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Agnatha

Fish Guide

Agnatha

Hagfish | Lamprey

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Agnatha
 
Fossil range: Early Cambrian - Recent

 
 
Scientific classification
Kingdom: Animalia
 
Phylum: Chordata
 
Subphylum: Vertebrata
 
Superclass: Agnatha
 
Groups
Myxini (hagfish)
Hyperoartia
  • Petromyzontidae (lampreys)

Pteraspidomorphi
Thelodonti
Anaspida
Cephalaspidomorphi

  • Galeaspida
    Pituriaspida
    Osteostraci

Agnatha (Greek, "no jaws") is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 60 species between them. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. The branchial arches supporting the gill pouches lies close to the body surface. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). There is no identifiable stomach. Fertilization is external. The Agnatha are ectothermic, with a cartilaginous skeleton, and the heart contains 2 chambers.

Although they are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates, and modern classifications tend to move the hagfish into a separate group (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) being more closely related to the jawed fishes.

Fossil agnathans

Haikouichthys is a fossil agnathan.
Haikouichthys is a fossil agnathan.
Cephalaspis is another 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: 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 plates. The first armored agnathans—the Ostracoderms, precursors to the bony fish and hence to the tetrapods (including human beings)—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.


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