Osteostraci Temporal range:Wenlock[1]–Late Devonian,433–359 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Cephalaspis species
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Agnatha
Superclass:	†Cephalaspidomorphi
Class:	†Osteostraci Lankester, 1868
Orders
Zenaspida Benneviaspidida Thyestiida
Synonyms
Osteostracida Cephalaspidiformes

TheclassOsteostraci (meaning "bony shells") is an extinct taxon of bony-armoredjawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from theMiddle Silurian toLate Devonian.

Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all knownagnathans.This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar tolampreys 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 topituriaspids, and together, these two taxa of jawless vertebrates are the sister-group ofgnathostomes. 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.^[3]

Most osteostracans had a massive cephalothorac shield, but all Middle and Late Devonian species appear to have had a reduced, thinner, and often micromeric dermal skeleton. This reduction may have occurred at least three times independently because the pattern of reduction is different in each taxon.^[4] The largest known osteostracan isParameteoraspis, its crescent-shaped headshield was 35 to 40 cm wide.^[5]

They were probably relatively good swimmers, possessingdorsal fins, pairedpectoral fins, and a strong tail. The shield of bone covering the head formed a single piece, and so presumably did not grow during adult life. However, the way in which the bone was laid down makes it possible to examine the imprints of nerves and other soft tissues. This reveals the presence of complex sensory organs and the sides and upper surface of the head, which may have been used to sense vibrations.^[6]

Below is acladogram showing thephylogenetic relationships of osteostracans from Sansom (2009):^[3]

• Janvier, Philippe.Early Vertebrates Oxford, New York: Oxford University Press, 1998.ISBN 0-19-854047-7
• Long, John A.The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996.ISBN 0-8018-5438-5

• Natural History of Vertebrates Lecture Notes: Chapter 3 - Jawless Vertebrates and the Origin of Jawed Vertebrates

• Osteostraci
• Paleozoic jawless fish
• Silurian first appearances
• Famennian extinctions
• Prehistoric fish classes

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• Short description is different from Wikidata
• Articles with 'species' microformats