Craniate
| Craniates Temporal range:
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| A Pacific 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[1] |
| Included groups | |
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| Synonyms | |
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A craniate is a member of the Craniata (sometimes called the Craniota), a proposed
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 the vertebral column. The clade was thus composed of the Hyperoartia and the vertebrates, and any extinct chordates with skulls.
However recent studies using
Characteristics
In the simplest sense, craniates are chordates with well-defined heads, thus excluding members of the chordate subphyla
In addition to distinct crania (sing.
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, whose
Systematics and taxonomy
Linnaeus (1758)[9] used the terms Craniata and Vertebrata interchangeably to include lampreys, jawed fishes, and terrestrial vertebrates (or tetrapods). Hagfishes were classified as Vermes, possibly representing a transitional form between 'worms' and fishes.
Løvtrup (1977)[9] 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 the heart
- Nervous regulation of the heart by the vagus nerve
- A typhlosole (a spirally coiled valve of the intestinal wall)
- True lymphocytes
- 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 vertical semicircular canals
- Neuromastorgans (composed of vibration sensitive hair cells) in the laterosensory canals
- An electroreceptive lateral line (with voltage sensitive hair cells)
- Electrosensory lateral line nerves
- A cerebellum, 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 basis Janvier (1978)[9] proposed to use the names Vertebrata and Craniata as two distinct and nested taxa.
Validity
The validity of the taxon "Craniata" was recently examined by Delarbre et al. (2002) using
The new evidence removes support for the hypothesis for the evolutionary sequence by which (from among tunicate-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 Janvier stated:
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.[10]
Classification
Chordata
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See also
- Haikouella, extinct genus
- Haikouichthys, extinct genus
Notes
- S2CID 83266247.
- PMID 30670644.
- ^ a b c Campbell & Reece 2005 p. 676
- ^ Cracraft & Donoghue 2004 p. 390
- S2CID 17652802.
- ^ Campbell & Reece 2005 pp. 675–7
- ^ Parker & Haswell 1921
- ^ Chordates OpenStax, 9 May 2019.
- ^ a b c d e f Janvier, 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]
- PMID 18563158.
- PMID 21672842.
- PMID 29653534.
References
- 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. ISBN 978-0-19-517234-8.
- 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. PMID 11820840.
- Parker, T. J.; Haswell, W. A. (1921). A Text-book of Zoology. Macmillan & Co.