User:Abyssal/Portal:Cambrian

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The Cambrian Portal

Introduction

The Cambrian ( /ˈkæmbri.ən, ˈkeɪm-/ KAM-bree-ən, KAYM-; sometimes symbolized Ꞓ) is the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran period 538.8 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 for 'Cymru' (Wales), where Britain's Cambrian rocks are best exposed. Sedgwick identified the layer as part of his task, along with Roderick Murchison, to subdivide the large "Transition Series", although the two geologists disagreed for a while on the appropriate categorization. (Full article...)

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Cnidaria is a phylum of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids. Cnidarians' activities are coordinated by a decentralized nerve net. Many cnidarians have complex lifecycles with asexual polyp stages and sexual medusae, but some omit either the polyp or the medusa stage. Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms.

Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. Fossils of cnidarians that do not build mineralized structures are very rare. Scientists currently think that cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians. (see more...)

Did you know...

... that the 500-million-year-old Cambrian predator Hurdia was thought to be a number of separate organisms for 100 years, until the complete animal was reconstructed in March 2009?
... that the shrimp-like 510-million-year-old arthropod Waptia was named after two mountains?
... that Orsten, fossil-bearing lagerstätten in Sweden and elsewhere, are called "stinking stones" from organic content that has been preserved since the Cambrian Period?
... that the fordilloid Camya asy is one of four accepted Cambrian bivalves?
...that despite being known from many specimens, the fossil Fuxianhuia (pictured) remains one of the most controversial Cambrian arthropods?

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Size comparison of selected Burgess Shale fauna and a human.
Photo credit: Matt Martyniuk

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On the Origin of Species, published in 1859, is a work of scientific literature by Charles Darwin which is considered to be the foundation of evolutionary biology. Darwin's book introduced the scientific theory that populations evolve over the course of generations through a process of natural selection. It presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution. Darwin included evidence that he had gathered on the Beagle expedition in the 1830s and his subsequent research findings. Ideas about the transmutation of species were controversial as they conflicted with beliefs that species were unchanging parts of a designed hierarchy and that humans were unique and unlike animals.

The book was written for non-specialist readers and attracted widespread interest upon its publication. As Darwin was an eminent scientist, his findings were taken seriously and the evidence he presented generated scientific, philosophical, and religious discussion. Within two decades there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, but scientists were slow to give natural selection the significance that Darwin thought appropriate. During the "eclipse of Darwinism" from the 1880s to the 1930s, various other mechanisms of evolution were given more credit. With the development of the modern evolutionary synthesis in the 1930s and 1940s, Darwin's concept of evolutionary adaptation through natural selection became central to modern evolutionary theory, and it has now become the unifying concept of the life sciences. (see more...)