Acraniateis a member of theCraniata(sometimes called theCraniota), a proposedcladeofchordateanimalswith askullof hardboneorcartilage.Living representatives are theMyxini(hagfishes),Hyperoartia(includinglampreys), and the much more numerousGnathostomata(jawed vertebrates).[4][5]Formerly distinct fromvertebratesby excludinghagfish,molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.
| Craniates | |
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| APacific hagfish,an example of (what was thought to be) a "non-vertebrate craniate" | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Olfactores |
| Clade: | Craniata Lankester, 1877[2] |
| Included groups | |
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| Synonyms | |
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The clade was conceived largely on the basis of the Hyperoartia (lampreys and kin) being more closely related to the Gnathostomata (jawed vertebrates) than the Myxini (hagfishes). This, combined with an apparent lack of vertebral elements within the Myxini, suggested that the Myxini were descended from a more ancient lineage than the vertebrates, and that the skull developed before thevertebral column.The clade was thus composed of the Hyperoartia and thevertebrates,and any extinct chordates with skulls.
However recent studies usingmolecular phylogeneticshave contradicted this view, with evidence that theCyclostomata(Hyperoartiaand Myxini) ismonophyletic;this suggests that the Myxini are degenerate vertebrates, and therefore the vertebrates and craniates are cladistically equivalent, at least for the living representatives. The placement of the Myxini within the vertebrates has been further strengthened by recent anatomical analysis, with vestiges of a vertebral column being discovered in the Myxini.[6]
Characteristics
editIn the simplest sense, craniates are chordates with well-defined heads, thus excluding members of the chordate subphylaTunicata(tunicates) andCephalochordata(lancelets), but includingMyxini,which have cartilaginous crania and tooth-like structures composed ofkeratin.Craniata also includes alllampreys and armoured jawless fishes,armoured jawed fish,sharks, skates, and rays,andteleostomians:spiny sharks,bony fish,lissamphibians,temnospondylsandprotoreptiles,sauropsidsandmammals.The craniate head consists of a three-partbrain,neural crestwhich gives rise to many cell lineages, and a cranium.[7][8]
In addition to distinct crania (sing.cranium), craniates possess many derived characteristics, which have allowed for more complexity to follow.Molecular-geneticanalysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of manygenefamilies that are involved incell signaling,transcription,andmorphogenesis(seehomeobox).[4]
In general, craniates are much more active than tunicates and lancelets and, as a result, have greater metabolic demands, as well as several anatomical adaptations. Aquatic craniates have gill slits, which are connected to muscles to pump water through the slits, engaging in both feeding and gas exchange (as opposed to lancelets, whosepharyngeal slitsare used only for suspension feeding, chiefly by cilia-mucus rather than muscles). Muscles line the alimentary canal,movingfood through the canal, allowing higher craniates such as mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems that include a heart with at least two chambers,red blood cells,oxygen transportinghemoglobinas well asmyoglobin,liversandkidneys.[4]
Systematics and taxonomy
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Linnaeus(1758) classified hagfishes asVermes,aclassfor non-arthropod invertebrates (in modern nomenclature).[10]
Dumeril(1806)[11]grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills (Marspibranchii).Cyclostomeswere regarded as either degenerate cartilaginous fishes or primitive vertebrates.Cope(1889)[11]coined the nameAgnatha( "jawless" ) for a group that included the cyclostomes and a number of fossil groups in which jaws could not be observed. Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata (jawed vertebrates).Stensiö(1927)[11]suggested that the two groups of living agnathans (i.e. the cyclostomes) arose independently from different groups of fossil agnathans.
Løvtrup(1977)[11]argued that lampreys are more closely related to gnathostomes based on a number of uniquely derived characters, including:
- Arcualia (serially arranged paired cartilages above the notochord)
- Extrinsic eyeball muscles
- Radial muscles in the fins
- A closely set atrium and ventricle of theheart
- Nervous regulation of the heart by thevagus nerve
- Atyphlosole(a spirally coiled valve of the intestinal wall)
- Truelymphocytes
- A differentiated anterior lobe of the pituitary gland (adenohypophysis)
- Three inner ear maculae (patches of acceleration sensitive 'hair cells' used in balance) organized into two or three verticalsemicircular canals
- Neuromastorgans (composed of vibration sensitive hair cells) in the laterosensory canals
- An electroreceptivelateral line(with voltage sensitive hair cells)
- Electrosensory lateral line nerves
- Acerebellum,i.e. the multi-layered roof of the hindbrain with unique structure (characteristic neural architecture including direct inputs from the lateral line and large output Purkinje cells) and function (integrating sensory perception and coordinating motor control)
In other words, the cyclostome characteristics (e.g. horny teeth on a "tongue", gill pouches) are either instances of convergent evolution for feeding and gill ventilation in animals with an eel-like body shape, or represent primitive craniate characteristics subsequently lost or modified in gnathostomes. On this basisJanvier(1978)[citation needed]proposed to use the namesVertebrataandCraniataas two distinct and nested taxa.
Validity
editThe validity of the taxon "Craniata" was recently examined by Delarbre et al. (2002) usingmtDNAsequencedata, concluding that Myxini is more closely related toHyperoartiathan to Gnathostomata - i.e., that modern jawless fishes form a clade calledCyclostomata.The argument is that, if Cyclostomata is indeed monophyletic, Vertebrata would return to its old content (Gnathostomata+Cyclostomata) and the name Craniata, being superfluous, would become a junior synonym.
The new evidence removes support for the hypothesis for the evolutionary sequence by which (from amongtunicate-like chordates) first the hard cranium arose as it is exhibited by the hagfishes, then the backbone as exhibited by the lampreys, and then finally the hinged jaw that is now ubiquitous. In 2010,Philippe Janvierstated:
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.[12]
Classification
editPhylogenetic treeof the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with adagger,†. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported.[13][14][15]
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See also
edit- Haikouella,extinct genus
- Haikouichthys,extinct genus
Notes
edit- ^Yang, Chuan; Li, Xian-Hua; Zhu, Maoyan; Condon, Daniel J.; Chen, Junyuan (2018)."Geochronological constraint on the Cambrian Cheng gian g biota, South China"(PDF).Journal of the Geological Society.175(4): 659–666.Bibcode:2018JGSoc.175..659Y.doi:10.1144/jgs2017-103.ISSN0016-7649.S2CID135091168.Archived(PDF)from the original on 2022-10-09.
- ^Nielsen, C. (July 2012). "The authorship of higher chordate taxa".Zoologica Scripta.41(4): 435–436.doi:10.1111/j.1463-6409.2012.00536.x.S2CID83266247.
- ^Miyashita, Tetsuto (2019)."Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny".Proceedings of the National Academy of Sciences of the United States of America.116(6): 2146–2151.Bibcode:2019PNAS..116.2146M.doi:10.1073/pnas.1814794116.PMC6369785.PMID30670644.
- ^abcCampbell & Reece 2005 p. 676
- ^Cracraft & Donoghue 2004 p. 390
- ^Janvier, Philippe (2011)."Comparative Anatomy: All Vertebrates Do Have Vertebrae".Current Biology.21(17): R661–R663.Bibcode:2011CBio...21.R661J.doi:10.1016/j.cub.2011.07.014.ISSN0960-9822.PMID21920298.S2CID17652802.
- ^Campbell & Reece 2005 pp. 675–7
- ^Parker & Haswell 1921
- ^ChordatesOpenStax,9 May 2019.
- ^Linnaeus, Carolus (1758).Systema Naturae(in Latin) (10 ed.).
- ^abcdJanvier, Philippe."Craniata - Animals with skulls".Tree of Life Web Project (ToL).Tree of Life Web Project.
- ^"MicroRNAs revive old views about jawless vertebrate divergence and evolution." Proceedings of the National Academy of Sciences (USA) 107:19137-19138.[1]
- ^Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.; Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.; Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.; Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton, A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.; Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.; Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T. (19 June 2008)."The amphioxus genome and the evolution of the chordate karyotype".Nature.453(7198): 1064–1071.Bibcode:2008Natur.453.1064P.doi:10.1038/nature06967.PMID18563158.
- ^Ota, K. G.; Kuratani, S. (September 2007)."Cyclostome embryology and early evolutionary history of vertebrates".Integrative and Comparative Biology.47(3): 329–337.doi:10.1093/icb/icm022.PMID21672842.
- ^Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ (April 2018)."A phylogenomic framework and timescale for comparative studies of tunicates".BMC Biology.16(1): 39.doi:10.1186/s12915-018-0499-2.PMC5899321.PMID29653534.
References
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- Campbell, Neil A.;Reece, Jane B.(2005).Biology(Seventh ed.). San Francisco CA:Benjamin Cummings.
- Cleveland, P.; Hickman, J.; Roberts, L. S.; Keen, S. L.; Larson, A.; Eisenhour, D. J. (2007).Animal Diversity(Fourth ed.). New York:McGraw Hill.
- Cracraft, Joel; Donoghue, Michael J. (2004).Assembling the Tree of Life.New York: Oxford University Press US.ISBN978-0-19-517234-8.
- Delarbre, Christiane; Gallut, C; Barriel, V; Janvier, P; Gachelin, G; et al. (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): 184–192.Bibcode:2002MolPE..22..184D.doi:10.1006/mpev.2001.1045.PMID11820840.
- Parker, T. J.;Haswell, W. A.(1921).A Text-book of Zoology.Macmillan & Co.