the region of Cambria in North Wales where rocks of this age were first found. The name "Cambria" is a version of “Cumbria”, a latinisation the Welsh Cymry (roughly meaning countryman). During this period, the first hard-skinned and shelled invertebrates appear on Earth. This period saw a number of mass extinctions and radical life form changes. The most serious was the “Botomian-Toyonian” turnover which rivaled the Permian extinction in terms of percentage of overall diversity lost and killed off most small, shelled animals, the majority of Archaeocyathids, as well as the worldwide Archaeocyathid reefs. During the late Cambrian, there were also three mass-extinctions. The cause(s) of these is not clear but it may have been related to climate change as most of the animals affected were warm water species.
Tectonics and Paleoclimate
During the Cambrian period, most continents were located in the southern hemisphere near the equator. The super continent of Pannotia continued to assemble in some regions but fragmented into Gondwana, Laurentia, Baltica, and various mostly submerged Asian landmasses. Laurentia stretched across the Cambrian equator and was partly submerged by the Iapetus Ocean while the mostly submerged Baltica and Siberia continued their approach from the South-East. Gondwana remained the largest super continent. Other continents included Kazakhstania and China (actually China, Thailand, Malaysia, and Indochina). Seas were for the most part shallow, especially along the edges of the continents. Global transgressions occurred in the Middle and Late Cambrian as shallow seas repeatedly invaded the land providing a perfect habitat for many types of marine invertebrates. These shallow seas covered much of the continents except for Gondwana and parts of Eastern Siberia and Central Kazakhistan where there were highlands. The Cambrian climate was generally warm, wet and mild. Since there were no continental landmasses located at the poles; the ocean currents were able to circulate freely thus inhibiting significant ice formation. The Cambrian was in essence a benign spell between the great ice ages of the Proterozoic and Ordovician periods.
Plants of the Cambrian period mostly consisted of cyanophytes (blue-green “algae” including seaweeds). There were no known land plants however. Microbial "scum" comprised of fungi, algae and possibly lichens probably covered parts of the land.
During the Cambrian period, life was entirely confined to the oceans with the possible exception of microorganisms. During this period, there was an inexplicable appearance and diversification of almost every major group of aquatic animal. Creatures with shells and exoskeletons appeared for the first time including trilobites, brachiopods, and mollusks. This sudden evolutionary burst was so spectacular that it often referred to as the "Cambrian explosion" and is unique throughout all of Earth’s history. What triggered this event is not clear but one theory holds that during the Cambrian, the numbers of oxygen-depleting bacteria in the aquatic environments were reduced in number sufficiently to permit higher levels of atmospheric oxygen to be mixed into the world’s oceans which in turn made dissolved oxygen available to a much larger diversity of animals.
The most characteristic animals of the Cambrian period were the trilobites (a primitive form of arthropod remarkable for its highly developed eyes) which reached their fullest development in the middle Cambrian. Greatest of all the Cambrian animals were as the Anomalocarids which reached up to 2 meters on length and were equipped with twin grasping organs and a dreadful mouth full of teeth. Many trilobite exoskeletons have been found with large bites taken out of them which appear to be the result of an encounter with Anomalocaris.
Other very important groups of Cambrian animals were the sponges, echinoderms, gastropods (marine snails), cephalopods, Mollusks, the Halkeria (a scale covered creature with two mollusk-like limpet-shaped shells), the Wiwaxia (a scale and spine covered creature similar to the Halkeria but without shells), the Archaeocyathids (sponge-like, cup-shaped animal), and soft-bodied echiurians that were burrowing worm-like creatures which seemed to have been the most important and predominant carnivores of the time.
The first chordates (vertebrate ancestors) appeared during the Cambrian as did the first foraminifers (shelled amoebas).
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).