From Wikipedia, the free encyclopedia

Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (January 2007)

Acanthodii Fossil range: Early Silurian to Permian
Mesacanthus, Parexus, Ischnacanthus
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Class: Acanthodii
Orders
Climatiiformes Ischnacanthiformes Acanthodiformes

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 appeared in the early Silurian (430 mya) (becoming thus the earliest jawed fishes) and lasted until the late Permian (250 mya). 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.

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.

• List of acanthodians
• Placoderm

• 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.

• Acanthodii taxonomy
• †Ischnacanthiformes taxonomy
• †Climatiiformes taxonomy
• †Acanthodiformes taxonomy
• Ischnacanthus gracilis - A Devonian Fish by Neal Robbins
• PALAEOZOIC FISH UK
• Acanthodopsis wardi

The subject of this article has been identified by the Missing Encyclopedic Articles project as being a high priority for expansion.