Agnatha

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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 ?
The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention.
Among euryhaline fishes, most teleosts and petromyzontid agnathans (lampreys) are regulators, while chondrichthyans and the myxinoid agnathans (hagfishes) are conformers (see "Water Balance in Elasmobranchs" below).
The list of agnathans of the Himmiste Beds in the Himmiste-Kuigu quarry contains thelodonts Thelodus laevis (Pander), T.
As a rule, articulated skeletons or shields of other agnathans are not found on the same bedding plane with P.
Gross (1947) studied fish microremains from the erratic boulders of the North German Lowland, among other agnathans and fishes also Strosipherus indentatus.
In the last handbook on early agnathans and fishes (Novitskaya 2004), Oniscolepis was shortly discussed in the section on the order Tesseraspidiformes Halstead, family Tesseraspididae Berg, which contained two genera, Tesseraspis and ?Kallostrakon.
Patten (e.g., 1931), produced additional specimens, exhausting the quarry; in recent decades, no articulated agnathans have been found apart from a few shields of Tremataspis mammillata Patten and Dartmuthia gemmifera Patten.
Silurian-Devonian vertebrate dominated communities, with particular references to agnathans. In Paleocommunities: A Case Study from the Silurian and Lower Devonian (Boucot, A.
Exoskeletal elements such as scales, tesserae and bony plates of Palaeozoic jawless vertebrates (agnathans) and fishes with jaws (gnathostomes) have been shown to exhibit characteristic morphology, sculpture and microstructure, all of which have been useful for taxonomic identification, systematics and phylogenetic analyses of these fossils (Gross 1967; Karatajute-Talimaa 1978; Marss & Ritchie 1998; Wilson & Caldwell 1998; Wilson & Marss 2004, 2009; Marss et al.