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jawless fishies |
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{{paraphyletic group |
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Fwoggy the Fwog |
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|name = Agnathans |
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|image = Lampetra fluviatilis.jpg |
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|image_caption = ''[[Lampetra fluviatilis]]'' |
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| fossil_range = {{fossil range|530|0|ref=<ref>{{cite journal|doi=10.1038/46965|title=Lower Cambrian vertebrates from south China|year=1999|last1=Shu|first1=D-G.|last2=Luo|first2=H-L.|last3=Conway Morris|first3=S.|last4=Zhang|first4=X-L.|last5=Hu|first5=S-X.|last6=Chen|first6=L.|last7=Han|first7=J.|last8=Zhu|first8=M.|last9=Li|first9=Y.|last10=Chen|first10=L-Z.|journal=Nature|volume=402|issue=6757|pages=42}}</ref><ref name=Xian-guang2002>{{Cite journal| year = 2002 | title = New evidence on the anatomy and phylogeny of the earliest vertebrates | journal = Proceedings of the Royal Society B | volume = 269 | issue = 1503 | pages = 1865–1869 | doi = 10.1098/rspb.2002.2104| pmid = 12350247 | pmc = 1691108| last1 = Xian-Guang| first1 = H.| last2 = Aldridge| first2 = R. J.| last3 = Siveter| first3 = D. J.| last4 = Siveter| first4 = D. J.| last5 = Xiang-Hong| first5 = F.}}</ref>}} |
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| regnum = [[Animal]]ia |
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| phylum = [[Chordate|Chordata]] |
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| subphylum = [[Vertebrata]] |
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| superclassis = '''Agnatha''' |
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| superclassis_authority = [[Edward Drinker Cope|Cope]], 1889 |
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| excludes = [[Gnathostomata]]<ref>''[[Edwin Harris Colbert|Colbert, E.H.]] & Morales, M. (2001): [[Evolution of the Vertebrates|Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time]]''. 4th edition. John Wiley & Sons, Inc, New York, ISBN 978-0-471-38461-8.</ref> |
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}} |
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'''Agnatha''' ([[Greek Language|Greek]],<ref>[[Shorter Oxford English Dictionary]]</ref> "no jaws") is a [[superclass (biology)|superclass]] of jawless fish in the [[phylum (biology)|phylum]] [[Chordata]], subphylum [[Vertebrata]], consisting of both present ([[Cyclostomata|cyclostomes]]) and extinct ([[conodont]]s and [[ostracoderm]]s) species. The group excludes all vertebrates with jaws, known as [[gnathostome]]s. |
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The agnathans as a whole are [[paraphyletic]],<ref>{{cite book|last=Purnell|first=M. A.|authorlink=|editor=[[Derek Briggs|Derek E. G. Briggs]] and Peter R. Crowther|year=2001|title=Palaeobiology II|publisher=[[Blackwell Publishing]]|location=[[Oxford]]|isbn=0-632-05149-3|page=401}}</ref> because most extinct agnathans belong to the [[stem group]] of gnathostomes.<ref name="Zhao & Zhu 2007">{{cite journal | year = 2007 | title = Diversification and faunal shift of Siluro-Devonian vertebrates of China | journal = Geological Journal | volume = 42 | issue = (3–4) | pages = 351–369 | url = http://www3.interscience.wiley.com/journal/114129423/abstract | doi = 10.1002/gj.1072 | author = Zhao Wen-Jin, Zhu Min}}</ref><ref name="Sansom 2009">{{cite journal | author = Sansom, Robert S. | year = 2009 | title = Phylogeny, classification, & character polarity of the Osteostraci (Vertebrata) | journal = Journal of Systematic Palaeontology | volume = 7 | pages = 95–115 | url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=3978288 | doi = 10.1017/S1477201908002551}}</ref> Recent molecular data, both from rRNA<ref name="Mallatt, J., and J. Sullivan. 1998.">{{cite journal | author = Mallatt, J., and J. Sullivan. 1998. | year = 1998 | title = 28S and 18S ribosomal DNA sequences support the monophyly of lampreys and hagfishes | journal = Molecular Biology and Evolution | volume = 15 | pages = 1706–1718 | pmid = 9866205 | issue = 12 | doi = 10.1093/oxfordjournals.molbev.a025897}}</ref> and from mtDNA<ref name="DeLarbre et al. 2002">{{cite journal | author = DeLarbre Christiane ; Gallut Cyril ; Barriel Veronique ; Janvier Philippe ; Gachelin Gabriel | year = 2002 | title = Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly | journal = Molecular Phylogenetics & Evolution | volume = 22 | issue = 2 | pages = 184–192 | pmid = 11820840 | doi = 10.1006/mpev.2001.1045}}</ref> as well as embryological data<ref>{{cite journal|last=Oisi|first=Yasuhiro|author2=Ota, Kinya G. |author3=Kuraku, Shigehiro |author4=Fujimoto, Satoko |author5= Kuratani, Shigeru |title=Craniofacial development of hagfishes and the evolution of vertebrates|journal=Nature|date=19 December 2012|volume=493|issue=7431|pages=175–180|doi=10.1038/nature11794}}</ref> strongly supports the hypothesis that living agnathans, the cyclostomes, are [[monophyletic]].<ref name="janvier1">{{cite journal|doi=10.1073/pnas.1014583107 |quote=Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time.|title=MicroRNAs revive old views about jawless vertebrate divergence and evolution|year=2010|last1=Janvier|first1=P.|journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107|issue=45|pages=19137}}</ref> |
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The oldest fossil agnathans appeared in the [[Cambrian]], and two groups still survive today: the [[lamprey]]s and the [[hagfish]], comprising about 120 [[species]] in total. Hagfish are considered members of the subphylum [[Vertebrata]], because they secondarily lost vertebrae; before this event was inferred from molecular<ref name="Mallatt, J., and J. Sullivan. 1998."/><ref name="DeLarbre et al. 2002"/><ref>{{cite journal|last=Stock|first=David|author2=Whitt GS|title=Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group|journal=Science|year=1992|volume=257|issue=5071|doi=10.1126/science.1496398|pmid=1496398|pages=787–9}}</ref> and developmental<ref>{{cite journal|last=Ota|first=Kinya|title=Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish|journal=Nature Communications |year=2011|doi=10.1038/ncomms1355|volume=2|issue=6|pages=373|display-authors=etal}}</ref> data, the group [[Craniata]] was created by [[Linnaeus]] (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of [[Fish jaw|jaws]], modern agnathans are characterised by absence of paired [[fin]]s; the presence of a [[notochord]] both in larvae and adults; and seven or more paired [[gill]] pouches. Lampreys have a light sensitive [[Pineal gland#Other animals|pineal eye]] (homologous to the [[pineal gland]] in [[mammal]]s). All living and most extinct Agnatha do not have an identifiable [[stomach]] or any [[appendages]]. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are [[ectothermic]] or cold blooded, with a [[Cartilage|cartilaginous]] [[skeleton]], and the [[heart]] contains 2 chambers. |
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While a few scientists still regard the living agnathans as only superficially similar, and argue that many of these similarities are probably shared [[Basal (phylogenetics)|basal]] characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the [[lampreys]] (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.{{citation needed|date=April 2014}} |
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== Metabolism == |
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Agnathans are [[ectothermic]], meaning they do not regulate their own body temperature. Agnathan metabolism is slow in cold water, and therefore they do not have to eat very much. They have no distinct stomach, but rather a long gut, more or less homogenous throughout its length. Lampreys feed on other fish and mammals. They rely on a row of sharp teeth to shred their host. [[Anticoagulant]] fluids preventing blood clotting are injected into the host, causing the host to yield more blood. Hagfish are scavengers, eating mostly dead animals. They also use a sharp set of teeth to break down the animal. The fact that Agnathan teeth are unable to move up and down limits their possible food types. |
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== Body covering == |
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In modern agnathans, the body is covered in skin, with neither dermal or epidermal [[Scale (zoology)|scales]]. The skin of [[hagfish]] has copious slime glands, the slime constituting their defense mechanism. The slime can sometimes clog up enemy fishes' gills, causing them to die. In direct contrast, many extinct agnathans sported extensive exoskeletons composed of either massive, heavy dermal [[armour (zoology)|armour]] or small mineralized scales (see [[Agnatha#Fossil agnathans|below]]). |
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== Appendages == |
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Almost all agnathans, including all [[extant taxon|extant agnathans]], have no paired appendages, although most do have a dorsal or a [[caudal fin]]. Some fossil agnathans, such as [[osteostracan]]s and [[pituriaspida|pituriaspids]], did have paired fins, a trait inherited in their [[Gnathostomata|jawed descendants]].<ref name="Romer & Parson">[[Alfred Romer|Romer, A.S]]. & Parsons, T.S. (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia.</ref> |
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== Reproduction == |
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Fertilization in lampreys is external. Mode of fertilization in hagfishes is not known. Development in both groups probably is external. There is no known parental care. Not much is known about the hagfish reproductive process. It is believed that hagfish only have 30 eggs over a lifetime.<ref>{{cite web|url=http://www.aquaticcommunity.com/mix/hagfish.php |title=Hagfish |publisher=Aquaticcommunity.com|accessdate=2013-06-30}}</ref> Most species are [[hermaphrodites]]. There is very little of the larval stage that characterizes the lamprey. Lamprey are only able to reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. Lampreys reproduce in freshwater riverbeds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment. The resulting hatchlings go through four years of larval development before becoming adults. They also have a certain unusual form of reproduction. |
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== Evolution == |
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[[File:Evolution of jawless fish.png|thumb|260px|right|Evolution of jawless fishes. The diagram is based on [[Michael Benton]], 2005.<ref>Benton, M. J. (2005) [[Vertebrate Palaeontology (Benton)|''Vertebrate Palaeontology'']], Blackwell, 3rd edition, Figure 3.25 on page 73, ISBN 0-632-05637-1.</ref>]] |
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{{see also|Evolution of fish}} |
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Although a minor element of modern marine [[Fauna (animals)|fauna]], agnathans were prominent among the early fish in the early [[Paleozoic]]. Two types of Early [[Cambrian]] animal apparently having fins, [[vertebrate]] musculature, and gills are known from the early Cambrian [[Maotianshan shales]] of [[China]]: ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]''. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is ''[[Haikouella]]''. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian [[Burgess Shale]] of [[British Columbia]]. |
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Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the [[Ostracoderm]]s, precursors to the [[bony fish]] and hence to the [[tetrapods]] (including [[human]]s)—are known from the middle [[Ordovician]], and by the Late [[Silurian]] the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as [[thelodont]]s, [[osteostracan]]s, and [[galeaspid]]s, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including [[conodonts]].<ref name="Baker 2008">{{cite journal | year = 2008 | title = The evolution and elaboration of vertebrate neural crest cells | journal = Current Opinion in Genetics & Development | volume = 18 | pages = 536–543 | doi = 10.1016/j.gde.2008.11.006 | author = Baker, Clare V.H. | pmid = 19121930 | issue = 6}}</ref> Agnathans declined in the [[Devonian]] and never recovered. |
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{{clear}} |
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== Classification == |
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{| class="wikitable" |
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|- |
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! colspan=5| Subgroups of jawless fish |
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|- |
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! colspan=2 | Subgroup |
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! Example |
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! Comments |
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|- |
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! rowspan=2 | [[Cyclostomata|Cyclostomes]] |
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! [[Myxini]] |
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| [[File:Pacific hagfish Myxine.jpg|140px]]<center>[[hagfish]]</center> |
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| [[Myxini]] (hagfish) are [[eel]]-shaped slime-producing marine animals (occasionally called '''slime eels'''). They are the only known living animals that have a [[skull]] but not a [[vertebral column]]. Along with [[lampreys]], hagfish are jawless and are [[living fossils]]; hagfish are [[Basal (phylogenetics)|basal]] to vertebrates, and living hagfish remain similar to hagfish 300 million years ago.<ref>[http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html Myxini] – University of California Museum of Paleontology</ref> The classification of hagfish has been controversial. The issue is whether the hagfish is itself a degenerate type of vertebrate-fish (most closely related to lampreys), or else may represent a stage which precedes the evolution of the vertebral column (as do [[lancelets]]). The original scheme groups hagfish and lampreys together as [[cyclostomata|cyclostomes]] (or historically, Agnatha), as the oldest surviving clade of [[vertebrates]] alongside [[gnathostomes]] (the now-ubiquitous jawed-vertebrates). An alternative scheme proposed that jawed-vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduces the category [[craniata]] to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme.<ref name="janvier1"/> |
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|- |
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! [[Hyperoartia]] |
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| [[File:Eudontomyzon mariae Dunai ingola.jpg|140px]]<center>[[lamprey]]</center> |
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| [[Hyperoartia]] is a disputed group of vertebrates that includes the modern [[lamprey]]s and their [[fossil]] relatives. Examples of hyperoartians from early in their fossil record are ''[[Endeiolepis]]'' and ''[[Euphanerops]]'', fish-like animals with [[hypocercal tail]]s that lived during the Late [[Devonian]] Period. Some [[paleontology|paleontologists]] still place these forms among the "[[ostracoderm]]s" (jawless armored "fishes") of the [[class (biology)|class]] [[Anaspida]], but this is increasingly considered an artificial arrangement based on [[plesiomorph|ancestral traits]]. Placement of this group among the jawless vertebrates is a matter of dispute. While today enough fossil diversity is known to make a close relationship among the "ostracoderms" unlikely, this has muddied the issue of the Hyperoartia's closest relatives. Traditionally the group was placed in a superclass [[Cyclostomata]] together with the [[Myxini]] (hagfishes). More recently, it has been proposed that the Myxini are more [[basal (evolution)|basal]] among the [[Craniata|skull-bearing chordates]], while the Hyperoartia are retained among [[vertebrate]]s. But even though this may be correct, the lampreys represent one of the oldest divergences of the vertebrate lineage, and whether they are better united with some "ostracoderms" in the [[Cephalaspidomorphi]], or not closer to these than to e.g. to other "ostracoderms" of the [[Pteraspidomorphi]], or even the long-extinct [[conodont]]s, is still to be resolved. Even the very existence of the Hyperoartia is disputed, with some analyses favoring a treatment of the "basal Hyperoartia" as a [[monophyletic]] lineage [[Jamoytiiformes]] that may in fact be very close to the ancestral [[Gnathostomata|jawed vertebrate]]s. |
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|- |
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! rowspan=4 | [[Ostracoderms]] |
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! [[extinction|<sup>†</sup>]][[Pteraspidomorphi]]<br /><small>(extinct)</small> |
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| [[File:Pteraspis NT.jpg|140px]]<center> |
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| <sup>†</sup>[[Pteraspidomorphi]] is an extinct group of early jawless fish. The fossils show extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. |
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The taxon contains the subgroups [[Heterostraci]], [[Astraspida]], [[Arandaspida]]. |
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|- |
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! <sup>†</sup>[[Thelodonti]]<br /><small>(extinct)</small> |
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| [[File:Thelodonti.gif|140px]] |
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| [[Thelodonti]] ''(nipple teeth)'' are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids<ref name=Turner1982>{{cite journal|author=Turner, S.|author2= Tarling, D. H. |year=1982 |title=Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=39 |pages=295–311 |doi=10.1016/0031-0182(82)90027-X|issue=3–4}}</ref>) represent a [[Monophyly|monophyletic grouping]], or disparate stem groups to the major lines of jawless and [[Gnathostome|jawed fish]]. Thelodonts are united in possession of "[[thelodont scales]]". This defining character is not necessarily a result of shared ancestry, as it may have been [[Convergent evolution|evolved independently by different groups]]. Thus the thelodonts are generally thought to represent a polyphyletic group,<ref>{{cite book | url = https://books.google.com/?id=B3leE3TTeuIC&pg=PA146 | title = Vertebrate fossils and the evolution of scientific concepts: writings in tribute to Beverly Halstead | isbn = 978-2-88124-996-9 | author1 = Sarjeant, William Antony S. | author2 = L. B. Halstead | year = 1995}}</ref> although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.<ref>{{cite journal |author=Donoghue, P. C., P. L. Forey & R. J. Aldridge |year=2000 |title=Conodont affinity and chordate phylogeny |journal=Biological Reviews of the Cambridge Philosophical Society |volume=75 |issue=2 |pages=191–251 |doi=10.1017/S0006323199005472 |pmid=10881388}}</ref>{{rp|206}} "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes [[Heterostraci]] and [[Anaspida]], differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.<ref name=Turner1999>{{cite book |author=Turner, S. |year=1999 |chapter=Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance |editor=A. J. Boucot & J. Lawson |title=Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report |publisher=[[Cambridge University Press]] |pages=42–78}}</ref><ref name=palaeos>The early and mid Silurian. See {{cite web|author=Kazlev, M.A., White, T|title=Thelodonti|work=Palaeos.com |date=March 6, 2001 |accessdate=October 30, 2007 |url=http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.100.html#Thelodonti}}</ref> The fish lived in both freshwater and marine environments, first appearing during the [[Ordovician]], and perishing during the [[Late Devonian extinction|Frasnian–Famennian extinction event]] of the Late [[Devonian]]. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms. |
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! <sup>†</sup>[[Anaspida]]<br /><small>(extinct)</small> |
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| [[File:Anaspida.png|140px]] |
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| [[Anaspida]] ''(without shield)'' is an extinct group of primitive jawless vertebrates that lived during the [[Silurian]] and [[Devonian]] periods.<ref name="isbn0-415-23370-4">{{cite book |author=Ahlberg, Per Erik |title=Major events in early vertebrate evolution: palaeontology, phylogeny, genetics, and development |publisher=Taylor & Francis |location=Washington, DC |year=2001 |isbn=0-415-23370-4 |url=https://books.google.com/books?id=zeyRZNZl-74C&pg=PA188 |accessdate=|page= 188}}</ref> They are classically regarded as the ancestors of lampreys.<ref name="isbn0-521-32271-5">{{cite book |author=Patterson, Colin |title=Molecules and morphology in evolution: conflict or compromise? |publisher=Cambridge University Press |location=Cambridge, UK |year=1987 |isbn=0-521-32271-5 |url=https://books.google.com/books?id=DL_KQPX3AmIC&pg=PA142|page= 142}}</ref> Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly [[hypocercal]] tail. They first appeared in the early [[Silurian]], and flourished until the [[Late Devonian extinction]],<ref name="isbn0-226-31568-1">{{cite book |author=Hall, Brian Keith; [[James Hanken|Hanken, James]] |title=The Skull |publisher=University of Chicago Press |location=Chicago |year=1993 |isbn=0-226-31568-1 |url= https://books.google.com/books?id=fB-hO7i50esC&pg=PA131|page= 131}}</ref> where most species, save for [[lamprey]]s, went extinct due to the environmental upheaval during that time. |
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! <sup>†</sup>[[Cephalaspidomorphi|Cephalaspido-<br />morphi]]<br /><small>(extinct)</small> |
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| [[File:Cephalaspis Lyellii.jpg|140px]] |
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| [[Cephalaspidomorphi]] is a broad group of extinct armored agnathans found in Silurian and Devonian strata of North America, Europe, and China, and is named in reference to the [[osteostraci|osteostracan]] genus ''[[Cephalaspis]]''. Most biologists regard this [[taxon]] as extinct, but the name is sometimes used in the classification of [[lamprey]]s, as lampreys are sometimes thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the early [[Silurian]] period through the [[Mesozoic]], and into the present day. Cephalaspidomorphi were, like most contemporary fish, very well armoured. Particularly the head shield was well developed, protecting the head, [[gill]]s and the anterior section of the innards. The body was in most forms well armoured as well. The head shield had a series of grooves over the whole surface forming an extensive [[lateral line]] organ. The eyes were rather small and placed on the top of the head. There was no proper [[jaw]]. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite.<ref name=Colbert&Morales>{{cite book|last=Morales|first=Edwin H. Colbert, Michael|title=Evolution of the vertebrates : a history of the backboned animals through time|year=1991|publisher=Wiley-Liss|location=New York|isbn=978-0-471-85074-8|edition=4th}}</ref> Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina," include the [[class (biology)|classes]] [[Osteostraci]], [[Galeaspida]], and [[Pituriaspida]] |
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|} |
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== Groups == |
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*[[Cyclostomata|Cyclostomes]] |
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**[[Myxini]] (hagfish) |
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**[[Hyperoartia]] (Petromyzontida) |
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***Petromyzontidae ([[lamprey]]s) |
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*[[Ostracoderm]]s |
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**†[[Pteraspidomorphi]] |
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**†[[Thelodonti]] |
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**†[[Anaspida]] |
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**[[Cephalaspidomorphi]] |
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***†[[Galeaspida]] |
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***†[[Pituriaspida]] |
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***†[[Osteostraci]] |
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== See also == |
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{{wikispecies|Agnatha}} |
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* [[Gnathostomata]] |
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* [[Amphirhina]], an alternate name for the above parallel, or sister, classification |
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* [[Cyclostomata]] |
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== References == |
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{{reflist|33em}} |
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{{Chordata}} |
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{{evolution of fish}} |
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[[Category:Agnatha| ]] |
Revision as of 16:33, 19 January 2016
jawless fishies Fwoggy the Fwog