The oldest fossil agnathans appeared in theCambrian. Living jawless fish comprise about 120 species in total. Hagfish are considered members of thesubphylumVertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular[6][7][11] and developmental[12] data, theCraniata hypothesis was accepted (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates.
Agnathans areectothermic, 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.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. Because agnathan teeth are unable to move up and down it limits their possible food types.
In addition to the absence ofjaws, modern agnathans are characterised by absence of pairedfins; the presence of anotochord both in larvae and adults; and seven or more pairedgill pouches. Lampreys have a light sensitivepineal eye (homologous to thepineal gland inmammals). All living and most extinct Agnatha do not have an identifiablestomach or anyappendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha areectothermic or cold-blooded, with acartilaginousskeleton, and theheart contains 2 chambers.
In modern agnathans, the body is covered in skin, with neither dermal or epidermalscales. The skin ofhagfish 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, manyextinct agnathans sported extensive exoskeletons composed of either massive, heavy dermalarmour or small mineralized scales.
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.[14] 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.
Although a minor element of modern marinefauna, agnathans were prominent among the early fish in the earlyPaleozoic. Two types of EarlyCambrian animal apparently having fins,vertebrate musculature, and gills are known from the early CambrianMaotianshan shales ofChina:Haikouichthys andMyllokunmingia. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathan from the same region isHaikouella. A possible agnathan that has not been formally described was reported by Simonetti from the Middle CambrianBurgess Shale ofBritish Columbia.Conodonts, a class of agnathans which arose in the early Cambrian,[16] remained common enough until their extinction in theTriassic that their teeth (the only parts of them that were usually fossilized)are often used asindex fossils from the late Cambrian to the Triassic.[17]
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—theostracoderms, precursors to thebony fish and hence to thetetrapods (includinghumans)—are known from the middleOrdovician, and by the LateSilurian the agnathans had reached the high point of their evolution. Most of the ostracoderms, such asthelodonts,osteostracans, andgaleaspids, 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, includingconodonts.[18] Agnathans declined in theDevonian 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 onvariable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences.[19] 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.[20]
Myxini (hagfish) areeel-shaped slime-producing marine animals (occasionally calledslime eels). They are the only known living animals that have askull but not avertebral column. The group has gone through the most extensive gene loss of all vertebrates, with 1,386 missing gene families.[21] Along withlampreys, hagfish are jawless and areliving fossils; hagfish arebasal to vertebrates, and living hagfish remain similar to hagfish 300 million years ago.[22] 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 dolancelets). The original scheme groups hagfish and lampreys together ascyclostomes (or historically, Agnatha), as the oldest surviving clade ofvertebrates alongsidegnathostomes (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 categoryCraniata to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme.[9]
Petromyzontida, also called Hyperoartia, is a disputed group of vertebrates that includes the modernlampreys and theirfossil relatives. Examples of hyperoartians from early in their fossil record areEndeiolepis andEuphanerops, fish-like animals withhypocercal tails that lived during the LateDevonian Period. Somepaleontologists still place these forms among the "ostracoderms" (jawless armored "fishes") of theclassAnaspida, but this is increasingly considered an artificial arrangement based onancestral 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 superclassCyclostomata together with theMyxini (hagfishes). More recently, it has been proposed that the Myxini are morebasal among theskull-bearing chordates, while the Hyperoartia are retained amongvertebrates. 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 theCephalaspidomorphi, or not closer to these than to e.g. to other "ostracoderms" of thePteraspidomorphi, or even the long-extinctconodonts, 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 amonophyletic lineageJamoytiiformes that may in fact be very close to the ancestraljawed vertebrates.
Conodonts were eel like agnathans that lived from theCambrian up until the beginning of theJurassic period. They were very diverse in terms of lifestyles, with some species beingfilter feeders and others being macropredators. For over acentury, these animals were only known because of their microscopic,phosphatic tooth structures called "conodont elements". It wasn't until the mid-1980s that body fossils of conodonts were found inScotland andWisconsin, 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 theOrdovician-Silurian extinction event. They then reached another spike in diversity in themid-late Devonian before again declining in theCarboniferous. They were relatively rare in thePermian, but dramatically increased in numbers in theearly 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 thelarge extinction at the end of the Triassic. Instead, it is now thought that they were out competed by newerMesozoic taxa.[25][26][27][28][29][30]
†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.
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[31]) represent amonophyletic grouping, or disparate stem groups to the major lines of jawless andjawed fish. Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have beenevolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group,[32] although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.[33]: 206 "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classesHeterostraci andAnaspida, 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.[34][35] The fish lived in both freshwater and marine environments, first appearing during theOrdovician, and perishing during theFrasnian–Famennian extinction event of the LateDevonian. 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.[36]
Anaspida(without shield) is an extinct group of primitive jawless vertebrates that lived during theSilurian andDevonian periods.[37] They are classically regarded as the ancestors of lampreys.[38] Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highlyhypocercal tail. They first appeared in theEarly Silurian, and flourished until theLate Devonian extinction,[39] where most species, save forlampreys, became extinct due to the environmental upheaval during that time.
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 theosteostracan genusCephalaspis. Most biologists regard thistaxon as extinct, but the name is sometimes used in the classification oflampreys, 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 earlySilurian period through theMesozoic, 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 extensivelateral line organ. The eyes were rather small and placed on the top of the head. There was no properjaw. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite.[40] Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina", include theclassesOsteostraci,Galeaspida, andPituriaspida
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 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 Miyashitaet al. (2019) is considered compatible with both morphological and molecular evidence.[42][43]
^Märss, T.; Miller, C.G. (2004). "Thelodonts and distribution of associated conodonts from the Llandovery-lowermost Lochkovian of the Welsh Borderland".Palaeontology.47 (5):1211–1265.Bibcode:2004Palgy..47.1211M.doi:10.1111/j.0031-0239.2004.00409.x. [W. Kiessling/M. Krause/E. Ito]
^abcMallatt, J.; Sullivan, J. (December 1998). "28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes".Molecular Biology and Evolution.15 (12):1706–1718.doi:10.1093/oxfordjournals.molbev.a025897.PMID9866205.
^abcDelarbre C, Gallut C, Barriel V, Janvier P, Gachelin G (February 2002). "Complete mitochondrial DNA of the hagfish,Eptatretus burgeri: The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly".Molecular Phylogenetics and Evolution.22 (2):184–92.Bibcode:2002MolPE..22..184D.doi:10.1006/mpev.2001.1045.PMID11820840.
^abJanvier, P. (November 2010)."MicroRNAs revive old views about jawless vertebrate divergence and evolution".Proceedings of the National Academy of Sciences of the United States of America.107 (45):19137–19138.Bibcode:2010PNAS..10719137J.doi:10.1073/pnas.1014583107.PMC2984170.PMID21041649.Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberget 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.
^Stanley, Steven M.; Luczaj, John A. (2015).Earth System History (4th ed.).Macmillan Education. p. 311.Conodonts arose late in the Early Cambrian and diversified into the Ordovician. ... Similar small teeth in very early Cambrian faunas ... may represent conodont ancestors.
^Baker CV (December 2008). "The evolution and elaboration of vertebrate neural crest cells".Current Opinion in Genetics & Development.18 (6):536–543.doi:10.1016/j.gde.2008.11.006.PMID19121930.
^Speer, Brian R. (1997)."Introduction to the Myxini". U.C. Museum of Paleontology. University of California, Berkeley. Archived fromthe original on 2017-12-15. Retrieved2013-02-21.
^Turner S (1999). "Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance". In A. J. Boucot, J. Lawson (eds.). Palaeocommunities – Project Ecostratigraphy, Final Report (Report). International Geological Correlation Programme. Vol. 53.Cambridge University Press. pp. 42–78.
^The early and mid Silurian. SeeKazlev MA, White T (March 6, 2001)."Thelodonti".Palaeos.com. Archived fromthe original on 2007-10-28. RetrievedOctober 30, 2007.
^Colbert, Michael; Morales, Edwin H. (1991).Evolution of the Vertebrates : A history of the backboned animals through time (4th ed.). New York, NY: Wiley-Liss.ISBN978-0-471-85074-8.
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