Cephalaspidomorphi

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Para otros usos de este término, véase Piscis. «pez» redirige aquí. Para otras acepciones, véase pez (desambiguación). ? Peces Rango fósil: Cámbrico - Presente El grupo de los peces es parafilético y se define como todos los vertebrados que no son tetrápodos, es decir, por la exclusión de un taxón (los tetrápodos) de otro mayor (los vertebrados), y no por la posesión de características derivadas comunes (apomorfías). Las especies hoy existentes pertenecen a tres grupos (a veces considerados clases, a veces superclases): Agnatos o peces sin mandíbulas, que incluye unas pocas especies actuales (lampreas y mixines). Es un grupo parafilético. Condrictios o peces cartilaginosos, que incluyen a tiburones, rayas y quimeras, caracterizados por poseer hendiduras branquiales externamente visibles y un esqueleto compuesto sólo de cartílago. Son un grupo de vertebrados muy primitivos, pero muy exitosos evolutivamente, ya que los tiburones son animales antiquísimos que no han cambiado mucho desde su origen. Osteictios o peces óseos, con esqueleto óseo y branquias protegidas mediante un opérculo. Es un grupo parafilético. A su vez se subdividen en: Actinopterigios, peces óseos con aletas provistas de radios. Sarcopterigios, peces óseos con aletas lobuladas. Son el grupo hermano de los tetrápodos (veterbrados provistos de cuatro patas); los primeros anfibios se originaron a partir de sarcopterigios primitivos. El siguiente cladograma muestra las relaciones fil Oscar (Astronotus <b>...</b>

Cephalaspidomorphi

Cephalaspidomorphs are a group of jawless fishes named for the cephalaspids, a group of osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day.

Classification

In the 1920s, the biologists Johan Kiær and Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.

Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard the 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 the Cephalaspidomorphi. Many biolgists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and the affinities of the lampreys are also contested. Others have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida, that were largely unknown in the 1920s.

Lampreys

Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class whose sole living representatives are the lampreys. Evidence now suggests that lampreys acquired the characters they share with cephalaspids by convergent evolution. As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia.

External links

The Tree of Life discusses relations among jawless fish

References

Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7

Stensiö, E. A. (1927). The Devonian and Downtonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. Skrifter om Svalbard og Ishavet, 12, 1-391.

Category:Vertebrates without jaws

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