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| Rhizaria Fossil range: Neoproterozoic - Recent |
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 Live Ammonia tepida (Foraminifera)
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The Rhizaria are a species-rich supergroup of protists. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up the vast majority of protozoan fossils. Nearly all have mitochondria with tubular cristae. There are three main groups of Rhizaria:
Cercozoa - Various amoebae and flagellates, usually with filose pseudopods and common in soil
Foraminifera - Amoeboids with reticulose pseudopods, common as marine benthos
Radiolaria - Amoeboids with axopods, common as marine plankton
A few other groups may be included in the Cercozoa, but on some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals respectively, and the peculiar amoeba Gromia. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by Cavalier-Smith in 2002, who also included the centrohelids and Apusozoa.
Evolutionary relationship
Rhizaria is part of the bikont clade, which also comprises the Archaeplastida, the Chromalveolata, the Excavata, and some smaller, unresolved groups such as the Apusozoa and the Centrohelida. As bikonts, they all descend from a heterotrophic eukaryote with two flagella.
Historically, many rhizarians were considered animals, with their motility and heterotrophy as justification. However, when the five-kingdom system took prevalence over the animal-plant dichotomy, the rhizarians were put into the kingdom Protista. Then, after Woese published his three-domain system, because of the paraphyly of the kingdom Monera, taxonomists turned their attention to the eukaryote domain, and the inherent paraphyly of Protista. After much debate, which continues to this day, Rhizaria emerged as a monophyletic group.
References
- Cavalier-Smith, Thomas (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology 52 (2): 297–354. ISSN 1466-5026. PMID 11931142. Retrieved on 2007-06-08.
- Nikolaev, Sergey I.; Cédric Berney, José F. Fahrni, Ignacio Bolivar, Stephane Polet, Alexander P. Mylnikov, Vladimir V. Aleshin, Nikolai B. Petrov, and Jan Pawlowski (2004). "The twilight of the Heliozoa and rise of the Rhizaria, an emerging supergroup of amoeboid eukaryotes". Proceedings of the National Academy of Sciences 101 (21): 8066–8071. doi:. ISSN 0027-8424. PMID 15148395. Retrieved on 2007-06-08.
- Fabien, Burki; Kamran Shalchian-Tabrizi, Marianne Minge, Åsmund Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan Pawlowski (2007). "Phylogenomics Reshuffles the Eukaryotic Supergroups". PLoS ONE 2 (8): e790-. doi:. Retrieved on 2008-01-24.
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- This page was last modified on 21 June 2008, at 23:27.
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