Acanthodii

(paleontology) A class of extinct fusiform fishes, the first jaw-bearing vertebrates in the fossil record.

A subclass of importance, including the earliest known jawed fishes or gnathostomes, first appearing in the Lower Silurian and surviving until the Lower Permian. They were usually small, less than 8 in. (20 cm) in length, though a few may have been as much as 100 in. (250 cm) long. The body was fusiform, the mouth terminal or nearly so, the eyes large, and the nasal capsules small. The tail was heterocercal; there were one or two dorsal fins, and all fins except the caudal had a spine on the anterior edge (see illustration). The scales had a square or rhombic crown and typically were nonoverlapping; they grew by periodic additions all around, lacked a pulp chamber, and were composed superficially of dentine or mesodentine and basally of bone, often acellular. Acanthodii are best considered as a subclass of Teleostomi, collateral with Osteichthyes, and can be classified as follows:

Subclass Acanthodii

     Order Climatiida

          Family Climatiidae

          Family Diplacanthidae

          ?Family Gyracanthidae

     Order Ischnacanthida

          Family Ischnacanthidae

     Order Acanthodida

          Family Acanthodidae

Climatiidae), about 6 in. (15 cm) long.">
Lateral view of Climatius reticulatus (Climatiidae), about 6 in. (15 cm) long.

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). They appeared in the early Silurian (430 mya) and lasted until the late Permian (250 mya). 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.

See also

• List of acanthodians
• Placoderm

References

• 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.

External links

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

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)