Ostracoderm
Encyclopedia
Ostracoderms 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 fossil
s are found in the Ordovician
and Devonian Period strata
of North America
and Europe
. 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 respiration
. Earlier chordates with gill
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 operculum
. Unlike invertebrate
s that use cilia
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 were the first fossil fish to be discovered. The Swiss anatomist Louis Agassiz
received some fossils of bony armored fish from Scotland
in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish
and sturgeon
s but later realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms" which means "shell-skinned".
Ostracoderms existed in two major groups, the more primitive heterostracans
and the cephalaspids
. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.
After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates, however, that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed.
The Subclass Ostracodermi has been placed in the Superclass Agnatha
along with the extant Subclass Cyclostomata
, which includes lamprey
s and hagfish
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 armored
jawless fish
es of the Paleozoic
.
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
, primitive, jawless fishes that were covered in an armor of bony
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They 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 fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are found in the Ordovician
Ordovician
The 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 strata
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe 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 watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. 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 respiration
Respiration (physiology)
'In physiology, respiration is defined as 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 gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
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 operculum
Operculum (fish)
The 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....
. Unlike invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s that use cilia
Cilium
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
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 were the first fossil fish to be discovered. The Swiss anatomist Louis Agassiz
Louis Agassiz
Jean Louis Rodolphe Agassiz was a Swiss paleontologist, glaciologist, geologist and a prominent innovator in the study of the Earth's natural history. He grew up in Switzerland and became a professor of natural history at University of Neuchâtel...
received some fossils of bony armored fish from Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish
Catfish
Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...
and sturgeon
Sturgeon
Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common...
s but later realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms" which means "shell-skinned".
Ostracoderms existed in two major groups, the more primitive heterostracans
Heterostraci
Heterostraci 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 cephalaspids
Cephalaspidomorphi
Cephalaspidomorphs are a group of jawless fishes named for the cephalaspids, a group of osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys because lampreys were once thought to be related to cephalaspids...
. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.
After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period. More recent research indicates, however, that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed.
The Subclass Ostracodermi has been placed in the Superclass Agnatha
Agnatha
Agnatha 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 Cyclostomata
Cyclostomata
Cyclostomata is a group of chordates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have round mouths that lack jaws but have retractable horny teeth...
, which includes lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...
s and hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...
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 armored
Armour (zoology)
Armour 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 fish
Agnatha
Agnatha 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 Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
.
Major groups
Some major groups of ostracoderms are as follows.- †Pteraspida
- †ThelodontiThelodontiThelodonts are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armor....
- †AnaspidaAnaspidaThe Anaspida are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked scales and paired fins, but have a striking highly hypocercal tail...
- †GaleaspidaGaleaspidaGaleaspida is an extinct taxon of jawless marine and freshwater fish. Their name is derived from a Latin word for helmet, galea, and refers to their massive bone shield on the head...
- †PituriaspidaPituriaspidaThe Pituriaspida are a small group of extinct armored jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia...
- †OsteostraciOsteostraciThe class Osteostraci was a group of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian....