Agnatha

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Agnatha

[′ag·nə′thə]
(vertebrate zoology)
The most primitive class of vertebrates, characterized by the lack of true jaws.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
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.
Jawless fish, whose modern counterparts include lampreys and hagfish, have been considered the earliest known vertebrates, or creatures with a backbone, for more than a decade.
Friedman told Discovery News that some fossil jawless fishes "had broad, shovel-shaped heads with their eyes placed on top, while others had narrow bodies and skulls with their eyes on either side of the head."
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And just because the species is classified as agnaths, which means "jawless fishes," don't underestimate its bite.
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Thornton proposes that the gene duplication that created the progesterone receptor took place before the jawless fishes branched off the rest of the vertebrate line.
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.