Agnatha (Greek, "no jaws") is a class or superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Many recent textbooks regard the group as paraphyletic[1] but recent molecular data, both from rRNA[2] and from mtDNA[3] strongly supports living agnathans as monophyletic. It has existed since the Cambrian, and continues to live now. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 100 species in total.
Fossil agnathans[]
Haikouicthys is a fossil agnathan.
Cephalaspis is another fossil agnathan.
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: Haikouicthys 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. Agnathans declined in the Devonian and never recovered.
Groups[]
- Myxini (hagfish)
- Hyperoartia
- Petromyzontidae (lampreys)
- Ostracoderms
- †Pteraspidomorphi
- †Thelodonti
- †Anaspida
- Cephalaspidomorphi
- †Galeaspida
- †Pituriaspida
- †Osteostraci
References[]
- ^ Purnell, M. A. (2001). Derek E. G. Briggs and Peter R. Crowther. ed. Palaeobiology II. Oxford: Blackwell Publishing. p. 401. ISBN 0-632-05149-3.;
- ^ 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: 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.
See also[]
- Gnathostomata
- Amphirhina, an alternate name for the above parallel, or sister, classification