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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.
The latter has lost the paired fins, possibly as a consequence of an adaptation to burrowing habits. [ 2 ] The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish , termed " ostracoderms ", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian .
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 ostracoderms, among others).
Anaspids were small, primarily marine agnathans that lacked heavy bony shield and paired fins, but have highly exaggerated hypocercal tails. They first appeared in the Early Silurian , and flourished until the Late Devonian extinction , [ 26 ] where most species, save for lampreys , became extinct.
Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard Agnatha as paraphyletic, having given rise to the jawed fishes. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to Cephalaspidomorphi.
Furcacauda is a genus of thelodontid agnathan from the Lower Devonian of Canada, and is the type genus of the order Furcacaudiformes. [2] Furcacaudiform thelodontids were deep water jawless vertebrates with symmetrical fork and lobed-finned tails and scales smaller than typical loganellid and nikoliviid thelodonti scales. [1]
The common name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail – hence, "spiny sharks".
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.