- Adam Sedgwick in 1867
The Cambrian Period marked a profound change in life on Earth; prior to the Cambrian, living organisms on the whole were small, unicellular and simple.
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 lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of many modern phyla, representing the evolutionary stems of modern groups of species, such as the arthropods.
530 million years ago
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record, accompanied by major diversification of organisms including animals, phytoplankton, and calcimicrobes.
Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies.
Over the following 70 or 80 million years the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today.
Trilobites – 521 million years ago
- Trilobites were very common during this time
Trilobites form one of the earliest known groups of arthropods
Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton.
- Redlichida such as this Paradoxides may represent the ancestral trilobites
While diverse life forms prospered in the oceans, the land was comparatively barren – with nothing more complex than a microbial soil crust and a few forms that apparently emerged to browse on the microbial materials.
- Cryptobiotic soil in Hovenweep National Monument.
Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia.
The seas were relatively warm, and polar ice was absent for much of the Cambrian period.
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.
There were likely polar ice caps and a series of glaciations, as the planet was still recovering from the earlier Snowball Earth.
It became warmer towards the end of the period; with the glaciers receding and eventually disappearing, as sea levels rose dramatically.
This trend would continue into the next Ordovician period.
Most animal life during the Cambrian was aquatic.
The Cambrian period marked a steep change in the diversity and composition of Earth’s biosphere.
This behavior had a profound and irreversible effect on the substrate which transformed the seabed ecosystems.
- An aerial photo of microbial mats around the Grand Prismatic Spring of Yellowstone National Park
Before the Cambrian, the sea floor was covered by microbial mats. By the end of the period, burrowing animals had destroyed the mats through bioturbation, and gradually turned the seabeds into what they are today.
As a consequence, many of those organisms who were dependent on the mats went extinct, while other species adapted to the changed environment who now offered new ecological niches.
Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla.
However, many of these 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.
Fossil evidence suggests that euthycarcinoids, an extinct group of arthropods, produced at least some of the Protichnites.
Fossils of the maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusk.
- Climactichnites wilsoni – trackways across the surface of the Cambrian sediment, Blackberry Hill, Wisconsin.
In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor; and mineralizing animals were rarer than in future periods, in part due to the unfavorable ocean chemistry. (Most Cambrian carbonates were formed by microbial or non-biological processes.)
Many modes of preservation are unique to the Cambrian, resulting in an abundance of lagerstätte.
This Botomian age of the Early Cambrian epoch lasted from about 524 to 517 million years ago.
At the end of the Botomian age there was a mass extinction that wiped out a high percentage of the organisms for which fossils were found.
The End-Botomian mass extinction event occurred around 517 million years ago.
Organisms which produced small shell fossils were almost exterminated.
- Late Cambrian 514 million years ago
The Dresbachian extinction event occurred around 502 mya
The Dresbachian stage spans about 4 million years, from 501 to 497 million years ago.
This term comes from the town of Dresbach which is located in southeastern Minnesota on the Mississippi River.
The Dresbachian extinction event was the second of two severe extinctions that slashed approximately 40 percent of marine life.
Earth’s Ozone layer developed around 500 MYA
The Dresbachian event was followed by the Cambro-Ordovician extinction event of about 488 million years ago.
The Cambrian–Ordovician extinction event ended the Cambrian Period, and led into the Ordovician Period in the Paleozoic Era.
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