Ostracoderms ("shell-skinned") are any of several groups of
extinctIn biology and ecology, extinction is the end of an organism or group of taxa. The moment of extinction is generally considered to be the death of the last individual of that species...
, primitive, jawless fishes that were covered in an armor of
bonyBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
plates. They belong to the taxon Ostracodermi, and their
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
s are found in the
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
and Devonian Period
strataStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
. They were often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling animals.
Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for
respirationIn animal physiology, respiration is the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
.
Ostracoderms ("shell-skinned") are any of several groups of
extinctIn biology and ecology, extinction is the end of an organism or group of taxa. The moment of extinction is generally considered to be the death of the last individual of that species...
, primitive, jawless fishes that were covered in an armor of
bonyBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
plates. They belong to the taxon Ostracodermi, and their
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
s are found in the
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
and Devonian Period
strataStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
. They were often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling animals.
Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for
respirationIn animal physiology, respiration is the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
. Earlier chordates with
gillA gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide...
s used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective
operculumThe operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body. The operculum is composed of four fused bones; the opercle, preopercle, interopercle, and subopercle...
. Unlike
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s that use
cilliaA cilium is an organelle found in eukaryotic cells. Cilia are tail-like projections.There are two types of cilia: motile cilia and non-motile, or primary, cilia, which typically serve as sensory organelles...
ted motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow moving prey into their mouths.
Ostracoderms existed in two major groups, the more primitive
heterostracansHeterostraci is an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Ordovician, and all, save for the Psammosteids, became extinct during the late Devonian...
and the
cephalaspidsCephalaspidomorphs are a taxon of jawless fishes named for the cephalaspids, a group of osteostracans. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys...
. 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.
The Subclass Ostracodermi has been placed in the Superclass
AgnathaAgnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
along with the extant Subclass
CyclostomataCyclostomata is a group of chordates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have have round mouths that lack jaws but have retractable horny teeth. The name Cyclostomata means "round mouths",...
, which includes
lampreyA lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. Translated directly, their name means stone lickers . While lampreys are well known for those species which bore into the flesh of other fish to suck their blood, these species make up the minority...
s and
hagfishHagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the subphylum Vertebrata...
es. The term does not often appear in classifications today because it is paraphyletic or polyphyletic. However, "ostracoderm" is still used as an informal term for the
armoredArmour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
jawless fishAgnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
es of the
PaleozoicThe Paleozoic or Palaeozoic Era is the earliest of three geologic eras of the Phanerozoic eon...
.
Some major groups of ostracoderms are as follows.
- †Pteraspidomorphi
- †Thelodonti
- †Anaspida
- †Galeaspida
- †Pituriaspida
- †Osteostraci