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Agnatha (/ ˈ æ ɡ n ə θ ə, æ ɡ ˈ n eɪ θ ə /; [3] from Ancient Greek ἀ-(a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum [4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ...
Furcacaudidae, the "'fork-tailed' agnathans," is an extinct family of thelodontid agnathans from the Lochkovian stage of the Early Devonian epoch and Wenlockian epoch of the Silurian, known from fossils found in Northern Canada.
Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, [17] where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Agnatha, the jawless vertebrates was formerly a superclass of jawless fishes. Most species are prehistoric or extinct, however there are about 60 modern species in the Agnaths superclass, divided into two classes: Class Myxini (family Myxinidae) - the hagfish
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Their classification as agnathans places hagfish as elementary vertebrates in between invertebrates and gnathostomes. However, discussion has long occurred in scientific literature about whether the hagfish were even invertebrate. Using fossil data, paleontologists posited that lampreys are more closely related to gnathostomes than hagfish.
Cyathaspidida is a taxon of extinct cyathaspidiform heterostracan agnathans whose fossils are found in Silurian to Lower Devonian marine strata of Europe and North America. [1] In life, they are thought to be benthic animals that lived most of their lives either mostly buried in or resting directly on top of the substrate.
The heterostracans differed from other Paleozoic agnathan taxa both in the arrangement and histology of their scales.. Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another.