Difference between revisions of "Ostracoderm" - New World Encyclopedia
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Revision as of 18:25, 27 December 2006
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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. Their fossils are found in the North American and European strata of the Ordovician, Silurian, and Devonian periods of the Paleozoic era, approximately 400 million years ago. It was one of the earliest chordates and fish to appear in the geologic record. The ostracoderms were one stage in the stepwise development of life on Earth, with new stages coming on the foundation of earlier stages.
| Paleozoic era (542 - 251 mya) | |||||
|---|---|---|---|---|---|
| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian |
Characteristics
The ostracoderms once were considered the oldest and most primitive of the known chordates (Gregory 1935). Their fossils have been found in strata from the Middle Ordovician to the Upper Devonian, and in particular the Upper Silurian and Lower Devonian (roughly 375 to 425 million years ago) revealed rich ostracoderm faunas (Gregory 1935). More recently, fossils of other fish-like creatures, the 530-million-year-old Early Cambrian fossil dubbed Haikouellaand the 515-million-year-old middle Cambrian animal Pikaia have been promoted as the world's earliest chordate (Heeren 2000). The ostracoderms have been considered the ancestors of the jawless Agnathans and jawed Gnathostomatans.
Ostracoderms were a small fish, often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling animals. Their fins were small, and they lacked lateral fins, but did have medial fins. They had permanently open mouths.
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 ciliated motion to move food, ostracoderms used their muscular gill pouch to create a suction that pulled in small and slow moving prey.
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.
Classification
Ostracoderms are placed in the taxon Ostracodermi. Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. Unlike the heterostracans, the cephalaspids had lateral stabilizers for more control of their swimming.
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). In some classifications, Ostracodermi is a subclass placed within the Superclass Agnatha along with the extant (living) 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 used for a superfamily that is also called by the more common designation Ostracioidea. This superfamily is part of the Tetraodontiformes order (Plectognathi) of the Superclass Gnathostomata. Tetraodontiformes is an order that includes the extant puffers, boxfishes, and porcupine fish, and Ostraciodea or Ostracodermi is the superfamily in which the extant boxfishes are placed.
ReferencesISBN links support NWE through referral fees
- Gregory, W. K. 1935. On the evolution of the skulls of vertebrates with special reference to heritable changes in proportional diameters (anisomerism). Proceedings of the National Academy of Sciences 21(1):1-8.
- Heeren, F. 2000. [Challenging fossil of a little fish. The Boston Globe May 30, 2000. Retrieved December 16, 2006.
- Nelson, J. S. 1994. Fishes of the World, 3rd edition. New York: John Wiley & Sons. ISBN 0-471-54713-1.
- Robertson, G. M. 1938. New genera of ostracoderms from the Upper Silurian of Oesel. Journal of Paleontology 12(5): 486-493.
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