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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.
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, [39] where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
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.
Ateleaspis possibly is the most basal vertebrate with paired fins. Ateleaspis was a small fish (about 15 – 20 cm) and had a flat headshield and a narrow trunk covered by brick-like scales. References
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.
This last theory was based on the fact that they seem to have a paired olfactory organ and a sensory-line pattern which is quite similar to that of the gnathostomes. These characteristics are, however, likely to be general for either the vertebrates or, at any rate, for the ensemble of all ostracoderms and the gnathostomes.