Craniata
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- For the class of brachiopods, see Craniforma.
| Craniata Fossil range: Early Cambrian - Recent |
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 A Pacific Hagfish, an example of a craniate
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Cephalaspidomorphi (disputed) |
Craniata (sometimes Craniota) is a proposed clade of chordate animals that contains the Myxini (hagfish), Cephalaspidomorphi (including lampreys), and Gnathostomata (jawed vertebrates)[1][2] as living representatives. Craniata includes all animals with a skull, or cranium, as the name suggests.
Craniata as an unranked taxon replaces the former use of Vertebrata (Vertebrata sensu lato). The main difference of the old and new (Vertebrata sensu stricto) interpretation of Vertebrata is that Myxini and sometimes the Cephalaspidomorphi (see Systematics and taxonomy, below) are now not included in Vertebrata anymore. The Myxini lack proper vertebrae, which are characteristic for vertebrates according to the new interpretation, whereas traditionally, and confusingly, they were not (Hickman et al., 2007). When Vertebrata is defined as excluding the Cephalaspidomorphi as well, it becomes another name for Gnathostomata.
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[edit] Characteristics
In the simplest sense craniates are chordates with heads, thus excluding members of chordate subphyla Urochordata (tunicates) and Cephalochordata (lancelets), but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin. Craniata also includes all lampreys and armored jawless fishes, sharks and rays, bony fish, amphibians, reptiles and mammals. The craniate head consists of a brain, sense organs including eyes, and a skull[3][4].
In addition to distinct crania (sing. cranium), craniates possess many derived characteristics which have allowed for more complexity to follow. Molecular-genetic analysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of many gene families that are involved in cell signaling, transcription, and morphogenesis (see homeobox)[1].
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 and nerves which pump water through the slits (as opposed to lancelets, whose pharyngeal slits are used only for suspension feeding), engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving food through the canal, allowing higher craniates like mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems which include a heart with two or more chambers, red blood cells, and O2 transporting hemoglobin, as well as kidneys[1].
[edit] Systematics and taxonomy
The taxonomy of this clade is unclear and disputed. Craniates can generally be split into the Myxini (hagfish), the Cephalaspidomorphi (including lampreys), and the Gnathostomata (jawed vertebrates), but it's not clear whether lampreys should be grouped with the jawed vertebrates under vertebrata or with the hagfish under agnatha/cyclostomata. It's also not clear whether lampreys should be considered true Cephalaspidomorphi or not.
[edit] Validity
The validity of this taxon is in doubt. mtDNA sequence analysis (Delarbre et al. 2002) suggests that Myxini are as close to the Hyperoartia as long believed - i.e., that the Agnatha are a valid clade. The Cephalaspidomorphs (lampreys) are traditionally allied with the Hyperoartia, which may or may not be correct. Note however that mtDNA is not very reliable in phylogenetic analysis that reach very deep into time, due to its accelerated rate of mutation, and that the fossil record is quite equivocal.[clarification needed]
If the Acrania are indeed not paraphyletic, Vertebrata would return to its old content (Gnathostomata + Agnatha) and Craniata, being superfluous, would become its synonym.
[edit] See also
- Extinct genera Haikouella and Haikouichthys
[edit] References
- ^ a b c Campbell & Reece 2005 p. 676
- ^ Cracraft & Donoghue 2004 p. 390
- ^ Campbell & Reece 2005 pp. 675-7
- ^ Parker & Haswell 1921
- CAMPBELL, NEIL A. & REECE, JANE B. (2005). Biology, Seventh Edition. San Francisco CA: Benjamin Cummings.
- CLEVELAND P. HICKMAN, J., ROBERTS, L. S., KEEN, S. L., LARSON, A. & EISENHOUR, D. J. (2007). Animal Diversity, Fourth Edition. New York: McGraw Hill.
- CRACRAFT, JOEL & DONOGHUE, MICHAEL J. (2004). Assembling the Tree of Life. New York: Oxford University Press US.
- Delarbre 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. doi:.
- PARKER, T. J. & HASWELL, W. A. (1921). A Text-book of Zoology. Macmillian & Co., Ltd..
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