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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.
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).
They appeared in the Cambrian, about 510 million years ago, and became extinct towards the end of the Devonian, about 377 million years ago. Initially Ostracoderms had poorly formed fins, and paired fins, or limbs, first evolved within this group. They were covered with a bony armour or scales and were often less than 30 cm (12 in) long.
Chondrichthyes (cartilage-fish) or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii ( sharks , rays and skates ) and Holocephali ( chimaeras , sometimes called ghost sharks ...
Priscomyzon provides evidence that agnathans close to modern lampreys had existed before the end of the Devonian period.Lampreys have ancient origins, have endured for 360 million years and lived through four major extinction events - even so, their evolutionary history is obscure.
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 .
The fish also has much longer lower pectoral-fin rays, the fins on the side of the body. The fins are “extended” and more full compared to other scorpionfish that live in the area, the ...
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.