Cambrian period

The Cambrian Period was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte
 sedimentary deposits, sites of exceptional preservation where "soft"
parts of organisms are preserved as well as their more resistant shells.
 As a result, our understanding of the Cambrian biology surpasses that
of some later periods.
The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms
 gradually became more common in the millions of years immediately
preceding the Cambrian, but it was not until this period that
mineralized—hence readily fossilized—organisms became common. The rapid
 diversification of life forms in the Cambrian, known as the Cambrian explosion,
 produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian
radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.
Although
 diverse life forms prospered in the oceans, the land is thought to have
 been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continentswere probably dry and rocky due to a lack of vegetation. …

Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian
 rocks, it was not until 1994 that the Cambrian system/period was
internationally ratified. The base of the Cambrian lies atop a complex
assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil
 to mark the lower boundary of the Cambrian, is difficult since the
occurrence of very similar trace fossils belonging to the Treptichnids
group are found well below the T. pedum in Namibia, Spain and Newfoundland, and possibly in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.
Subdivisions
The Cambrian Period followed the Ediacaran Period and was followed by the Ordovician Period. The Cambrian is divided into four epochs (series) and ten ages (stages). Currently only three series and six stages are named and have a GSSP (an internationally agreed-upon stratigraphic reference point).
Because
 the international stratigraphic subdivision is not yet complete, many
local subdivisions are still widely used. In some of these subdivisions
the Cambrian is divided into three series (epochs) with locally
differing names – the Early Cambrian (Caerfai or Waucoban, 541 ± 1.0 to 509 ± 1.7 mya), Middle Cambrian (St Davids or Albertan, 509 ± 1.0 to 497 ± 1.7 mya) and Furongian (497 ± 1.0 to 485.4
 ± 1.7 mya; also known as Late Cambrian, Merioneth or Croixan). Rocks of
 these epochs are referred to as belonging to the Lower, Middle, or
Upper Cambrian.
Trilobite zones allow biostratigraphic correlation in the Cambrian.
Each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:

Plate reconstructions suggest a global supercontinent, Pannotia, was in the process of breaking up early in the period, with Laurentia (North America), Baltica, and Siberia having separated from the main supercontinent of Gondwana
 to form isolated land masses. Most continental land was clustered in
the Southern Hemisphere at this time, but was drifting north. Large,
high-velocity rotational movement of Gondwana appears to have occurred
in the Early Cambrian.
With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth
 were long melted – the sea level was high, which led to large areas of
 the continents being flooded in warm, shallow seas ideal for sea life.
 The sea levels fluctuated somewhat, suggesting there were 'ice ages',
associated with pulses of expansion and contraction of a south polar ice cap.
In Baltoscandia a Lower Cambrian transgression transformed large swathes of the Sub-Cambrian peneplain into an epicontinental sea.

The Earth was generally cold during the early Cambrian, probably due to the ancient continent of Gondwana covering the South Pole
 and cutting off polar ocean currents. However, average temperatures
were 7 degrees Celsius higher than today. There were likely polar ice
caps and a series of glaciations, as the planet was still recovering
from an earlier Snowball Earth.
 It became warmer towards the end of the period; the glaciers receded
and eventually disappeared, and sea levels rose dramatically. This trend
 would continue into the Ordovician period.

Although there were a variety of macroscopic marine plants no land plant (embryophyte)
 fossils are known from the Cambrian. However, biofilms and microbial
mats were well developed on Cambrian tidal flats and beaches 500 mya.,
and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation.

Most animal life during the Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time, but this has proven to be incorrect. Arthropods
 were by far the most dominant animals in the ocean, but trilobites were
 only a minor part of the total arthropod diversity. What made them so
apparently abundant was their heavy armor reinforced by calcium
carbonate (CaCO3), which fossilized far more easily than the fragile chitinous exoskeletons of other arthropods, leaving numerous preserved remains.
The period marked a steep change in the diversity and composition of Earth's biosphere. The Ediacaran biota
 suffered a mass extinction at the start of the Cambrian Period, which
corresponded to an increase in the abundance and complexity of burrowing
 behaviour. This behaviour had a profound and irreversible effect on the substrate which transformed the seabed ecosystems. Before the Cambrian, the sea floor was covered by microbial mats. By the end of the Cambrian, burrowing animals had destroyed the mats in many areas through bioturbation,
 and gradually turned the seabeds into what they are today. As a
consequence, many of those organisms that were dependent on the mats
became extinct, while the other species adapted to the changed
environment that now offered new ecological niches. Around the same
time there was a seemingly rapid appearance of representatives of all
the mineralized phyla except the Bryozoa, which appeared in the Lower Ordovician.
 However, many of those phyla were represented only by stem-group
forms; and since mineralized phyla generally have a benthic origin, they
 may not be a good proxy for (more abundant) non-mineralized phyla.
While
 the early Cambrian showed such diversification that it has been named
the Cambrian Explosion, this changed later in the period, when there
occurred a sharp drop in biodiversity. About 515 million years ago…