View source for Biology:Agnatha

{{Short description|Infraphylum of jawless fish}}
{{Paraphyletic group
| fossil_range = <br />[[Earth:Cambrian Stage 3|Cambrian Stage 3]]&ndash;[[Earth:Holocene|Present]],<br />{{fossil range|518|0||earliest=Early Cambrian|ref=<ref>{{Cite journal|last1=Yang|first1=Chuan|last2=Li|first2=Xian-Hua|last3=Zhu|first3=Maoyan|last4=Condon|first4=Daniel J.|last5=Chen|first5=Junyuan|date=2018|title=Geochronological constraint on the Cambrian Chengjiang biota, South China|journal=Journal of the Geological Society|language=en|volume=175|issue=4|pages=659–666|doi=10.1144/jgs2017-103|bibcode=2018JGSoc.175..659Y |s2cid=135091168 |issn=0016-7649|url=http://nora.nerc.ac.uk/id/eprint/521412/1/2018-JGS-Chuan%20Yang%20et%20al.pdf}}</ref>}}
| image = Lampetra fluviatilis.jpg
| image_alt = "Lampetra fluviatilis"
| image_caption = ''Lampetra fluviatilis''
| auto = yes
| taxon = Agnatha
| authority = Cope, 1889
| includes = *{{Extinct}}Myllokunmingiida <small></small>
*{{Extinct}}''[[Biology:Metaspriggina|Metaspriggina]]''?
*{{Extinct}}Haikouellidae <small></small>
*{{Extinct}}Conodonta <small></small>
*Ostracoderms
**{{Extinct}}[[Biology:Pteraspidomorphi|Pteraspidomorphi]] <small></small>
**{{Extinct}}Thelodontomorphi
***{{Extinct}}[[Biology:Thelodonti|Thelodonti]] <small></small><ref>{{cite journal | last1 = Märss | first1 = T. | last2 = Miller | first2 = C.G. | year = 2004 | title = Thelodonts and distribution of associated conodonts from the Llandovery-lowermost Lochkovian of the Welsh Borderland | journal = Palaeontology | volume = 47 | issue = 5 | pages = 1211–1265 | doi = 10.1111/j.0031-0239.2004.00409.x | bibcode = 2004Palgy..47.1211M | doi-access =  }} [W. Kiessling/M. Krause/E. Ito]</ref>
**{{extinct}}[[Biology:Anaspidomorphi|Anaspidomorphi]]
**{{Extinct}}[[Biology:Cephalaspidomorphi|Cephalaspidomorphi]] <small></small>
*[[Biology:Cyclostomi|Cyclostomi]]
*{{extinct}}''[[Biology:Palaeospondylus|Palaeospondylus]]''?
| excludes = *[[Biology:Gnathostomata|Gnathostomata]] <small>(from Cephalaspidomorphi)</small>
}}

'''Agnatha''' ({{IPAc-en|'|æ|g|n|ə|θ|ə|,_|æ|g|'|n|eɪ|θ|ə}};<ref>Shorter Oxford English Dictionary</ref> {{etymology|grc|''{{wikt-lang|grc|ἀ-}}'' ({{grc-transl|ἀ-}})|without||''{{wikt-lang|grc|γνάθος}}'' ({{grc-transl|γνάθος}})|jaws}}) is an infraphylum<ref>{{cite journal |journal=PLOS One |date=April 2015 |title=A Higher Level Classification of All Living Organisms |doi=10.1371/journal.pone.0119248|pmc=4418965 |first1=Michael|last1=Ruggiero |first2=Dennis P.|last2=Gordon |first3=Thomas M.|last3=Orrell |first4=Nicolas|last4=Bailly |volume=10 |issue=4 |pages=e0119248 |pmid=25923521 |bibcode=2015PLoSO..1019248R |doi-access=free}}</ref> of '''jawless fish''' in the [[Biology:Phylum|phylum]] [[Biology:Chordata|Chordata]], [[Biology:Subphylum|subphylum]] Vertebrata, consisting of both living ([[Biology:Cyclostomata|cyclostomes]]) and extinct ([[Biology:Conodont|conodont]]s, anaspids, and [[Biology:Ostracoderm|ostracoderm]]s) species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.<ref>{{Cite journal |last1=Heimberg |first1=Alysha M. |last2=Cowper-Sal·lari |first2=Richard |last3=Sémon |first3=Marie |last4=Donoghue |first4=Philip C.J. |last5=Peterson |first5=Kevin J. |date=2010-11-09 |title=microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=45 |pages=19379–19383 |doi=10.1073/pnas.1010350107 |pmc=2984222 |pmid=20959416|doi-access=free }}</ref>

Recent molecular data, both from rRNA<ref name="Mallatt, J., and J. Sullivan. 1998.">{{cite journal |last1=Mallatt |first1=J. |last2=Sullivan |first2=J. | date = December 1998 | title = 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes | journal = Molecular Biology and Evolution | volume = 15 | issue = 12 | pages = 1706–1718 | pmid = 9866205 | doi = 10.1093/oxfordjournals.molbev.a025897 | doi-access =  }}</ref> and from mtDNA<ref name="DeLarbre et al. 2002">{{cite journal | vauthors = Delarbre C, Gallut C, Barriel V, Janvier P, Gachelin G | title = Complete mitochondrial DNA of the hagfish, ''Eptatretus burgeri'': The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly | journal = Molecular Phylogenetics and Evolution | volume = 22 | issue = 2 | pages = 184–92 | date = February 2002 | pmid = 11820840 | doi = 10.1006/mpev.2001.1045 }}</ref> as well as embryological data,<ref>{{cite journal | vauthors = Oisi Y, Ota KG, Kuraku S, Fujimoto S, Kuratani S | title = Craniofacial development of hagfishes and the evolution of vertebrates | journal = Nature | volume = 493 | issue = 7431 | pages = 175–80 | date = January 2013 | pmid = 23254938 | doi = 10.1038/nature11794 | bibcode = 2013Natur.493..175O | s2cid = 4403344 | hdl = 20.500.14094/D1005717 | hdl-access = free }}</ref> strongly supports the hypothesis that both groups of living agnathans, [[Biology:Hagfish|hagfishes]] and [[Biology:Lamprey|lampreys]], are more closely related to each other than to jawed fish, forming the clade Cyclostomi.<ref name="janvier1">{{cite journal |last=Janvier |first=P. | title = MicroRNAs revive old views about jawless vertebrate divergence and evolution | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 45 | pages = 19137–19138 | date = November 2010 | pmid = 21041649 | pmc = 2984170 | doi = 10.1073/pnas.1014583107 | bibcode = 2010PNAS..10719137J  | 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&nbsp;century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time.|doi-access=free }}</ref>

The oldest fossil agnathans appeared in the [[Earth:Cambrian|Cambrian]], and two groups still survive today: the [[Biology:Lamprey|lamprey]]s and the [[Biology:Hagfish|hagfish]], comprising about 120&nbsp;[[Biology:Species|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 |last1=Stock |first1=D.W. |last2=Whitt |first2=G.S. | title = Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group | journal = Science | volume = 257 | issue = 5071 | pages = 787–9 | date = August 1992 | pmid = 1496398 | doi = 10.1126/science.1496398 | bibcode = 1992Sci...257..787S }}</ref> and developmental<ref>{{cite journal | vauthors = Ota KG, Fujimoto S, Oisi Y, Kuratani S | date = June 2011 | title = Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish | journal = Nature Communications | volume = 2 | issue = 6 | page = 373 | pmid = 21712821 | pmc = 3157150 | doi = 10.1038/ncomms1355 | bibcode = 2011NatCo...2..373O }}</ref> data, the group Craniata was created by [[Biography:Carl Linnaeus|Linnaeus]] (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates.

While a few scientists still regard the living agnathans as only superficially similar, and argue that many of these similarities are probably shared [[Biology:Basal (phylogenetics)|basal]] characteristics of ancient vertebrates, recent taxonomic studies 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.<ref name="Mallatt, J., and J. Sullivan. 1998."/><ref name="DeLarbre et al. 2002"/><ref>{{cite journal |last1=Stock |first1=D.W. |last2=Whitt |first2=G.S. | title = Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group | journal = Science | volume = 257 | issue = 5071 | pages = 787–789 | date = August 1992 | pmid = 1496398 | doi = 10.1126/science.1496398 | bibcode = 1992Sci...257..787S }}</ref>

== Metabolism ==
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 homogeneous throughout its length. Lampreys feed on other fish and mammals. [[Medicine:Anticoagulant|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 use a row of sharp teeth to break down the animal. The fact that Agnathan teeth are unable to move up and down limits their possible food types.

==Morphology==
In addition to the absence of [[Biology:Fish jaw|jaws]], modern agnathans are characterised by absence of paired [[Biology:Fin|fin]]s; the presence of a notochord both in larvae and adults; and seven or more paired [[Biology:Gill|gill]] pouches. Lampreys have a light sensitive [[Biology:Pineal gland#Other animals|pineal eye]] (homologous to the [[Biology:Pineal gland|pineal gland]] in [[Biology:Mammal|mammal]]s). All living and most extinct Agnatha do not have an identifiable [[Biology:Stomach|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 , with a [[Biology:Cartilage|cartilaginous]] [[Biology:Skeleton|skeleton]], and the [[Biology:Heart|heart]] contains 2 chambers.

===Body covering===

In modern agnathans, the body is covered in skin, with neither dermal or epidermal [[Biology:Scale (zoology)|scales]]. The skin of [[Biology:Hagfish|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 [[Biology:Armour (zoology)|armour]] or small mineralized scales.

===Appendages===

Almost all agnathans, including all extant agnathans, have no paired appendages, although most do have a dorsal or a caudal fin. Some fossil agnathans, such as osteostracans and [[Biology:Pituriaspida|pituriaspids]], did have paired fins, a trait inherited in their [[Biology:Gnathostomata|jawed descendants]].<ref name="Romer & Parson">Romer, A.S. & Parsons, T.S. (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia.</ref>

== Reproduction ==

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|access-date=2013-06-30}}</ref> 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 [[Biology:Cloaca|cloaca]]s 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.

== Evolution ==
[[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'', Blackwell, 3rd edition, Figure 3.25 on page 73, {{ISBN|0-632-05637-1}}.</ref>]]

Although a minor element of modern marine fauna, agnathans were prominent among the early fish in the early [[Earth:Paleozoic|Paleozoic]]. Two types of Early [[Earth:Cambrian|Cambrian]] animal apparently having fins, [[Biology:Vertebrate|vertebrate]] musculature, and gills are known from the early Cambrian Maotianshan shales of {{wipe|China}}: ''[[Biology:Haikouichthys|Haikouichthys]]'' and ''[[Biology:Myllokunmingia|Myllokunmingia]]''. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is ''[[Biology:Haikouella|Haikouella]]''. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian [[Earth:Burgess Shale|Burgess Shale]] of British Columbia. [[Biology:Conodont|Conodont]]s, a class of agnathans which arose in the early Cambrian,<ref>{{cite book |title=Earth System History |edition=4th |publisher=Macmillan Education |year=2015 |page=311 |quote=Conodonts arose late in the Early Cambrian and diversified into the Ordovician. ... Similar small teeth in very early Cambrian faunas ... may represent conodont ancestors. |first1=Steven M. |last1=Stanley |first2=John A. |last2=Luczaj }}</ref> remained common enough until their extinction in the [[Earth:Triassic|Triassic]] that their teeth (the only parts of them that were usually fossilized) [[Earth:Conodont biostratigraphy|are often used]] as index fossils from the late Cambrian to the Triassic.<ref>{{Cite journal | doi = 10.1666/0022-3360(2001)075<1174:CPPF>2.0.CO;2 | last1 = Sweet | first1 = W. C. | last2 = Donoghue | first2 = P. C. J. | year = 2001 | title = Conodonts: past, present and future | journal = Journal of Paleontology | volume = 75 | issue = 6| pages = 1174–1184 | s2cid = 53395896 | issn = 0022-3360 | url = https://donoghue.blogs.bristol.ac.uk/files/2019/08/Sweet_Donoghue_2001.pdf | url-status = live | archive-url = https://web.archive.org/web/20221030162538/https://cpb-eu-w2.wpmucdn.com/blogs.bristol.ac.uk/dist/3/589/files/2019/08/Sweet_Donoghue_2001.pdf | archive-date = 2022-10-30 }}</ref>

Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the [[Biology:Ostracoderm|Ostracoderm]]s, precursors to the bony fish and hence to the tetrapods (including [[Biology:Human|human]]s)—are known from the middle [[Earth:Ordovician|Ordovician]], and by the Late [[Earth:Silurian|Silurian]] the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, 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 [[Biology:Conodont|conodont]]s.<ref name="Baker 2008">{{cite journal | vauthors = Baker CV | title = The evolution and elaboration of vertebrate neural crest cells | journal = Current Opinion in Genetics & Development | volume = 18 | issue = 6 | pages = 536–543 | date = December 2008 | pmid = 19121930 | doi = 10.1016/j.gde.2008.11.006 }}</ref> Agnathans declined in the [[Earth:Devonian|Devonian]] and never recovered.

Approximately 500 million years ago, two types of recombinatorial adaptive immune systems (AISs) arose in vertebrates. The jawed vertebrates diversify their repertoire of immunoglobulin domain-based T and B cell antigen receptors mainly through the rearrangement of V(D)J gene segments and somatic hypermutation, but none of the fundamental AIS recognition elements in jawed vertebrates have been found in jawless vertebrates. Instead, the AIS of jawless vertebrates is based on [[Biology:Variable lymphocyte receptor|variable lymphocyte receptor]]s (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences.<ref>{{cite book |last1=Hirano |first1=Masayuki |last2=Das |first2=Sabyasachi |last3=Guo |first3=Peng |last4=Cooper |first4=Max D. |date=2011-01-01 |chapter=Chapter 4 – The Evolution of Adaptive Immunity in Vertebrates |title=Advances in Immunology |volume=109 |pages=125–157 |editor-last=Alt |editor-first=Frederick W. |publisher=Academic Press |doi=10.1016/b978-0-12-387664-5.00004-2 |pmid=21569914 |isbn=9780123876645 |chapter-url=http://www.sciencedirect.com/science/article/pii/B9780123876645000042 |access-date=2019-12-03}}</ref> Three VLR genes (VLRA, VLRB, and VLRC) have been identified in lampreys and hagfish, and are expressed on three distinct lymphocytes lineages. VLRA+ cells and VLRC+ cells are T-cell-like and develop in a thymus-like lympho-epithelial structure, termed thymoids. VLRB+ cells are B-cell-like, develop in hematopoietic organs, and differentiate into "VLRB antibody"-secreting plasma cells.<ref>{{Cite journal |last1=Wu |first1=Fenfang |last2=Chen |first2=Liyong |last3=Ren |first3=Yong |last4=Yang |first4=Xiaojing |last5=Yu |first5=Tongzhou |last6=Feng |first6=Bo |last7=Chen |first7=Shangwu |last8=Xu |first8=Anlong |date=October 2016 |title=An inhibitory receptor of VLRB in the agnathan lamprey |journal=Scientific Reports |volume=6 |issue=1 |page=33760 |doi=10.1038/srep33760 |pmid=27762335 |pmc=5071834 |issn=2045-2322 |bibcode=2016NatSR...633760W}}</ref>

{{clear}}

== Classification ==
{| class="wikitable"
|+ Subgroups of jawless fish
|-
! colspan=2 | Subgroup
! Example
! Comments
|-
! rowspan=2 | [[Biology:Cyclostomata|Cyclostomes]]
! Myxini
| [[File:Eptatretus hexatrema 15727711.jpg|170x170px]]
{{center|[[Biology:Hagfish|hagfish]]}}
| Myxini (hagfish) are [[Biology:Eel|eel]]-shaped slime-producing marine animals (occasionally called '''slime eels'''). They are the only known living animals that have a [[Biology:Skull|skull]] but not a [[Biology:Vertebral column|vertebral column]]. Along with lampreys, hagfish are jawless and are living fossils; hagfish are [[Biology:Basal (phylogenetics)|basal]] to vertebrates, and living hagfish remain similar to hagfish 300&nbsp;million years ago.<ref>{{cite web |last = Speer |first = Brian R. |year= 1997 |title = Introduction to the Myxini |department= U.C. Museum of Paleontology |publisher = University of California, Berkeley |url = http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html |access-date = 2013-02-21 |url-status = dead |archive-url = https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html |archive-date = 2017-12-15}}</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 [[Biology: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"/>
|-
! [[Biology:Hyperoartia|Hyperoartia]]
| [[File:Pacific lamprey facing right.jpg|170x170px]]{{center|[[Biology:Lamprey|lamprey]]}}
| [[Biology:Hyperoartia|Hyperoartia]] is a disputed group of vertebrates that includes the modern [[Biology:Lamprey|lamprey]]s and their [[Biology:Fossil|fossil]] relatives. Examples of hyperoartians from early in their fossil record are ''[[Biology:Endeiolepis|Endeiolepis]]'' and ''Euphanerops'', fish-like animals with hypocercal tails that lived during the Late [[Earth:Devonian|Devonian]] Period. Some [[Biology:Paleontology|paleontologists]] still place these forms among the "[[Biology:Ostracoderm|ostracoderm]]s" (jawless armored "fishes") of the [[Biology:Class|class]] [[Biology:Anaspida|Anaspida]], but this is increasingly considered an artificial arrangement based on 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 [[Biology:Cyclostomata|Cyclostomata]] together with the Myxini (hagfishes). More recently, it has been proposed that the Myxini are more basal among the skull-bearing chordates, while the Hyperoartia are retained among [[Biology:Vertebrate|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 [[Biology:Cephalaspidomorphi|Cephalaspidomorphi]], or not closer to these than to e.g. to other "ostracoderms" of the [[Biology:Pteraspidomorphi|Pteraspidomorphi]], or even the long-extinct [[Biology:Conodont|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 [[Biology:Monophyletic|monophyletic]] lineage [[Biology:Jamoytiiformes|Jamoytiiformes]] that may in fact be very close to the ancestral [[Biology:Gnathostomata|jawed vertebrate]]s.
|-
! rowspan 4 | Myllokunmingiida
![[Biology:Extinction|<sup>†</sup>]] [[Biology:Myllokunmingiidae|Myllokunmingiidae]]<br /><small>(extinct)</small>
| [[File:Haikouichthys NT.jpg|140px]]{{center|''[[Biology:Haikouichthys|Haikouichthys]]''}}
|The [[Biology:Myllokunmingiidae|myllokunmingiids]] were a primitive order of agnathans that were endemic to the [[Earth:Cambrian|Cambrian]] aged  [[Earth:Maotianshan Shales|Maotianshan Shales]] lagerstätte in {{wipe|China}}. These creatures are the earliest known group of [[Biology:Craniate|craniate]]s (chordates with a skull of [[Biology:Bone|hard bone]] or [[Biology:Cartilage|cartilage]]). Currently the group includes 3 known genera, ''[[Biology:Haikouichthys|Haikouichthys]]'', ''[[Biology:Myllokunmingia|Myllokunmingia]]'', and ''Zhongjianichthys''.<ref name="myl2">{{cite journal |last=Shu |first=Degan |date=April 2003 |title=A paleontological perspective of vertebrate origin |journal=Chinese Science Bulletin |volume=48 |issue=8 |pages=725–735 |bibcode=2003ChSBu..48..725S |doi=10.1007/BF03187041 |s2cid=85163902}}</ref><ref name="van der Laan">{{cite journal |last=van der Laan |first=Richard |year=2016 |title=''Family-group names of fossil fishes'' |url=https://www.researchgate.net/publication/303911230}}</ref>
|-
! rowspan 5 | [[Biology:Conodont|Conodont]]a
! [[Biology:Extinction|<sup>†</sup>]] 
[[Biology:Conodont|Conodont]]<br /><small>(extinct)</small>
| [[File:Panderodus.png|154x154px]]
{{center|''[[Biology:Panderodus|Panderodus]]''}}
|[[Biology:Conodont|Conodont]]s were eel like agnathans that lived from the [[Earth:Cambrian|Cambrian]] up until the beginning of the [[Earth:Jurassic|Jurassic]] period. They were very diverse in terms of lifestyles, with some species being [[Biology:Filter feeder|filter feeder]]s and others being macropredators. For over a [[Physics:Century|century]], these animals were only known because of their microscopic, [[Chemistry:Phosphate|phosphatic]] tooth structures called "Conodont elements". It wasn't until the mid-1980s that body fossils of conodonts were found in Scotland and Wisconsin, showing these animals true appearance. Their teeth make great index fossils, as many species lived and died out in a relatively short period of time. These fish reached their peak in diversity during the middle of the Ordovician, but were hit hard by the [[Earth:Late Ordovician mass extinction|Ordovician-Silurian extinction]] event. They then reached another spike in diversity in the [[Earth:Devonian|mid-late Devonian]] before again declining in the [[Earth:Carboniferous|Carboniferous]]. They were relatively rare in the [[Earth:Permian|Permian]], but dramatically increased in numbers in the [[Earth:Early Triassic|early Triassic]]. Despite this, they went extinct during the lower Jurassic period, with some of the last surviving populations being in Japan. They possibly survived longer there due to the relative remoteness of the area. Originally, it was thought that they  were wiped out by the [[Biology:Triassic–Jurassic extinction event|large extinction]] at the end of the Triassic. Instead, it is now thought that they were out competed by newer [[Earth:Mesozoic|Mesozoic]] taxa.<ref name="C.H. Pander's introduction to conodonts, 1856">{{cite web |author1=Sweet, Walter C. |author2=Cooper, Barry J. |date=December 2008 |title=C.H. Pander's introduction to conodonts, 1856 |url=https://www.researchgate.net/publication/286568271 |access-date=3 January 2019}}</ref><ref name="Gabbott1995">{{cite journal |last=Gabbott |first=S.E. |author2=R. J. Aldridge |author3=J. N. Theron |year=1995 |title=A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa |journal=Nature |volume=374 |issue=6525 |pages=800–803 |bibcode=1995Natur.374..800G |doi=10.1038/374800a0 |s2cid=4342260}}</ref><ref>{{cite journal |last1=Quinton |first1=Page C. |year=2016 |title=Effects of extraction protocols on the oxygen isotope composition of conodont elements |journal=Chemical Geology |volume=431 |pages=36–43 |bibcode=2016ChGeo.431...36Q |doi=10.1016/j.chemgeo.2016.03.023}}</ref><ref>{{Cite journal |last1=Bergström |first1=S. M. |last2=Carnes |first2=J. B. |last3=Ethington |first3=R. L. |last4=Votaw |first4=R. B. |last5=Wigley |first5=P. B. |year=1974 |title=Appalachignathus, a New Multielement Conodont Genus from the Middle Ordovician of North America |journal=Journal of Paleontology |volume=48 |issue=2 |pages=227–235 |doi=10.1666/0022-3360(2001)075<1174:CPPF>2.0.CO;2 |jstor=1303249|s2cid=53395896 }}</ref><ref>{{Cite journal |last1=Ginot |first1=Samuel |last2=Goudemand |first2=Nicolas |date=December 2020 |title=Global climate changes account for the main trends of conodont diversity but not for their final demise |journal=Global and Planetary Change |language=en |volume=195 |pages=103325 |bibcode=2020GPC...19503325G |doi=10.1016/j.gloplacha.2020.103325 |s2cid=225005180|doi-access=free |url=https://univ-lyon1.hal.science/hal-02996040/file/S0921818120302162.pdf }}</ref><ref>{{Cite journal |last1=Du |first1=Yixing |last2=Onoue |first2=Tetsuji |last3=Tomimatsu |first3=Yuki |last4=Wu |first4=Qiangwang |last5=Rigo |first5=Manuel |date=2023 |title=Lower Jurassic conodonts from the Inuyama area of Japan: implications for conodont extinction |journal=Frontiers in Ecology and Evolution |volume=11 |doi=10.3389/fevo.2023.1135789 |issn=2296-701X|doi-access=free |hdl=11577/3479836 |hdl-access=free }}</ref>
|-
! rowspan=6 | Ostracoderms
! [[Biology:Extinction|<sup>†</sup>]][[Biology:Pteraspidomorphi|Pteraspidomorphi]]<br /><small>(extinct)</small>
| [[File:Astraspis desiderata paleoart with light rays.tif|170x170px]]{{center|''[[Biology:Astraspis|Astraspis]]''}}
| <sup>†</sup>[[Biology:Pteraspidomorphi|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.
The taxon contains the subgroups [[Biology:Heterostraci|Heterostraci]], [[Biology:Astraspida|Astraspida]], [[Biology:Arandaspida|Arandaspida]].
|-
! <sup>†</sup>[[Biology:Thelodonti|Thelodonti]]<br/><small>(extinct)</small>
| [[File:Thelodonti.gif|140px]]
| [[Biology:Thelodonti|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| vauthors = Turner S, Tarling DH |year=1982 |title=Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=39 |pages=295–311 |doi=10.1016/0031-0182(82)90027-X|issue=3–4|bibcode=1982PPP....39..295T }}</ref>) represent a [[Biology:Monophyly|monophyletic grouping]], or disparate stem groups to the major lines of jawless and 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 [[Biology:Convergent evolution|evolved independently by different groups]]. Thus the thelodonts are generally thought to represent a polyphyletic group,<ref>{{cite book | title = Vertebrate fossils and the evolution of scientific concepts: Writings in tribute to Beverly Halstead | isbn = 978-2-88124-996-9 | vauthors = Sarjeant WA, Halstead LB | year = 1995 | publisher = Gordon and Breach | url = https://books.google.com/books?id=B3leE3TTeuIC&pg=PA146}}</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 | vauthors = Donoghue PC, Forey PL, Aldridge RJ | title = Conodont affinity and chordate phylogeny | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 75 | issue = 2 | pages = 191–251 | date = May 2000 | pmid = 10881388 | doi = 10.1111/j.1469-185X.1999.tb00045.x | s2cid = 22803015 }}</ref>{{rp|206}} "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes [[Biology:Heterostraci|Heterostraci]] and [[Biology:Anaspida|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 report | vauthors = Turner S |year=1999 |chapter=Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance |editor1=A. J. Boucot |editor2=J. Lawson |title=Palaeocommunities – Project Ecostratigraphy, Final Report |series=International Geological Correlation Programme |volume=53 |publisher={{wipe|Cambridge University Press}} |pages=42–78}}</ref><ref name=palaeos>The early and mid Silurian. See {{cite web |vauthors=Kazlev MA, White T |title=Thelodonti |website=Palaeos.com |date=March 6, 2001 |access-date=October 30, 2007 |url-status=dead |url=http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.100.html#Thelodonti |archive-url=https://web.archive.org/web/20071028153634/http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.100.html#Thelodonti |archive-date=2007-10-28}}</ref> The fish lived in both freshwater and marine environments, first appearing during the [[Earth:Ordovician|Ordovician]], and perishing during the [[Earth:Late Devonian extinction|Frasnian–Famennian extinction event]] of the Late [[Earth:Devonian|Devonian]]. They occupied a large variety of ecological niches, with a large amount of species preferring reef ecosystems, where their flexible bodies were more at ease than the heavily armoured bulks of other jawless fish.<ref>{{cite journal | vauthors = Ferrón HG, Botella H | title = Squamation and ecology of thelodonts | journal = PLOS ONE | volume = 12 | issue = 2 | page = e0172781 | year = 2017 | pmid = 28241029 | pmc = 5328365 | doi = 10.1371/journal.pone.0172781 | bibcode = 2017PLoSO..1272781F | doi-access = free }}</ref>
|-
! <sup>†</sup>[[Biology:Anaspida|Anaspida]]<br /><small>(extinct)</small>
| [[File:Anaspida.png|140px]]
|-
! <sup>†</sup>[[Biology:Cephalaspidomorphi|Cephalaspido-<br />morphi]]<br /><small>(extinct)</small>
| [[File:Thyestes verrucosus22DB.jpg|226x226px]]{{center|''[[Biology:Thyestes (fish)|Thyestes]]''}}
| [[Biology:Cephalaspidomorphi|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 [[Biology:Osteostraci|osteostracan]] genus ''[[Biology:Cephalaspis|Cephalaspis]]''. Most biologists regard this [[Biology:Taxon|taxon]] as extinct, but the name is sometimes used in the classification of [[Biology:Lamprey|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 [[Earth:Silurian|Silurian]] period through the [[Earth:Mesozoic|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, [[Biology:Gill|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 [[Biology:Lateral line|lateral line]] organ. The eyes were rather small and placed on the top of the head. There was no proper [[Medicine:Jaw|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 |last1=Colbert |first1=Michael |last2=Morales |first2=Edwin H. |year=1991 |title=Evolution of the Vertebrates : A history of the backboned animals through time |edition=4th |publisher=Wiley-Liss |location=New York, NY |isbn=978-0-471-85074-8}}</ref> Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina", include the [[Biology:Class|classes]] [[Biology:Osteostraci|Osteostraci]], [[Biology:Galeaspida|Galeaspida]], and [[Biology:Pituriaspida|Pituriaspida]]
|}

==Groups==
Phylogeny based on the work of Mikko Haaramo and Delsuc et al.<ref name="mikko">{{cite web |last=Haaramo |first=Mikko |year=2007 |title=''Chordata'' – lancets, tunicates, and vertebrates |website=Mikko's Phylogeny Archive |url=http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/chordata.html#Vertebrata |access-date=30 December 2016}}</ref><ref>{{cite journal |vauthors=Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ |date=April 2018 |title=A phylogenomic framework and timescale for comparative studies of tunicates |journal=BMC Biology |volume=16 |issue=1 |page=39 |pmid=29653534 |pmc=5899321 |doi=10.1186/s12915-018-0499-2 |doi-access=free }}</ref>

{{clade|{{clade
|label1=[[Biology:Cyclostomata|Cyclostomata]]
|1=
{{clade 
  |1=Hyperotreti/Myxini (hagfishes)
    |2=[[Biology:Hyperoartia|Petromyzontomorpha]] (lampreys)
}}

    |2={{clade
      |1=?†Conodonta
      |2={{clade
        |1=†Pteraspidomorpha
        |label2=Myopterygii
        |2={{clade
          |label1=[[Biology:Anaspidomorphi|Anaspidomorphi]]
          |1={{clade
            |1=?†[[Biology:Jamoytiiformes|Jamoytiiformes]]
            |2=?†[[Biology:Euphanerida|Euphanerida]]
            |3=†[[Biology:Anaspida|Anaspida]]
             }}
          |2={{clade
            |1=†[[Biology:Thelodonti|Thelodonti]]
            |2={{clade
              |grouplabel1={{clade labels|label1=Cephalaspidomorpha |top1=35%}}
              |1={{clade
                |1=†[[Biology:Galeaspida|Galeaspida]] |bar1=red
                |2={{clade
                  |1=†[[Biology:Pituriaspida|Pituriaspida]] |bar1=red 
                  |2={{clade
                    |1=†[[Biology:Osteostraci|Osteostraci]] |bar1=red
                    |2=[[Biology:Gnathostomata|Gnathostomata]] (vertebrates with jaws)
                     }}
                   }}
                 }}
               }}
             }}
           }}
         }}
       }}
     }}|style=font-size:80%;line-height:80%|label1=Vertebrata/|sublabel1=Craniata}}

While the "agnatha" Conodonta was indeed jawless, if it would have continued to live, its descendants would still be closer related to e.g. humans than to lampreys, and also contempory it was closer related to the ancestor of humans. Due to such considerations, Agnatha can not be consolidated into a coherent grouping without either removing any non-cyclostomata, or by including all vertebrata thus rendering it into a junior synonym of vertebrata.

The new phylogeny from Miyashita ''et al''. (2019) is considered compatible with both morphological and molecular evidence.<ref>{{cite journal |first1=Tetsuto |last1=Miyashita |first2=Michael I. |last2=Coates |first3=Robert |last3=Farrar |first4=Peter |last4=Larson |first5=Phillip L. |last5=Manning |first6=Roy A. |last6=Wogelius |first7=Nicholas P. |last7=Edwards |first8=Jennifer |last8=Anné |first9=Uwe |last9=Bergmann |first10=A. Richard |last10=Palmer |first11=Philip J. |last11=Currie |display-authors=6 |year=2019 |title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=116 |issue=6 |pages=2146–2151 |doi=10.1073/pnas.1814794116 |pmid=30670644 |pmc=6369785|bibcode=2019PNAS..116.2146M |doi-access=free }}</ref>

{{clade|{{clade
  |grouplabel1={{clade labels|label1=[[Biology:Agnatha|Agnatha]] |top1=35%}}
  |1=†[[Biology:Myllokunmingiidae|Myllokunmingiidae]] [[File:Haikouichthys_3d.png|50 px]] |bar1=red
  |2={{clade
       |bar1=red
       |1={{clade 
         |label1=†[[Biology:Anaspidomorphi|Anaspidomorphi]]
         |1={{clade      
            |1=†[[Biology:Euphanerida|Euphanerida]] [[File:Cornovichthys_blaauweni.jpg|50 px]]
            |2=†[[Biology:Jamoytiiformes|Jamoytiiformes]] [[File:Jamoytius_kerwoodi_model.jpg|50 px]]
            |label3=†[[Biology:Anaspida|Anaspida]] 
            |3={{clade
               |1=†[[Biology:Endeiolepis|Endeiolepis]]
               |2=†[[Biology:Birkeniiformes|Birkeniiformes]] [[File:Lasanius_NT_small.jpg|50 px]] }} }}
         |2={{clade
            |label1=†Conodonta
            |1= {{clade
               |1=†[[Biology:Paraconodontida|Paraconodontida]]
               |2={{clade
                  |1=†[[Biology:Proconodontida|Proconodontida]] 
                  |label2=†Euconodonta
                  |2={{clade
                     |1={{clade
                        |1=†Protopanderodontida
                        |2=†Panderontida }}
                     |label2=†[[Biology:Prioniodontida|Prioniodontida]]
                     |2={{clade
                        |1=†[[Biology:Paracordylodus|Paracordylodus]]
                        |2={{clade
                           |1=†[[Biology:Balognathidae|Balognathidae]] [[File:Promissum_NT_small.jpg|50 px]]
                           |2={{clade
                              |1=†[[Biology:Prioniodinida|Prioniodinida]]
                              |2=†[[Biology:Ozarkodinida|Ozarkodinida]] }} }} }} }} }} }}
            |label2=[[Biology:Cyclostomata|Cyclostomi]]
            |2={{clade
               |1=[[Biology:Hagfish|Hagfish]] [[File:Eptatretus_hexatrema_15727711.jpg|50 px]]  
               |2=Lampreys [[File:Jõesilmud2.jpg|50 px]] }} }} }}
       |2={{clade 
         |label1=†[[Biology:Pteraspidomorphi|Pteraspidomorphi]]  |bar1=red
         |1={{clade
            |1=†Anatolepis
            |2={{clade
               |1=†[[Biology:Arandaspida|Arandaspida]] [[File:Arandaspis_NT.jpg|50 px]]
               |label2=†Heterostracomorphi
               |2={{clade
                  |label1=†[[Biology:Astraspida|Astraspida]]
                  |1={{clade
                     |1=†Astraspidiformes [[File:Astraspis_desiderata.jpg|50 px]]
                     |2=†Tesakoviaspidida }}
                  |2=†[[Biology:Eriptychiida|Eriptychiida]]
                  |label3=†[[Biology:Heterostraci|Heterostraci]]
                  |3={{clade
                     |1=†Tesseraspidiformes
                     |2=†[[Biology:Cardipeltiformes|Cardipeltiformes]] [[File:Cardipeltis_bryanti.jpg|50 px]]
                     |3=†Lepidaspidiformes [[File:Lepidaspis_NT.jpg|50 px]]
                     |4=†Corvaspidida
                     |5={{clade
                        |1=†[[Biology:Traquairaspidiformes|Traquairaspidiformes]] [[File:Phialaspis_Traquairaspis_symmondsi.jpg|50 px]]
                        |2=†[[Biology:Tolypelepidida|Tolypelepidida]] [[File:Tolypelepis_undulata.jpg|50 px]]
                        |3={{clade
                           |1=†[[Biology:Cyathaspidiformes|Cyathaspidiformes]] [[File:Ctenaspis_NT_small.jpg|50 px]]
                           |2=†[[Biology:Pteraspidiformes|Pteraspidiformes]] [[File:Larnovaspis_stensioei.jpg|50 px]] }} }} }} }} }} }}         |2={{clade
            |label1=†[[Biology:Thelodonti|Thelodonti]]  |bar1=red
            |1={{clade
               |1=†[[Biology:Thelodontiformes|Thelodontiformes]] [[File:Thelodus_parvidens.JPG|50 px]]  
               |2=†[[Biology:Archipelepidiformes|Archipelepidiformes]] [[File:Archipelepis_turbinata.jpg|50 px]]  
               |3=†[[Biology:Furcacaudiformes|Furcacaudiformes]] [[File:Furcacauda_heintzae.jpg|50 px]]  }}
            |2={{clade
               |label1=†[[Biology:Galeaspida|Galeaspida]]  |bar1=red
               |1={{clade
                     |1=†[[Biology:Eugaleaspidiformes|Eugaleaspidiformes]] [[File:Dunyu_longiforus.jpg|50 px]]
                     |label2=†Polybranchiaspidida
                     |2={{clade
                        |1=†Huananaspidiformes
                        |2=†Polybranchiaspidiformes }} }}
               |2={{clade
                  |1=†[[Biology:Pituriaspida|Pituriaspida]] [[File:Pituriaspida.jpg|50 px]]  |bar1=red
                  |2={{clade
                     |label1=†[[Biology:Osteostraci|Osteostraci]]  |bar1=red
                     |1={{clade
                        |1=†[[Biology:Ateleaspididae|Ateleaspididae]] [[File:Ateleaspis_small.jpg|50 px]]
                        |2={{clade
                           |1=†[[Biology:Cephalaspidida|Cephalaspidida]] [[File:Cephalaspis_tenuicornis.jpg|50 px]]
                           |2={{clade
                              |1=†[[Biology:Zenaspida|Zenaspida]] [[File:Zenaspis_NT_small.jpg|50 px]]
                              |2={{clade 
                                 |1=†[[Biology:Benneviaspidida|Benneviaspidida]] [[File:Boreaspis_NT.jpg|50 px]]
                                 |2=†[[Biology:Thyestiida|Thyestiida]] [[File:Osteostraci_Janvier.gif|50 px]] }} }} }} }}
                     |2=[[Biology:Gnathostomata|Gnathostomes]]  [[File:Bothriolepis_canadensis.jpg|50 px]]
}} }} }} }} }} }} }}|style=font-size:85%;line-height:85%;|label1=Vertebrata/|sublabel1=Craniata}}

== See also ==
* [[Biology:Gnathostomata|Gnathostomata]]
* [[Biology:Amphirhina|Amphirhina]], an alternate name for the above parallel, or sister, classification
* [[Biology:Cyclostomata|Cyclostomata]]

== References ==
{{Reflist|25em}}

{{Chordata}}
{{Taxonbar|from=Q161095}}

[[Category:Agnatha| ]]
[[Category:Paraphyletic groups]]
[[Category:Extant Cambrian first appearances]]

{{Sourceattribution|Agnatha}}