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Daemon
Posted: Monday, September 18, 2017 12:00:00 AM
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Radiolarians

Radiolarians are a highly diverse group of marine protozoa widely recognized for their intricately ornate mineral skeletons. Part of the plankton community, radiolarians have a fossil record that extends back to the early Paleozoic era. Because they have undergone continuous evolutionary change, they are often used to analyze the layers of the marine sedimentary record. What German biologist produced exquisite drawings of radiolarians that helped to popularize them among Victorian microscopists? More...
raghd muhi al-deen
Posted: Monday, September 18, 2017 3:52:58 AM

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Radiolarian
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Radiolarian
Radiolaria
Temporal range: Cambrian – Recent
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Radiolaria illustration from the Challenger Expedition 1873–76.
Scientific classification
Domain: Eukaryota
Kingdom: Rhizaria
Superphylum: Retaria
Phylum: Radiolaria
Müller 1858 emend.
Classes

Polycystinea
Acantharea
Sticholonchea

The Radiolaria are amoeboid protozoa (diameter 0.1–0.2 mm) that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the ocean bottom as radiolarian ooze. Due to their rapid turn-over of species, they represent an important diagnostic fossil found from the Cambrian onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.
Description
Circogonia icosahedra, a species of Radiolaria, shaped like a regular icosahedron

Radiolarians have many needle-like pseudopodia supported by bundles of microtubules, called axopods, which aid in the Radiolarian's buoyancy. The nuclei and most other organelles are in the endoplasm, while the ectoplasm is filled with frothy vacuoles and lipid droplets, keeping them buoyant. Often it also contains symbiotic algae, especially zooxanthellae, which provide most of the cell's energy. Some of this organization is found among the heliozoa, but those lack central capsules and only produce simple scales and spines.

Some radiolarians are known for their resemblance to regular polyhedra, such as with the icosahedron-shaped Circogonia icosahedra pictured to the left.
Taxonomy

The radiolarians belongs to the supergroup Rhizaria together with Cercozoa and Foraminifera.[1] Traditionally the radiolarians have been divided into four groups—Acantharea, Nassellaria, Spumellaria and Phaeodaria. Phaeodaria is however now considered to be a Cercozoan.[2][3] Nassellaria and Spumellaria both produce siliceous skeletons and were therefore grouped together in the group Polycystina. Despite some initial suggestions to the contrary, this is also supported by molecular phylogenies. The Acantharea produce skeletons of strontium sulfate and is closely related to a peculiar genus, Sticholonche (Taxopodida), which lacks an internal skeleton and was for long time considered a heliozoan. The Radiolaria can therefore be divided into two major lineages: Polycystina (Spumellaria + Nassellaria) and Spasmaria (Acantharia + Taxopodida).[4][5]

There are several higher-order groups that have been detected in molecular analyses of environmental data. Particularly, groups related to Acantharia[6] and Spumellaria.[7] These groups are so far completely unknown in terms of morphology and physiology and the radiolarian diversity is therefore likely to be much higher than what is currently known.

The relationship between the Foraminifera and Radiolaria is also debated. Molecular trees supports their close relationship—a grouping termed Retaria.[8] But whether they are sister lineages or if the Foraminifera should be included within the Radiolaria is not known.
Fossil record

The earliest known radiolaria date to the very start of the Cambrian period, appearing in the same beds as the first small shelly fauna—they may even be terminal Precambrian in age. They have significant differences from later radiolaria, with a different silica lattice structure and few, if any, spikes on the test.[9] Ninety percent of radiolarian species are extinct. The skeletons, or tests, of ancient radiolarians are used in geological dating, including for oil exploration and determination of ancient climates.[10]
Haeckel's radiolarians

German biologist Ernst Haeckel produced finely detailed drawings of radiolaria in Kunstformen der Natur (1904), helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.
References

with my pleasure
ChristopherJohnson
Posted: Monday, September 18, 2017 7:12:34 AM

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They are responsible of layers of deposits and metamorphic rocks.
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