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The agnathans as a whole are paraphyletic, because most extinct agnathans belong to the stem group of gnathostomes. Recent molecular data, both from rRNA and from mtDNA strongly supports the theory that living agnathans, known as cyclostomes, are monophyletic.
The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, with about 100 species in total. Hagfish are not members of the subphylum Vertebrata, as hagfish do not have vertebrae; they are rather classified in the more inclusive group Craniata. 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. There is 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 any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are exothermic, with a cartilaginous skeleton, and the heart contains 2 chambers.
While a few scientists still regard the living agnaths as only superficially similar, and argue that many of these similarities are probably shared basal characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.
Agnathans are ectothermic or cold blooded, meaning they do not regulate their own body temperature. Therefore, Agnathan metabolism is slow in cold water, and therefore do not have to eat as much. They have no distinct stomach, but rather a long gut, more or less homogenous throughout its length. Lampreys are parasitic, feeding off of other fish and mammals. They rely on a row of sharp teeth to shred their host. Fluids preventing clotting are injected into the host,causing the host to yield more blood. Hagfish are decomposers, eating mostly dead animals. They also use a sharp set of teeth to break down the animal. Agnathans feeding habits have limited their ability to advance evolutionarily. The fact that all Agnathan's teeth are not able to move up and down limit their possible food types. They have never been known to attack humans, without water they would die so when they are taken out of the water with their host they will release immediately.
The only modern Agnathan body covering is skin, with neither dermal or epidermal scales. The skin of hagfish has copious slime glands, the slime constituting their defence mechanism. Many extinct agnathans sported heavy dermal armour or small mineralized scales (see below).
Most agnathans, including all those living today have no paired appendages, although they do have a tail and a caudal fin. Some fossil agnathans, such as osteostracans, did have paired fins, a trait inherited in their jawed descendants.
The internal skeleton of the Agnatha is not bony but rather cartilaginous (made up of dense connective tissue). The somewhat rudimentary skull never ossifies and remains a chondrocranium throughout life. Also, Agnathans retain a notochord in adulthood, a characteristic distinctive of the class. This notochord is a cartilagious rod that forms the basis of the vertebral column in highter vertebrates.
Fertilization is external, as is development. There is no parental care. Not much is known about the hagfish reproductive process. It is believed that hagfish only have 30 eggs over a lifetime. Most species are hermaphrodites. There is very little of the larval stage that characterizes the lamprey. Lampreys can only reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. Lampreys reproduce in freshwater river beds, and bury their eggs about two centimeters underground. Lampreys work in pairs buildings the egg nests. Lampreys go through four years of larval development before becoming adults. They also have a certain unusual form of reproduction.
Although a minor element of modern marine fauna, Agnatha 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 agnathid from the same region is Haikouella. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.
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 ostraoderms, 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. Agnathans declined in the Devonian and never recovered.
- ↑ Purnell, M. A. (2001). Derek E. G. Briggs and Peter R. Crowther Palaeobiology II, Oxford: Blackwell Publishing.
- ↑ Zhao Wen-Jin, Zhu Min (2007). Diversification and faunal shift of Siluro-Devonian vertebrates of China. Geological Journal 42 ((3-4)): 351–369.
- ↑ Sansom, Robert S. (2009). Phylogeny, classification, & character polarity of the Osteostraci (Vertebrata). Journal of Systematic Palaeontology 7: 95–115.
- ↑ Mallatt, J., and J. Sullivan. 1998. (1998). 28S and 18S ribosomal DNA sequences support the monophyly of lampreys and hagfishes.. Molecular Biology and Evolution 15 (12): 1706–1718.
- ↑ DeLarbre Christiane ; Gallut Cyril ; Barriel Veronique ; Janvier Philippe ; Gachelin Gabriel (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.
- ↑ 6.0 6.1 6.2 Romer, A.S. & Parsons, T.S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
- ↑ Baker, Clare V.H. (2008). The evolution and elaboration of vertebrate neural crest cells. Current Opinion in Genetics & Development 18 (6): 536–543.
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