Difference between revisions of "Ostracoderm" - New World Encyclopedia

From New World Encyclopedia
Line 10: Line 10:
 
{{Taxobox end}}
 
{{Taxobox end}}
  
'''Ostracoderms''' ("shell-skinned") are any of several groups of [[extinction|extinct]], primitive, jawless [[fish]]es that were covered in an armor of [[Bone|bony]] plates. Their [[fossil]]s 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.
+
'''Ostracoderms''' ("shell-skinned") are any of several groups of [[extinction|extinct]], primitive, jawless [[fish]]es that were covered in an armor of [[Bone|bony]] plates. Their [[fossil]]s 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 [[chordate]]s and [[fish]] to appear in the geologic record.
 
   
 
   
 
{{Paleozoic Footer}}
 
{{Paleozoic Footer}}
  
==Classification==
+
==Characteristics==
They belong to the taxon Ostracodermi, and  
+
The ostracoderms once were considered the oldest and most primative of the known [[chordate]]s (Gregory 1935). Their [[fossil]]s 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 ''Haikouella''and the 515-million-year-old middle Cambrian animal ''Pikaia'' have been promoted as the world's earliest chordate. The ostracoderms have been considered the ancestors of the jawless Agnathans and jawed Gnathostomatans.
  
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 [[lamprey]]s and [[hagfish]]es.  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]].
+
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.
  
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.
+
Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for [[Respiration (physiology)|respiration]]. In all previous life that had them, [[gill]]s 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 [[invertebrate]]s 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.
  
==Characteristics==
+
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.
The ostracoderms have been considered the oldest and most primative of the known [[chordate]]s (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.
 
  
 +
==Classification==
 +
Ostracoderms are placed in the taxon Ostracodermi. Ostracoderms existed in two major groups, the more primitive [[Heterostraci|heterostracans]]* and the [[cephalaspidomorphi|cephalaspids]]*. Unlike the heterostracans, the cephalaspids had lateral stabilizers for more control of their swimming.
  
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.
+
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 [[lamprey]]s and [[hagfish]]es. 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]].
Had median fins, but no lateral fins.
 
  
Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for [[Respiration (physiology)|respiration]]. In all previous life that had them, [[gill]]s 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 (fish)|operculum]]. Unlike [[invertebrate]]s that use [[Cilium|cillia]]ted motion to move food, ostracoderms used their muscular gill pouch to create a suction that pulled in small and slow moving [[prey]].
+
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.  
  
Ostracoderms existed in two major groups, the more primitive [[Heterostraci|heterostracans]] and the [[cephalaspidomorphi|cephalaspids]]. The cephalaspids improved over the heterostracans because they had lateral stabilizers for more control of their swimming.
+
==References==
 
+
*
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.
 
  
  
 
{{credit|90807930}}
 
{{credit|90807930}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]

Revision as of 03:22, 17 December 2006

Ostracodermi
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Agnatha
Class: Ostracodermi

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.

Paleozoic era (542 - 251 mya)
Cambrian Ordovician Silurian Devonian Carboniferous Permian

Characteristics

The ostracoderms once were considered the oldest and most primative 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. 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.

References
ISBN links support NWE through referral fees


Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.