10-525: Zenaspida Benneviaspidida Thyestiida The class Osteostraci (meaning "bony shells") is an extinct taxon 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 . Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans . This
20-480: A prehistoric jawless fish is a stub . You can help Misplaced Pages by expanding it . Boreaspis Boreaspis (meaning " Boreas 's Shield") is an extinct genus of osteostracan agnathan vertebrate that lived in the Devonian period. Fourteen different species of Boreaspis have been found in sandstone of the lagoons and estuaries of Devonian Spitsbergen ; however, some of these likely do not belong to
30-438: 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. The largest known osteostracan is Parameteoraspis , its crescent-shaped headshield was 35 to 40 cm wide. They were probably relatively good swimmers, possessing dorsal fins , paired pectoral fins , and a strong tail. The shield of bone covering
40-403: Is an extinct order of osteostracans , a group of jawless stem - gnathostomes . They possessed a distinct headshield, which varied in width to length ratio by species. The head shield is dome-shaped and extremely large in comparison to the main body. The abdominal section of this shield has a less developed median dorsal crest. As a rule for this order, the nasohypophysial opening is larger than
50-412: 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 to pituriaspids , and together, these two taxa of jawless vertebrates are
60-446: Is often used to speciate. The cladogram below is adapted from a 2014 article by Scott and Wilson: Spangenhelmaspis Parameteoraspis Balticaspis Trewinia Escuminaspis Levesquaspis Wladysagitta Glabrapelta Dentapelta Superciliaspis Machairaspis Scolenaspis Ukrainaspis Tegaspis Stensiopelta Diademaspis Zenaspis This article about
70-419: The genus. The species B. robusta and B. costata have been reassigned to Spatulaspis ; and B. circinus , B. curtirostris , and B. gracilis now belong to Dicranaspis . Species of Boreaspis were very small, with head shields about 2 centimetres (0.79 in) long. All species possessed a long spathe -like rostrum derived from the anterior-most end of the head shield, which would have enhanced
80-1324: 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. Below is a cladogram showing the phylogenetic relationships of osteostracans from Sansom (2009): Hirella Aceraspis Ateleaspis Hemicyclaspis Cephalaspis Spangenhelmaspis Wladysagitta Parameteoraspis Balticaspis Trewinia Escuminaspis Levesquaspis Tegaspis Stensiopelta Diademaspis Zenaspis Scolenaspis Ukrainaspis Machairaspis Superciliaspis Pattenaspis Zychaspis Hildenaspis Mimetaspis Waengsjoeaspis Camptaspis Yvonaspis Ectinaspis Securiaspis "Benneviaspis" longicornis "Benneviaspis" anglica "Benneviaspis" lankesteri Benneviaspis Hoelaspis Severaspis "Boreaspis" ceratops "Boreaspis" intermedia Boreaspis Dicranaspis Spatulaspis Belonaspis Hapilaspis Tauraspis Zenaspida Zenaspida Zenaspidida
90-423: The nasal division. The pineal plate seen in other osteostracans is barely developed or completely absent. The median dorsal field is notably broad, and the lateral fields are widened in the posterior, but reach back no further than the proximal section of the dorsal surface of the cornual processes. The ornamentation on the head shield can have singular, large tubercles, or groups of tubercles which range in size. This
100-421: 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. Most osteostracans had a massive cephalothorac shield, but all Middle and Late Devonian species appear to have had
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