View source for Agnatha

{{Short description|Paraphyletic infraphylum of fish}}
{{About|jawless vertebrates|the absence of one or two jaws|Agnathia|the municipality in Lebanon|Aynata}}
{{Paraphyletic group
| fossil_range = <br />[[Cambrian Stage 3]]&ndash;[[Holocene|Present]]<br />{{fossil range|518|0|earliest= 525|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>}}
| name = Jawless fish
| image = Одинока мінога.JPG
| image_caption = A [[lamprey]], a jawless fish belonging to [[Cyclostomi]]
| auto = yes
| taxon = Agnatha
| authority = [[Edward Drinker Cope|Cope]], 1889
| includes = *†[[Anaspidomorphi]]
*†[[Conodonta]]
*[[Cyclostomi]] (hagfish and lampreys)
*†[[Galeaspida]]
*†[[Myllokunmingiida]]?
*†''[[Metaspriggina]]''?
*†''[[Nuucichthys]]''?
*†[[Osteostraci]]
*†''[[Palaeospondylus]]''?
*†[[Pituriaspida]]
*†[[Pteraspidomorphi|Pteraspidomorpha]] 
*†[[Thelodonti]]<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>
*†[[Yunnanozoon]]idae?                                                                                              
*†''[[Zhongxiniscus]]''?
*{{Extinct}}[[Emmonaspis]]?
| image2 = Pteraspis Wiki2.png
| image2_caption = [[Life reconstruction]] of extinct agnathan genus, ''[[Pteraspis]]'' ([[Pteraspidomorphi]])
| excludes = *[[Gnathostomata]] (jawed vertebrates)
}}

'''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}}) or '''jawless fish''' is a [[paraphyletic]] [[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 |article-number=e0119248 |pmid=25923521 |bibcode=2015PLoSO..1019248R |doi-access=free}}</ref> of [[animal]]s in the [[subphylum]] [[Vertebrata]] of the [[phylum]] [[Chordata]], characterized by the lack of [[jaw]]s. The group consists of both [[extant taxon|living]] ([[Cyclostomi|cyclostomes]] such as [[hagfish]]es and [[lamprey]]s) and [[Extinction|extinct]] [[clade]]s (e.g. [[conodont]]s and [[Cephalaspidomorphi|cephalaspidomorphs]], among others). They are [[sister taxon|sister]] to vertebrates with [[jaw]]s known as [[gnathostome]]s,<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> who [[evolution|evolve]]d from jawless ancestors during the early [[Silurian]] by developing folding [[joint|articulation]]s in the first pairs of [[gill arch]]es.

[[Sequencing|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 | bibcode = 2002MolPE..22..184D }}</ref> as well as [[Embryology|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, [[hagfish]]es and [[lamprey]]s, are more closely related to each other than to [[Gnathostomata|jawed fish]], forming the [[Class (biology)|superclass]] 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><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. |date=August 1992 |title=Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group |journal=Science |volume=257 |issue=5071 |pages=787–789 |bibcode=1992Sci...257..787S |doi=10.1126/science.1496398 |pmid=1496398}}</ref>

The oldest fossil agnathans appeared in the [[Cambrian]]. Living jawless fish comprise about 120&nbsp;[[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 [[Craniata]] hypothesis was accepted (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates.

== 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 carrion, as well as other fish and marine mammals, although some species are non-carnivorous.<ref name=":9spr">{{Citation |last1=Potter |first1=Ian C. |title=The Taxonomy, Phylogeny, and Distribution of Lampreys |date=2014-11-25 |url=https://www.springer.com/cda/content/document/cda_downloaddocument/9789401793056-c1.pdf?SGWID=0-0-45-1486832-p176846628 |work=Lampreys: Biology, Conservation and Control |pages=35–73 |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-017-9306-3_2 |isbn=978-94-017-9305-6 |access-date=2018-10-21 |last2=Gill |first2=Howard S. |last3=Renaud |first3=Claude B. |last4=Haoucher |first4=Dalal |archive-date=3 August 2018 |archive-url=https://web.archive.org/web/20180803014120/https://www.springer.com/cda/content/document/cda_downloaddocument/9789401793056-c1.pdf?SGWID=0-0-45-1486832-p176846628 }}</ref> [[Anticoagulant]] fluids preventing blood clotting are injected into the host, causing the host to yield more blood. Hagfish are scavengers, eating mostly dead animals, although they have also been observed [[predation|hunting]].<ref>{{Cite journal | last1 = Zintzen | first1 = V. | last2 = Roberts | first2 = C. D. | last3 = Anderson | first3 = M. J. | last4 = Stewart | first4 = A. L. | last5 = Struthers | first5 = C. D. | last6 = Harvey | first6 = E. S. | title = Hagfish predatory behaviour and slime defence mechanism | doi = 10.1038/srep00131 | journal = Scientific Reports | volume = 1 | page = 131 | year = 2011 | pmid =  22355648| pmc = 3216612| bibcode = 2011NatSR...1E.131Z }}</ref> They use a row of sharp teeth to break down the animal. Because agnathan teeth are unable to move up and down it limits their possible food types.

==Morphology==
In addition to the absence of [[Fish jaw|jaws]], modern agnathans are characterised by absence of paired [[Fish fin|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 paired [[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.

===Body covering===

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 [[#Evolution|extinct agnathans]] sported extensive exoskeletons composed of either massive, heavy dermal [[armour (zoology)|armour]] or small mineralized scales.

===Appendages===

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>

== 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. 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 (Benton)|''Vertebrate Palaeontology'']], Blackwell, 3rd edition, Figure 3.25 on page 73, {{ISBN|0-632-05637-1}}.</ref>]]
{{See also|Evolution of fish}}

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 agnathan from the same region is ''[[Haikouella]]''. A possible agnathan that has not been formally described was reported by Simonetti from the Middle Cambrian [[Burgess Shale]] of [[British Columbia]]. [[Conodont]]s, a class of agnathans which arose in the early Cambrian,<ref>{{cite book |title=Earth System History |edition=4th |publisher=[[Macmillan Publishers|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 [[Triassic]] that their teeth (the only parts of them that were usually fossilized) [[Conodont biostratigraphy|are often used]] as [[index fossil]]s 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 | bibcode = 2001JPal...75.1174S | 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 [[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 [[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 [[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 [[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=978-0-12-387664-5 |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 |article-number=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 | [[Cyclostomi]]
! [[Myxini]]
| [[File:Eptatretus hexatrema 15727711.jpg|170x170px]]
{{center|''[[Eptatretus hexatrema]]'' (sixgill hagfish)}}
| [[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]]. The group has gone through the most extensive gene loss of all vertebrates, with 1,386 missing gene families.<ref>{{Cite journal |last1=Marlétaz |first1=Ferdinand |last2=Timoshevskaya |first2=Nataliya |last3=Timoshevskiy |first3=Vladimir A. |last4=Parey |first4=Elise |last5=Simakov |first5=Oleg |last6=Gavriouchkina |first6=Daria |last7=Suzuki |first7=Masakazu |last8=Kubokawa |first8=Kaoru |last9=Brenner |first9=Sydney |last10=Smith |first10=Jeramiah J. |last11=Rokhsar |first11=Daniel S. |date=2024 |title=The hagfish genome and the evolution of vertebrates |journal=Nature |language=en |volume=627 |issue=8005 |pages=811–820 |doi=10.1038/s41586-024-07070-3 |pmid=38262590 |bibcode=2024Natur.627..811M |issn=1476-4687|pmc=10972751 }}</ref> 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&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 |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 [[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"/>
|-
! [[Petromyzontida]]
| [[File:Pacific lamprey facing right.jpg|170x170px]]{{center|''[[Entosphenus tridentatus]]'' 
(Pacific lamprey) }}
| [[Petromyzontida]], also called 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.
|-
! rowspan 4 | [[Myllokunmingiida]]
![[extinction|<sup>†</sup>]] [[Myllokunmingiidae]]<br /><small>(extinct)</small>
| [[File:Haikouichthys NT.jpg|140px]]{{center|''[[Haikouichthys ercaicunensis]]''}}
|The [[Myllokunmingiidae|myllokunmingiids]] were a primitive order of agnathans that were endemic to the [[Cambrian]] aged  [[Maotianshan Shales]] [[lagerstätte]] in [[China]]. These creatures are the earliest known group of [[craniate]]s (chordates with a skull of [[Bone|hard bone]] or [[cartilage]]). Currently the group includes 3 known genera, ''[[Haikouichthys]]'', ''[[Myllokunmingia]]'', and ''[[Zhongjianichthys rostratus|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'' |doi=10.5852/ejt.2018.466 |journal=European Journal of Taxonomy |issue=466 }}</ref>
|-
! rowspan 5 | [[Conodont]]a
! [[extinction|<sup>†</sup>]] 
[[Conodont]]i and [[Conodont|Cavidonti]]<br /><small>(extinct)</small>
| [[File:Panderodus.jpg|154x154px]]
{{center|''[[Panderodus|Panderodus unicostatus]]''}}
|[[Conodont]]s were eel like agnathans that lived from the [[Cambrian]] up until the beginning of the [[Jurassic]] period. They were very diverse in terms of lifestyles, with some species being [[filter feeder]]s and others being macropredators. For over a [[century]], these animals were only known because of their microscopic, [[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 [[Late Ordovician mass extinction|Ordovician-Silurian extinction]] event. They then reached another spike in diversity in the [[Devonian|mid-late Devonian]] before again declining in the [[Carboniferous]]. They were relatively rare in the [[Permian]], but dramatically increased in numbers in the [[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 [[Triassic–Jurassic extinction event|large extinction]] at the end of the Triassic. Instead, it is now thought that they were out competed by newer [[Mesozoic]] taxa.<ref name="C.H. Pander's introduction to conodonts, 1856">{{cite journal |author1=Sweet, Walter C. |author2=Cooper, Barry J. |date=December 2008 |title=C.H. Pander's introduction to conodonts, 1856 |journal=Episodes |volume=31 |issue=4 |pages=429–432 |url=https://www.researchgate.net/publication/286568271 |access-date=3 January 2019 |doi=10.18814/epiiugs/2008/v31i4/009 |doi-access=free }}</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|bibcode=2001JPal...75.1174S |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 |article-number=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 | "[[Ostracoderm]]i"
! [[extinction|<sup>†</sup>]][[Pteraspidomorphi]]<br /><small>(extinct)</small>
| [[File:Astraspis desiderata paleoart with light rays.tif|170x170px]]{{center|''[[Astraspis desiderata]]''}}
| <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.
The taxon contains the subgroups [[Heterostraci]], [[Astraspida]], [[Arandaspida]].
|-
! <sup>†</sup>[[Thelodonti]]<br/><small>(extinct)</small>
| [[File:Thelodonti.gif|140px]]
| [[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=[[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 [[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 | 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 [[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 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=[[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=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 [[Ordovician]], and perishing during the [[Late Devonian extinction|Frasnian–Famennian extinction event]] of the Late [[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 | article-number = e0172781 | year = 2017 | pmid = 28241029 | pmc = 5328365 | doi = 10.1371/journal.pone.0172781 | bibcode = 2017PLoSO..1272781F | doi-access = free }}</ref>
|-
! <sup>†</sup>[[Anaspida]]<br /><small>(extinct)</small>
| [[File:Anaspida.png|140px]]
| [[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 | last = Ahlberg | first = Per Erik | name-list-style = vanc |title=Major events in early vertebrate evolution: Palaeontology, phylogeny, genetics, and development |publisher=Taylor & Francis |location=Washington, DC |year=2001 |isbn=978-0-415-23370-5 |url=https://books.google.com/books?id=zeyRZNZl-74C&pg=PA188 |page= 188}}</ref> They are classically regarded as the ancestors of lampreys.<ref name="isbn0-521-32271-5">{{cite book |last = Patterson |first = Colin |title=Molecules and Morphology in Evolution: Conflict or compromise? |publisher=Cambridge University Press |location=Cambridge, UK |year=1987 |isbn=978-0-521-32271-3 |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 [[Llandovery epoch|Early Silurian]], and flourished until the [[Late Devonian extinction]],<ref name="isbn0-226-31568-1">{{cite book | last1 = Hall | first1 = Brian Keith | last2 = Hanken | first2 = James |author2-link=James Hanken |title=The Skull |publisher=University of Chicago Press |location=Chicago, IL |year=1993 |isbn=978-0-226-31568-3 |url= https://books.google.com/books?id=fB-hO7i50esC&pg=PA131|page= 131}}</ref> where most species, save for [[lamprey]]s, became extinct due to the environmental upheaval during that time.
|-
! <sup>†</sup>[[Cephalaspidomorphi|Cephalaspido-<br />morphi]]<br /><small>(extinct)</small>
| [[File:Thyestes verrucosus22DB.jpg|226x226px]]{{center|''[[Thyestes (fish)|Thyestes verrucosus]]''}}
| [[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 |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 [[class (biology)|classes]] [[Osteostraci]], [[Galeaspida]], and [[Pituriaspida]].
|}

==Phylogeny==
{{See also|Placodermi#Cladogram}}
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 name="Delsuc-2018">{{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=[[Cyclostomata]]
|1=
{{clade 
  |1=[[Hyperotreti]]/[[Myxini]] (hagfishes)
    |2=[[Hyperoartia|Petromyzontomorpha]] (lampreys)
}}

    |2={{clade
      |1=?†[[Conodonta]]
      |2={{clade
        |1=†[[Pteraspidomorpha]]
        |label2=Myopterygii
        |2={{clade
          |label1=[[Anaspidomorphi]]
          |1={{clade
            |1=?†[[Jamoytiiformes]]
            |2=?†[[Euphanerida]]
            |3=†[[Anaspida]]
             }}
          |2={{clade
            |1=†[[Thelodonti]]
            |2={{clade
              |grouplabel1={{clade labels|label1=[[Cephalaspidomorpha]] |top1=35%}}
              |1={{clade
                |1=†[[Galeaspida]] |bar1=red
                |2={{clade
                  |1=†[[Pituriaspida]] |bar1=red 
                  |2={{clade
                    |1=†[[Osteostraci]] |bar1=red
                    |2=[[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 contemporary it was more closely 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 name="Delsuc-2018" /><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
|grouplabel1={{clade labels|label1=Agnatha |top1=45%}}
|1={{clade
   |1=†[[Myllokunmingiida]] [[File:Haikouichthys_3d.png|50 px]] |bar1=red
   |2={{clade
       |bar1=red
       |1={{clade 
         |label1=†[[Anaspidomorphi]]
         |1={{clade      
            |1=†[[Euphanerida]] [[File:Achanarella reconstruction.jpg|50 px]]
            |2=†[[Jamoytiiformes]] [[File:Jamoytius_kerwoodi_model.jpg|50 px]]
            |label3=†[[Anaspida]] 
            |3={{clade
               |1=†[[Endeiolepidiformes]]
               |2=†[[Birkeniiformes]] [[File:Lasanius_NT_small.jpg|50 px]] }}
}}
         |2={{clade
            |label1=†[[Conodonta]]
            |1={Conodonta}
            |label2=[[Cyclostomata|Cyclostomi]]
            |2={{clade
               |1=[[Hagfish]] [[File:Eptatretus_hexatrema_15727711.jpg|50 px]]  
               |2=[[Lampreys]] [[File:Jõesilmud2.jpg|50 px]] }} }} }}
       |2={{clade 
         |label1=†[[Pteraspidomorphi]]  |bar1=red
         |1={Pteraspidomorphi}
         |2={{clade
            |label1=†[[Thelodonti]]  |bar1=red
            |1={{clade
               |1=†[[Thelodontiformes]] [[File:Thelodus_parvidens.JPG|50 px]]  
               |2=†[[Archipelepidiformes]] [[File:Archipelepis_turbinata.jpg|50 px]]  
               |3=†[[Furcacaudiformes]] [[File:Furcacauda_heintzae.jpg|50 px]]  }}
            |2={{clade
               |label1=†[[Galeaspida]]  |bar1=red
               |1={{clade
                     |1=†[[Eugaleaspidiformes]] [[File:Dunyu_longiforus.jpg|50 px]]&nbsp;&nbsp;
                     |label2=†[[Polybranchiaspidida]]
                     |2={{clade
                        |1=†[[Huananaspidiformes]]
                        |2=†[[Polybranchiaspidiformes]] }} }}
               |2={{clade
                  |1=†[[Pituriaspida]] [[File:Pituriaspida.jpg|50 px]]  |bar1=red
                  |2={{clade
                     |label1=†[[Osteostraci]]  |bar1=red
                     |1={{clade
                        |1=†[[Atelaspidiformes]] [[File:Ateleaspis_small.jpg|50 px]]
                        |2={{clade
                           |1=†[[Cephalaspidida]] [[File:Cephalaspis_tenuicornis.jpg|50 px]]
                           |2={{clade
                              |1=†[[Zenaspida]] [[File:Zenaspis_NT_small.jpg|50 px]]
                              |2={{clade 
                                 |1=†[[Benneviaspidida]] [[File:Boreaspis_NT.jpg|50 px]]
                                 |2=†[[Thyestiida]] [[File:Osteostraci_Janvier.gif|50 px]] }} }} }} }}
                     |2=[[Gnathostomata|Gnathostomes]]  [[File:Phymolepis_cuifengshanensis.PNG|50 px]]
}} }} }} }} }} }} }}|style=font-size:85%;line-height:85%;|label1=[[Vertebrata]]/|sublabel1=[[Craniata]]|targetB={Conodonta}|subcladeB={{clade hidden |id=2 |mode=left
            |1= {{clade
               |1=†[[Paraconodontida]]
               |2={{clade
                  |1=†[[Proconodontida]] 
                  |label2=†[[Euconodonta]]
                  |2={{clade
                     |1={{clade
                        |1=†[[Protopanderodontida]]
                        |2=†[[Panderontida]] }}
                     |label2=†[[Prioniodontida]]
                     |2={{clade
                        |1=†[[Paracordylodus]]
                        |2={{clade
                           |1=†[[Balognathidae]] [[File:Promissum_NT_small.jpg|50 px]]
                           |2={{clade
                              |1=†[[Prioniodinida]]
                              |2=†[[Ozarkodinida]] }} }} }} }} }} }}
}}|targetC={Pteraspidomorphi}|subcladeC={{clade hidden |id=3 |mode=left
            |1=†[[Anatolepis]]
            |2={{clade
               |1=†[[Arandaspida]] [[File:Arandaspis_NT.jpg|50 px]]
               |label2=†[[Heterostracomorphi]]
               |2={{clade
                  |label1=†[[Astraspida]]
                  |1={{clade
                     |1=†[[Astraspidiformes]] [[File:Astraspis_desiderata.jpg|50 px]]
                     |2=†[[Tesakoviaspidida]] }}
                  |2=†[[Eriptychiida]]
                  |label3=†[[Heterostraci]]
                  |3={{clade
                     |1=†[[Tesseraspidiformes]]
                     |2=†[[Cardipeltiformes]] [[File:Cardipeltis_bryanti.jpg|50 px]]
                     |3=†[[Lepidaspidiformes]] [[File:Lepidaspis_NT.jpg|50 px]]
                     |4=†[[Corvaspidida]]
                     |5={{clade
                        |1=†[[Traquairaspidiformes]] [[File:Phialaspis_Traquairaspis_symmondsi.jpg|50 px]]
                        |2=†[[Tolypelepidida]] [[File:Tolypelepis_undulata.jpg|50 px]]
                        |3={{clade
                           |1=†[[Cyathaspidiformes]] [[File:Ctenaspis_NT_small.jpg|50 px]]
                           |2=†[[Pteraspidiformes]] [[File:Larnovaspis_stensioei.jpg|50 px]] }} }} }} }} }}
}}}}

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

== References ==
{{Wikispecies|position=left|Agnatha}}
{{Reflist|25em}}

{{Chordata}}
{{Evolution of fish}}
{{Taxonbar|from=Q161095}}
{{Authority control}}

[[Category:Agnatha| ]]
[[Category:Paraphyletic groups]]
[[Category:Infraphyla]]
[[Category:Terreneuvian first appearances]]
[[Category:Extant Cambrian first appearances]]
[[Category:Taxa named by Edward Drinker Cope]]