Acanthodii

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"Spiny shark" redirects here. For the spiny true sharks, see Echinorhinus.
Acanthodii
Temporal range: Early Silurian–Permian
Acanthodes BW.jpg
Acanthodes sp.
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Class: Acanthodii
Owen, 1846
Orders

Climatiiformes
Ischnacanthiformes
Acanthodiformes

Acanthodii (sometimes called spiny sharks) is a class of extinct fish, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from generalized or basal forms similar to primitive members of the still-extant Chondrichthyes.

The popular 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, largely immovable bony spines supporting all the fins except the tail - hence, "spiny sharks". Fossilized spines and scales are often all that remains of these fishes in ancient sedimentary rocks. Although not sharks or cartilaginous fish, acanthodians did, in fact, have a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. The earliest acanthodians were marine, but during the Devonian, freshwater species became predominant.

There are three orders recognized: Climatiiformes, Ischnacanthiformes and Acanthodiformes. Climatiiforma had shoulder armor and many small sharp spines, Ischnacanthiforma with teeth fused to the jaw, and the Acanthodiforma were filter feeders, with no teeth in the jaw, but long gill rakers. Overall, the acanthodians' jaws are presumed to have evolved from the first gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage.

Characteristics[edit]

Diplacanthus longispinus impression at the Museum für Naturkunde, Berlin

The scales of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.

Despite being called "spiny sharks," acanthodians predate sharks. Scales that have been tentatively identified as belonging to acanthodians, or "shark-like fishes" have been found in various Ordovician strata, though, they are ambiguous, and may actually belong to jawless fishes such as thelodonts. The earliest unequivocal acanthodian fossils date from the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Later, the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the Devonian and in the coal swamps of Carboniferous. But the first bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in Permian times (approximately 250 MYA).

Many paleonthologists consider that the acanthodians were close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings analogous to the operculum in later bony fishes.

Taxonomy and phylogeny[edit]

In a study of early jawed vertebrate relationships, Davis et al. (2012) found acanthodians to be split among the two major clades Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). The well-known acanthodian Acanthodes was placed within Osteichthyes, despite the presence of many chondrichthyan characteristics in its braincase. Below is a cladogram from the analysis, with bold lines representing the evolutionary grade of acanthodians:[1] A newly described Silurian placoderm, Entelognathus, which has jaw anatomy shared with bony fish and tetrapods will lead to revisions of his phylogeny [2]

Vertebrata 

 Galeaspida



 Osteostraci


Gnathostomata


 Bothriolepis 



 Pterichthyodes 





 Brindabellaspis 



 Macropetalichthys 





 Dicksonosteus 




 Buchanosteus 



 Coccosteus 







 Cowralepis 




 Austroptyctodus 



 Campbellodus 



 Rhamphodopsis 







 Tetanopsyrus 




 Culmacanthus 




 Diplacanthus 




 Gladiobranchus 



 Rhadinacanthus 









 Ptomacanthus 




 Climatius 




 Brachyacanthus 



 Parexus 






Osteichthyes

 Euthacanthus 





 Cassidiceps 





 Mesacanthus 



 Promesacanthus 





 Cheiracanthus 




 Acanthodes 



 Homalacanthus 









 Ischnacanthus 



 Poracanthodes 





 Dialipina




 Ligulalepis



 to Crown group Osteichthyes







Chondrichthyes

 Vernicomacanthus 




 Brochoadmones 




 Kathemacanthus 




 Obtusacanthus 



 Lupopsyrus 




 Pucapampella





 Doliodus



 Tamiobatis







 Cladodoides



 Orthacanthus





 Cobelodus




 Akmonistion



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Cladoselache







 Tristychius



 to Crown group Chondrichthyes


















Placodermi
Acanthodii
Acanthodii

See also[edit]

References[edit]

  1. ^ Davis, S. P.; Finarelli, J. A.; Coates, M. I. (2012). "Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes". Nature 486 (7402): 247. doi:10.1038/nature11080.  edit
  2. ^ "Ancient fish face shows roots of modern jaw". Nature. September 25, 2013. Archived from the original on 2013-10-31. Retrieved September 26, 2013. 

Further reading[edit]

  • Beneš, Josef (1979), Prehistoric Animals and Plants, New York: Hamlyn, ISBN 978-0-600-30341-1 
  • Janvier, Phillipe (1996), Early vertebrates, Oxford: Clarendon Press, ISBN 978-0-19-854047-2 
  • Long, John A. (1995), The Rise of Fishes: 500 Million Years of Evolution, Baltimore: Johns Hopkins University Press, ISBN 978-0-8018-4992-3 
  • Palmer, Douglas, ed. (1999), The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. A Visual Who's Who of Prehistoric Life, New York: Simon & Schuster, ISBN 978-0-684-86411-2 

External links[edit]