In biology, a phylum (English pronunciation: /ˈfaɪləm/; plural: phyla)[note 1] is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division. The kingdom Animalia contains approximately 35 phyla; the kingdom Plantae contains 12 divisions. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.
- 1 General description and familiar examples
- 2 Defining a phylum
- 3 Lists
- 4 See also
- 5 Notes
- 6 References
- 7 External links
General description and familiar examples
Informally, phyla can be thought of as grouping organisms based on general specialization of body plan, as well as developmental or internal organizations. For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although Article 3.1 of the International Code of Botanical Nomenclature allows the use of the term "phylum" in reference to plants, the term "division" is almost always used by botanists.
The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum to which humans belong, along with all other vertebrate species. Although there are 35 animal phyla, these nine include over 96% of animal species. Many phyla are exclusively marine, and only one phylum, the Onychophora (velvet worms) is entirely absent from the world's oceans—although ancestral onycophorans were marine.
Defining a phylum
At the most basic level, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is an unsatisfactory approach, but the phenetic definition is more useful when addressing questions of a morphological nature—such as how successful different body plans were.
Definition based on genetic relation
The largest objective measure in the above definitions is the "certain degree"—how unrelated do organisms need to be to be members of different phyla? The minimal requirement is that all organisms in a phylum should be related closely enough for them to be clearly more closely related to one another than to any other group. However, even this is problematic, as the requirement depends on our current knowledge about organisms' relationships: As more data becomes available, particularly from molecular studies, we are better able to judge the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not; for example, since Onychophora and Tardigrada have now been accepted as stem groups of the arthropods, these three phyla should be combined.
This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of cladistics, a method in which groups are placed on a "family tree" without any formal ranking of group size. So as to provide a handle on the size and significance of groups, a "body-plan" based definition of a phylum has been proposed by paleontologists Graham Budd and Sören Jensen. The definition was posited by paleontologists because it is extinct organisms that are typically hardest to classify, because they can be extinct off-shoots that diverged from a phylum's history before the characters that define the modern phylum were all acquired.
Definition based on body plan
By Budd and Jensen's definition, phyla are defined by a set of characters shared by all their living representatives. This has a couple of small problems—for instance, characters common to most members of a phylum may be secondarily lost by some members. It is also defined based on an arbitrary point of time (the present). However, as it is character based, it is easy to apply to the fossil record. A more major problem is that it relies on an objective decision of which group of organisms should be considered a phylum.
Its utility is that it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group. Further, organisms in the stem group to a phylum can bear all the aspects of the "body plan" of the phylum without all the characters necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.
Based upon this definition, which some say is unreasonably affected by the chance survival of rare groups, which vastly increase the size of phyla, representatives of many modern phyla did not appear until long after the Cambrian.
Phylum Meaning Common name Distinguishing characteristic Species described Acanthocephala Thorny headed worms Thorny-headed worms Reversible spiny proboscis 756 approx. Acoelomorpha Without gut Acoels No mouth or alimentary canal (alimentary canal = digestive tract in digestive system) Annelida Little ring Segmented worms Multiple circular segment 17,000+ extant Arthropoda Jointed foot Arthropods Chitin exoskeleton 1,134,000+ Brachiopoda Arm foot Lamp shells Lophophore and pedicle 300-500 extant Bryozoa Moss animals Moss animals, sea mats Lophophore, no pedicle, ciliated tentacles 5,000 extant Chaetognatha Longhair jaw Arrow worms Chitinous spines either side of head, fins 100 extant approx. Chordata Cord Chordates Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail 100,000+ approx. Cnidaria Stinging nettle Coelenterates Nematocysts (stinging cells) 11,000 approx. Ctenophora Comb bearer Comb jellies Eight "comb rows" of fused cilia 100 extant approx. Cycliophora Wheel carrying Symbion Circular mouth surrounded by small cilia 3+ Echinodermata Spiny skin Echinoderms Fivefold radial symmetry in living forms, mesodermal calcified spines 7,000 extant approx.; 13,000 extinct approx. Entoprocta Inside anus Goblet worm Anus inside ring of cilia 150 approx. Gastrotricha Hair stomach Meiofauna Two terminal adhesive tubes 690 approx. Gnathostomulida Jaw orifice Jaw worms 100 approx. Hemichordata Half cord Acorn worms, pterobranchs Stomochord in collar, pharyngeal slits 100 extant approx. Kinorhyncha Motion snout Mud dragons Eleven segments, each with a dorsal plate 150 approx. Loricifera Corset bearer Brush heads Umbrella-like scales at each end 122 approx. Micrognathozoa Tiny jaw animals — Accordion like extensible thorax 1 Mollusca Soft Mollusks / molluscs Muscular foot and mantle round shell 112,000 Nematoda Thread like Round worms Round cross section, keratin cuticle 80,000–1,000,000 Nematomorpha Thread form Horsehair worms 320 approx. Nemertea A sea nymph Ribbon worms 1,200 approx. Onychophora Claw bearer Velvet worms Legs tipped by chitinous claws 200 extant approx. Orthonectida Straight swim Single layer of ciliated cells surrounding a mass of sex cells 20 approx. Phoronida Zeus's mistress Horseshoe worms U-shaped gut 20 Placozoa Plate animals 1 Platyhelminthes Flat worms Flat worms 25,000 approx. Porifera* Pore bearer Sponges Perforated interior wall 5,000+ extant Priapulida Little Priapus 16 Rhombozoa Lozenge animal — Single axial cell surrounded by ciliated cells 75 Rotifera Wheel bearer Rotifers Anterior crown of cilia 2,000 approx. Sipuncula Small tube Peanut worms Mouth surrounded by invertible tentacles 144–320 Tardigrada Slow step Water bears Four segmented body and head 1,000+ Xenoturbellida Strange flatworm — Ciliated deuterostome 2 Total: 35 2,000,000- Protostome Bilateria Deuterostome Basal/disputed Other
Groups formerly ranked as phyla
Name as phylum Common name Current consensus Aschelminthes Pseudocoelomates Divided into several pseudocoelomate phyla. Craniata — Subgroup of phylum Chordata; perhaps synonymous with Vertebrata. Cephalochordata Lancelets Subphylum of phylum Chordata. Cephalorhyncha — Superphylum Scalidophora. Echiura Spoon worms Class of phylum Annelida. Enterepneusta Acorn worms Class of phylum Hemichordata. Gephyra Peanut worms and spoon worms Divided into phyla Sipuncula and Echiura. Mesozoa Mesozoans Divided into phyla Orthonectida and Rhombozoa. Myxozoa Severely modified Cnidarians. Pentastomida Tongue worms Subclass of Maxillopoda of phylum Arthropoda. Pogonophora Beard worms Part of family Siboglinidae of phylum Annelida. Pterobranchia — Class of phylum Hemichordata. Symplasma Glass sponges Class Hexactinellida of phylum Porifera. Urochordata Tunicates Subphylum of phylum Chordata. Vestimentifera Vent worms Part of family Siboglinidae of phylum Annelida.
Division Meaning Common name Distinguishing characteristics Anthocerotophyta Flower-horn plants Hornworts Horn-shaped sporophytes, no vascular system Bryophyta Moss plants Mosses Persistent unbranched sporophytes, no vascular system Marchantiophyta Marchantia plants Liverworts Ephemeral unbranched sporophytes, no vascular system Lycopodiophyta Wolf foot plants Clubmosses & Spikemosses Microphyll leaves, vascular system Pteridophyta Fern plants Ferns & Horsetails Prothallus gametophytes, vascular system Pteridospermatophyta Fern with seeds plant Seed ferns Only known from fossils, mostly Devonian, ranking in dispute Coniferophyta Sap/pitch plants Conifers Cones containing seeds and wood composed of tracheids Cycadophyta Palm plants Cycads Seeds, crown of compound leaves Ginkgophyta Ginkgo plants Ginkgo, Maidenhair Seeds not protected by fruit (single species) Gnetophyta Gnetophytes Seeds and woody vascular system with vessels Anthophyta (or Magnoliophyta) Flower plant Flowering plants Flowers and fruit, vascular system with vessels
Phylum Meaning Common name Distinguishing characteristics Chytridiomycota Little pot mushroom Chytrids Cellulose in cell walls, flagellated gametes Deuteromycota Second mushroom Imperfect fungi Unclassified fungi; only asexual reproduction observed Zygomycota Yolk mushroom Zygomycetes Blend gametangia to form a zygosporangium Glomeromycota Ball mushroom None Form arbuscular mycorrhizae with plants Ascomycota Bag/Wineskin Mushroom Sac fungi Produce spores in an 'ascus' Basidiomycota Basidium Mushroom Club Fungi Produce spores from a 'basidium'
Currently there are 29 phyla accepted by LPSN
- Acidobacteria, phenotipically diverse and mostly uncultured
- Actinobacteria, High-G+C Gram positive species
- Aquificae, only 14 thermophilic genera, deep branching
- Caldiserica, formerly candidate division OP5, Caldisericum exile is the sole representative
- Chlamydiae, only 6 genera
- Chlorobi, only 7 genera
- Chrysiogenetes, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
- Cyanobacteria, also known as the blue-green algae
- Deinococcus-Thermus, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
- Elusimicrobia, formerly candidate division Thermite Group 1
- Firmicutes, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
- Lentisphaerae, formerly clade VadinBE97
- Planctomycetes ANo ito?
- Proteobacteria, the most known phyla, containing species such as Escherichia coli or Pseudomonas aeruginosa
- Spirochaetes, species include Borrelia burgdorferi, which causes Lyme disease
- Tenericutes, alternatively class Mollicutes in phylum Firmicutes (notable genus: Mycoplasma)
- Thermotogae, deep branching
- Crenarchaeota, Second most common archaeal phylum
- Euryarchaeota, most common archaeal phylum
- Nanoarchaeota, ultra-small symbiotes
- ^ The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan."
- ^ "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. http://dictionary.reference.com/browse/phylum. Retrieved 2008-10-04. "Phyla in the plant kingdom are frequently called divisions."
- ^ Valentine, James W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. pp. 7. ISBN 0226845486. "Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their what? morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy."
- ^ Parker, Andrew (2003). In the blink of an eye: How vision kick-started the big bang of evolution. Sydney: Free Press. pp. 1–4. ISBN 0743257332. "The job of an evolutionary biologist is to create dinosaurs for the devil to consume, make sense of the conflicting diversity of form — there is not always a relationship between internal and external parts. Early in the history of the subject, it became obvious that internal organisations were generally more important to the higher classification of animals than are external shapes. The internal organisation puts general restrictions on how an animal can exchange gases, obtain nutrients and reproduce."
- ^ Davidson, E. H; Erwin, D. H (2006). "Gene Regulatory Networks and the Evolution of Animal Body Plans". Science (American Association for the Advancement of Science) 311 (5762): 796–800. doi:10.1126/science.1113832. PMID 16469913. http://www.sciencemag.org/cgi/content/abstract/311/5762/796.
- ^ a b c d e Budd, G.E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews 75 (02): 253–295. doi:10.1017/S000632310000548X. PMID 10881389. http://www.journals.cambridge.org/abstract_S000632310000548X. Retrieved 2007-05-26.
- ^ Budd, G.E. (1998). "Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle". Lethaia (Blackwell Synergy) 31 (3): 197–210. doi:10.1111/j.1502-3931.1998.tb00508.x. http://www.blackwell-synergy.com/doi/abs/10.1111/j.1502-3931.1998.tb00508.x.
- ^ Briggs, D. E. G; Fortey, R. A (2005). "Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation". Paleobiology 31 (2 (Suppl)): 94–112. doi:10.1666/0094-8373(2005)031[0094:WSSSGA]2.0.CO;2.
- ^ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
- ^ Species Register. "Flatworms — Phylum Platyhelminthes". Marine Discovery Centres. http://www.woodbridge.tased.edu.au/mdc/Species%20Register/phylum_platyhelminthes.htm. Retrieved 2007-04-09.
- ^ ""Kingdom Plantae Tree of Life"". http://www.fossilmuseum.net/Tree_of_Life/KingdomPlantae.htm.
- ^ J.P. Euzéby. "List of Prokaryotic names with Standing in Nomenclature: Phyla". http://www.bacterio.cict.fr/classifphyla.html. Retrieved 30 December 2010.
- Are phyla "real"? Is there really a well-defined "number of animal phyla" extant and in the fossil record?
- Major Phyla Of Animals
Taxonomic ranks Magnorder Domain/Superkingdom Superphylum/Superdivision Superclass Superorder Superfamily Supertribe Superspecies Kingdom Phylum/Division Class Legion Order Family Tribe Genus Species Subkingdom Subphylum Subclass Cohort Suborder Subfamily Subtribe Subgenus Subspecies Infrakingdom/Branch Infraphylum Infraclass Infraorder Section Infraspecies Microphylum Parvclass Parvorder Series Variety Form
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