Ostracoderms () are the
armored
Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or ...
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostome ...
of the
Paleozoic Era
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ''z ...
. The term does not often appear in classifications today because it is
paraphyletic (excluding
jawed fish
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
es) (may also be
polyphyletic if
anaspid
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and became extinct soon after the start of the Devonian. They were classically regarded as the ancestors of lampre ...
s are closer to
cyclostomes Cyclostome is a biological term (from the Greek for "round mouth") used in a few different senses:
* for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae). This was thought for ...
) and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.
An innovation of ostracoderms was the use of
gills not for feeding, but exclusively for
respiration. Earlier
chordates
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...
with
gill precursors 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
invertebrates that use
ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow moving
prey into their mouths.
Swiss anatomist
Louis Agassiz
Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history.
Spending his early life in Switzerland, he rec ...
received some fossils of bony armored fish from
Scotland in the 1830s. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as
catfish
Catfish (or catfishes; order Siluriformes or Nematognathi) 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 three largest species alive ...
and
sturgeon, but later realized that they lacked movable jaws. Hence, he classified them in 1844 as a new group, named "ostracoderms" to mean 'shell-skinned' (from Greek + ).
Ostracoderms have heads covered with a bony shield. They are among the earliest creatures with bony heads. The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth-like structures."
Neil Shubin
Neil Shubin (born December 22, 1960) is an American paleontologist, evolutionary biologist and popular science writer. He is the Robert R. Bensley Professor of Organismal Biology and Anatomy, Associate Dean of Organismal Biology and Anatomy an ...
writes: "Cut the bone of the
stracodermskull open…pop it under a microscope and…you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull--one of the earliest in the fossil record--is made entirely of little teeth. Teeth originally arose to bite creatures (see
Conodonts
Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...
); later a version of teeth was used in a new way to protect them."
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.
It was long assumed that pteraspidomorphs and thelodonts were the only ostracoderms with paired nostrils, while the other groups have just a single median nostril. It has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs.
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
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...
. More recent research indicates that fish with jaws had far less to do with the extinction of the ostracoderms than previously assumed, as they coexisted without noticeable decline for about 30 million years.
Vertebrate jaw design locked down early
/ref>
The Subclass Ostracodermi has been placed in the division Agnatha
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostom ...
along with the extant Subclass Cyclostomata
Cyclostomi, often referred to as Cyclostomata , is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth called cerato ...
, which includes lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like ...
s and hagfishes.
Major groups
See also
* Acanthodii
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
* Placoderm
References
{{evolution of fish
Paleozoic jawless fish
Paraphyletic groups
Polyphyletic groups