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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 ostracoderms, among others).
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.
They also had sucking mouth parts and some species may have lived in fresh water. Most pteraspidomorphs were marine, but lived very near to the shore, in lagoons and deltas. Some groups are thought to have been fresh water-dwelling. They were certainly bottom-dwellers, as shown by traces of abrasion of the ventral surfaces of their headshields.
The agnathans as a whole are paraphyletic, [13] because most extinct agnathans belong to the stem group of the gnathostomes, the jawed fish that evolved from them. [ 14 ] [ 15 ] Molecular data, both from rRNA [ 16 ] and from mtDNA [ 17 ] strongly supports the theory that living agnathans, known as cyclostomes , are monophyletic . [ 18 ]
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.
The fins are “extended” and more full compared to other scorpionfish that live in the area, the researchers said. It was this longer fin shape that the researchers were thinking about when ...
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 ...