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 = Cambrian Stage 3–Present {{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 = Cope, 1889 | includes = *†Anaspidomorphi *†Conodonta *Cyclostomi (hagfish and lampreys) *†Galeaspida *†Myllokunmingiida? *†''Metaspriggina''? *†''Nuucichthys''? *†Osteostraci *†Pituriaspida *†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> *†Yunnanozoonidae? *†''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 animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both living (cyclostomes such as hagfishes and lampreys) and extinct clades (e.g. conodonts and cephalaspidomorphs, among others). They are sister to 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> who evolved from jawless ancestors during the early Silurian by developing folding articulations in the first pairs of gill arches.

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 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, hagfishes and lampreys, are more closely related to each other than to jawed fish, forming the 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 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 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 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, their possible food types are limited.

==Morphology== In addition to the absence of jaws, modern agnathans are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. Lampreys have a light-sensitive pineal eye (homologous to the pineal gland in mammals). 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 cartilaginous skeleton, and the heart contains 2 chambers.

===Body covering===

In modern agnathans, the body is covered in skin, with neither dermal or epidermal 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 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 pituriaspids, did have paired fins, a trait inherited in their 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. 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>]] {{See also|Evolution of fish}}

Although a minor element of modern marine 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. Conodonts, 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 Triassic that their teeth (the only parts of them that were usually fossilized) 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 | 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 ostracoderms, precursors to the bony fish and hence to the tetrapods (including humans)—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 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 conodonts.<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 receptors (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>

== Classification == {| class="wikitable" |+ Subgroups of jawless fish |- ! colspan=2 | Subgroup ! Example ! Comments |- ! rowspan=2 | Cyclostomi ! Myxini | 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 to vertebrates, and living hagfish remain similar to hagfish 300 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 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 | 170x170px{{center|''Entosphenus tridentatus'' (Pacific lamprey) }} | Petromyzontida, also called Hyperoartia, is a disputed group of vertebrates that includes the modern lampreys and their fossil relatives. Examples of hyperoartians from early in their fossil record are ''Endeiolepis'' and ''Euphanerops'', fish-like animals with hypocercal tails that lived during the Late Devonian Period. Some paleontologists still place these forms among the "ostracoderms" (jawless armored "fishes") of the class 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 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 vertebrates. 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 conodonts, 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 jawed vertebrates. |- ! rowspan 4 | Myllokunmingiida !† Myllokunmingiidae (extinct) | 140px{{center|''Haikouichthys ercaicunensis''}} |The 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 craniates (chordates with a skull of hard bone or cartilage). Currently the group includes 3 known genera, ''Haikouichthys'', ''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'' |doi=10.5852/ejt.2018.466 |journal=European Journal of Taxonomy |issue=466 |doi-access=free }}</ref> |- ! rowspan 5 | Conodonta ! † Conodonti and Cavidonti (extinct) | 154x154px {{center|''Panderodus unicostatus''}} |Conodonts 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 feeders and others being macropredators. For over a century, these animals were only known because of their microscopic, phosphatic tooth structures called "conodont elements". It was not 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 Ordovician-Silurian extinction event. They then reached another spike in diversity in the 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 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 | "Ostracodermi" ! †Pteraspidomorphi (extinct) | 170x170px{{center|''Astraspis desiderata''}} | †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. |- ! †Thelodonti (extinct) | 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 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 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 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> |- ! †Anaspida (extinct) | 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 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 lampreys, became extinct due to the environmental upheaval during that time. |- ! †Cephalaspido- morphi (extinct) | 226x226px{{center|''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 osteostracan genus ''Cephalaspis''. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, 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, gills 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 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=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 |barbegin1=orange |2={{clade |1=†Pituriaspida |bar1=orange |2={{clade |1=†Osteostraci |barend1=orange |2=Gnathostomata (vertebrates with jaws) }} }} }} }} }} }} }} }} }}|style=font-size:85%;line-height:85%|label1=Vertebrata/|sublabel1=Craniata}}

While the "Agnatha" Conodonta was indeed jawless, if it would have continued to live, its descendants would still be more closely 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 50 px |barbegin1=red |2={{clade |bar1=red |1={{clade |label1=†Anaspidomorphi |1={{clade |1=†Euphanerida 50 px |2=†Jamoytiiformes 50 px |label3=†Anaspida |3={{clade |1=†Endeiolepidiformes |2=†Birkeniiformes 50 px }} }} |2={{clade |label1=†Conodonta |1={Conodonta} |label2=Cyclostomi |2={{clade |1=Hagfish 50 px |2=Lampreys 50 px }} }} }} |2={{clade |label1=†Pteraspidomorphi |bar1=red |1={Pteraspidomorphi} |2={{clade |label1=†Thelodonti |bar1=red |1={{clade |1=†Thelodontiformes 50 px |2=†Archipelepidiformes 50 px |3=†Furcacaudiformes 50 px }} |2={{clade |label1=†Galeaspida |bar1=red |1={{clade |1=†Eugaleaspidiformes 50 px   |label2=†Polybranchiaspidida |2={{clade |1=†Huananaspidiformes |2=†Polybranchiaspidiformes }} }} |2={{clade |1=†Pituriaspida 50 px |bar1=red |2={{clade |label1=†Osteostraci |barend1=red |1={{clade |1=†Atelaspidiformes 50 px |2={{clade |1=†Cephalaspidida 50 px |2={{clade |1=†Zenaspida 50 px |2={{clade |1=†Benneviaspidida 50 px |2=†Thyestiida 50 px }} }} }} }} |2=Gnathostomes 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 50 px |2={{clade |1=†Prioniodinida |2=†Ozarkodinida }} }} }} }} }} }} }}|targetC={Pteraspidomorphi}|subcladeC={{clade hidden |id=3 |mode=left |1=†Anatolepis |2={{clade |1=†Arandaspida 50 px |label2=†Heterostracomorphi |2={{clade |label1=†Astraspida |1={{clade |1=†Astraspidiformes 50 px |2=†Tesakoviaspidida }} |2=†Eriptychiida |label3=†Heterostraci |3={{clade |1=†Tesseraspidiformes |2=†Cardipeltiformes 50 px |3=†Lepidaspidiformes 50 px |4=†Corvaspidida |5={{clade |1=†Traquairaspidiformes 50 px |2=†Tolypelepidida 50 px |3={{clade |1=†Cyathaspidiformes 50 px |2=†Pteraspidiformes 50 px }} }} }} }} }} }}}}

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

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

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