Agnatha

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Related to Jawless fishes: superclass Agnatha, Cartilaginous fishes

Agnatha

[′ag·nə′thə]
(vertebrate zoology)
The most primitive class of vertebrates, characterized by the lack of true jaws.

Agnatha

 

a superclass of lower vertebrate animals. Agnatha are distinguished from all the remaining vertebrates, the Gnathostomata, by the absence of real jaws, and, in the ones living today, by the absence of paired extremities, as well as by the presence of an unpaired nostril. Agnatha are also called Marsipobranchia, because their gills look like pouches. The branchiate skeleton is located outside the pouches and has the appearance of a complete lattice (and not broken gill arches, as in fish) or is fused with the external shell. Agnatha is the most ancient group of vertebrates, widely distributed in the Silurian and Devonian periods. Fossil Agnatha (Ostracodermi) had a well-developed external and partially ossified internal skeleton. Their remains are the leading fossils for the Silurian and Devonian periods.

Of contemporary fauna, only representatives of the class Cyclostomata—the lampreys and hagfish—remain of the Agnatha.

REFERENCES

Berg, L. S. Sistema ryb. Moscow-Leningrad, 1940.
Osnovy paleontologii: Bescheliustnye, ryby. Moscow, 1964.
References in periodicals archive ?
Tiny scales of acanthodians often dominate fish microremains, particularly in drill core samples, and are much more common than macroremains of jawless fishes heterostracans (psammosteids) and/or placoderms.
Gareth Fraser, postdoctoral fellow at Georgia Tech's School of Biology, describes it as a common gene regulatory circuit that controls the development of all dentitions, from the first teeth in the throats of jawless fishes that lived half a billion years ago, to the incisors and molars of modern vertebrates.
These 18 Paleozoic-era snails, half of them new to science, did live on reefs some 420 million years ago, when jawless fishes spread throughout the seas and the ancestors of spiders and centipedes began creeping about on land.