Agnatha

(redirected from agnathan)
Also found in: Dictionary, Thesaurus, Medical.
Related to agnathan: Osteichthyes

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 ?
Some marl levels downstream along the Lemovzha River contain well-preserved remains of agnathans and fishes (Ivanov & Lebedev 2011; Skutschas et al.
The taxonomy and distribution of agnathans and fishes in southern Britain have been investigated, the regional vertebrate zonal scheme has been updated and characteristic vertebrate assemblages have been correlated with the conodont zonation (Miller & Marss 1999; Turner 2000; Marss & Miller 2004).
In middle Saaremaa, microlaminated argillaceous Eurypterus-dolostones or domerites contain often lingulid, eurypterid, and agnathan remains (Uduvere-968 core, depth 3.
Sostav i rasprostranenie beschelyustnykh i ryb v silurijskikh i devonskikh otlozheniyakh Latvii [Composition and distribution of agnathan and vertebrate assemblages in the Silurian and Devonian deposits of Latvia].
For example, the width of ridges and grooves on agnathan thelodont scales is 20-200 [micro]m, while the length may coincide with the length of the scale (up to 3.