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 to appear in the geologic record. The ostracoderms were a part of one stage in the stepwise development of life on Earth, with new stages forming on the foundation of earlier stages.
|Paleozoic era (542 - 251 mya)|
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. The Upper Silurian and Lower Devonian (roughly 375 to 425 million years ago) in particular revealed rich ostracoderm faunas (Gregory, 1935). More recently, fossils of other fish-like creatures, the 530-million-year-old Early Cambrian fossil dubbed Haikouella and the 515-million-year-old middle Cambrian animal Pikaia have been promoted as the world's earliest chordates (Heeren, 2000). The ostracoderms have been considered the ancestors of the jawless Agnathans and jawed Gnathostomatans.
Ostracoderms were small fishes, often less than 1 foot (30 cm) 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. The ostracoderms 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 fishes (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.
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, in his book Fishes of the World, calls the ostracoderms the "extinct heavily armored agnathans." Generally, ostracoderms are considered to be part of the Agnatha class or to be "pre-fish" that gave rise to the Agnatha class (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. Ostracodermi 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.
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