Ostracoderm

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Various ostracoderms of the class Osteostraci ('bony-shields')
Cardipeltis bryanti, a lower Devonian ostracoderm from the Bighorn Mountains of Wyoming. Ventral (underside) exposed.

Ostracodermi (lit. 'shell-skins') or ostracoderms is an unofficial

anaspids are closer to them) and thus does not correspond to one evolutionary lineage.[1]
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

gill precursors used them for both respiration and feeding.[2] 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 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 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 ὄστρακον óstrakon + δέρμα dérma).[3]

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."

Conodonts); later a version of teeth was used in a new way to protect them."[4]

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.[5][6]

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 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.[7]

The Subclass Ostracodermi has been placed in the division

Cyclostomata, which includes lampreys and hagfishes
.

Major groups

Major groups of ostracoderms
Group Class Image Description
Cephalaspido-
morphi 		Cephalaspidomorphi or cephalaspids ('head-shields'), like most contemporary fishes, were very well armoured. Particularly the head shield was well developed, protecting the head, gills and the anterior section of innards. The body were in most forms well armoured too. The head shield had a series of grooves over the whole surface forming an extensive lateral line organ. The eyes were rather small and placed atop the head. There was no jaw proper. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite.[8] 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. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day.
†Galeaspida
(extinct)
braincase
.
†Pituriaspida
(extinct)
Neeyambaspis enigmatica found in a single sandstone location of the Georgina Basin, in Western Queensland, Australia
.
†Osteostraci
(extinct)
Late Devonian. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group of pituriaspids. Together, these two taxa of jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of spongy bone, and a superficial layer of dentin), and perichondral bone.[10]
Other
groups 		Other groups
†Pteraspido-
morphi
(extinct) 		†Pteraspidomorphi ('wing-shield forms') have extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water.

The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida.

†Thelodonti
(extinct)
Frasnian–Famennian extinction event of the Late Devonian
. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms.
†Anaspida
(extinct)
Late Devonian extinction,[17] where most species, save for lampreys
, became extinct due to the environmental upheaval during that time.

See also

References

  1. ISBN 9781405144490
    .
  2. ISBN 0-03-096846-1
    . "Gills are not present in the pharyngeal pouches of protochordates as they are in fishes; rather the [pharangeal] slits of protochordates are part of their feeding mechanism." - p 32 "Water is drawn into the pharynx ... The pharynx wall is perforated by many vertically elongated, pharyngeal slits ... Cells in the endostyle of the pharyngeal floor secrete mucus that entraps minute food particles." - p 35 "Amphioxus also gains oxygen and discharges carbon dioxide from the water flowing through the pharynx even though gills are not present." - p 35
  3. ISBN 9780805043662
    .
  4. ^
    ISBN 9780307277459
    .
  5. ^ "Fossil fish reveals how jaws evolved - Planet Earth Online". Archived from the original on 2012-07-03. Retrieved 2014-04-11.
  6. ^ "GEOL 331 Invertebrate Paleontology". Archived from the original on 2016-10-20. Retrieved 2014-04-11.
  7. ^ Vertebrate jaw design locked down early
  8. ISBN 978-0-471-85074-8.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  9. ^ Dead fish breathes new life into the evolutionary origin of fins and limbs
  10. S2CID 85924210
    .
  11. doi:10.1016/0031-0182(82)90027-X
    .
  12. ISBN 978-2-88124-996-9
    .
  13. S2CID 22803015.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  14. ^ Turner, S. (1999). "Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance". In A. J. Boucot; J. Lawson (eds.). Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report. Cambridge University Press. pp. 42–78.
  15. ^ The early and mid Silurian. See Kazlev, M.A., White, T. (March 6, 2001). "Thelodonti". Palaeos.com. Archived from the original on October 28, 2007. Retrieved October 30, 2007.{{cite web}}: CS1 maint: multiple names: authors list (link)
  16. ISBN 0-415-23370-4
    .
  17. ISBN 0-226-31568-1
    .
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