Agnatha

(redirected from agnathans)
Also found in: Dictionary, Thesaurus, Medical.
Related to agnathans: Chondrichthyes, Ostracoderms, Placoderms, Chondrichthyans, Gnathostomes, Acanthodians

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 ?
In Scotland, the associated faunal list includes the protochordate Ainiktozoon loganense Scourfield and the agnathan Jamoytius kerwoodi White in addition to L.
Silurian-Devonian vertebrate dominated communities, with particular references to agnathans.
The fossil fishes that have so far been used for distinguishing zones are agnathans, placoderms and acanthodians (e.
Such a very fine pattern has been observed on the scales of both agnathans and gnathostome fishes (Marss 2006) in the Palaeozoic, and some details of the external surface of scales and tubercles have been illustrated in publications on histology (e.
During the last few decades the palaeontological studies in Estonia have been dealing with corals, stromatoporoids, brachiopods, trilobites, ostracods, conodonts, chitinozoans, scolecodonts, acritarchs, agnathans, early fishes and tetrapods, and various other groups of organisms.
The nektic, nektobenthic and planktic communities (fishes, agnathans, conodonts, graptolites, chitinozoans) of the East Baltic Silurian.