Ostracoderm
| Ostracodermi | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Scientific classification | ||||||||||
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Ostracoderms ("shell-skinned") are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the North American and European strata of the Ordovician, Silurian, and Devonian periods of the Paleozoic era.
first jawed fish in early Silurian
| Paleozoic era (542 - 251 mya) | |||||
|---|---|---|---|---|---|
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian |
Classification
Nelson (1994), in his book Fishes of the World, calls the ostracoderms the "extinct heavily armored agnathans." Generally, the ostracoderms are considered to be part of the Agnatha or to be "pre-fish" that gave rise to the Agnatha (Gregory 1935). The Subclass Ostracodermi has been placed in the Superclass Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes. The does not often appear in classifications today because it is paraphyletic or polyphyletic, but "ostracoderm" is still used as an informal term for the armored jawless fishes of the Paleozoic.
However, in Nelson's classification of fish, the term Ostracodermi is placed as a superfamily (more properly named Ostracioidea) of the Tetraodontiformes order (Plectognathi) of the Acanthopterygii of the Teleosts of the Superclass Gnathostomata. Tetraodontiformes is an order that includes the puffers, boxfishes, and porcupine fish. Also called the superfamily Ostracioidea, the Ostracodermi includes the boxfishes.
Characteristics
The ostracoderms have been considered the oldest and most primative of the known chordates (Gregory 1935). They were a small fish, often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling animals. Their fins wer small. and permanently open mouths.
They were prevelant from the Middle Ordovician, were preveleant in rich orstracoderm faunsas of the Upper Silurian and Lower Devoanian times (from about **8 to ***), and comes ot an end in the Pper Develonian (Gregory 1935). It is from these that the jawless Agnathans and jawed Gnathostomatans developed.
Had median fins, but no lateral fins.
Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. In all previous life that had them, gills were used for both respiration and feeding. They had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use cilliated motion to move food, ostracoderms used their muscular gill pouch to create a suction that pulled in small and slow moving prey.
Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids improved over the heterostracans because they had lateral stabilizers for more control of their swimming.
After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 400 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period.
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