Acanthodii
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| Acanthodii Fossil range: Early Silurian to Permian | ||||||||||
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| Image:Mesacanthus Parexus Ischnacanthus.JPG | ||||||||||
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Climatiiformes |
Acanthodii (sometimes called spiny sharks) is a class of extinct fishes, having features of both bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes). 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, whiched evolved from little-specialized forms close to Recent Chondrichthyes.
Acanthodians did, in fact, have a cartilaginous seleton, but their fins had a wide, bony base an were reinforced on their anterior margin with a dentine spine.
The earliest ancanthodians were marine, but during the Devonian, freshwater species became predominant. They are distinguished in two respects: they were the first known jawed vertebrates, and they had stout spines supporting their fins, fixed in place and non-movable (like a shark's dorsal fin).
There were three orders: 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 fieshes that had a gill skeleton made of pieces of jointed cartilage.
The popular name "spiny sharks" is really a misnomer for those early jawed fishes. The name was coined because they were generally shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks". Fossilized spines are often all that remains of these fishes in ancient sedimentary rocks.
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.
In fact, acanthodians were a much earlier group of fishes than sharks. They evolved in the sea at the beginning of the Silurian Period, some 50 MYA 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 and formed a bony covering on top of the head and over the lower shoulder girdle. Others formed a bony flap over the gill openins (the operculum in later bony fishes).
[edit] See also
[edit] References
- Benes, Josef. Prehistoric Animals and Plants. Artia. 1979.
- Janvier, P. Early vertebrates. Oxford University Press. 1996.
- Long, J.A. The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins Univ. Press. Baltimore and London. 1995.
- Palmer, Douglas, Ed. The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. A Visual Who's Who of Prehistoric Life. Marshall Editions Developments Limited. 1999.
[edit] External links
- Acanthodii taxonomy
- †Ischnacanthiformes taxonomy
- †Climatiiformes taxonomy
- †Acanthodiformes taxonomy
- Ischnacanthus gracilis - A Devonian Fish by Neal Robbins
- PALAEOZOIC FISH UK
- Acanthodopsis wardi
ca:Acantodi de:Stachelhaie et:Akantoodid es:Acanthodii fr:Acanthodii it:Acanthodii ja:棘魚類 no:Pigghaier pl:Fałdopłetwe pt:Acanthodii ru:Акантоды vi:Lớp Cá mập gai uk:Акантоди
