17-400: † Osteostraci † Galeaspida † Pituriaspida Gnathostomata ? Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the 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. If lampreys are included, they would extend
34-426: 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 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
51-441: 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 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
68-403: 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
85-634: 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 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
102-411: Is known, outside of the head shield. If they had a vertebral column at all, it would have been cartilage rather than bone. Likely, the axial skeleton consisted of an unsegmented notochord . A fleshy appendage emerged laterally on each side, behind the head shield, functioning as pectoral fins . The tail had a single, wrap-around tail-fin. Modern fishes with such a tail are rarely quick swimmers, and
119-446: 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
136-490: The Cephalaspidomorphs were not likely very active animals. They probably spent much of their time semi-submerged in the mud. They also lacked a swim bladder , and would not have been able to keep afloat without actively swimming. The head shield provided some lift though and would have made the Cephalaspidomorphs better swimmers than most of their contemporaries. The whole group were likely algae- or filter-feeders, combing
153-488: The affinities of the lampreys are also contested. Others have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida , that were largely unknown in the 1920s. Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class whose sole living representatives are the lampreys . Evidence now suggests that lampreys acquired
170-415: The anterior section of the viscera . The body was in most forms well armored as well. 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 on the top of 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. No internal skeleton
187-472: The bottom for small animals, much like the modern armored bottom feeders, such as Loricariidae or Hoplosternum catfish. In the 1920s, the biologists Johan Kiær and Erik Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids , and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening. Since then, opinions on
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#1732854996396204-428: The characters they share with cephalaspids by convergent evolution . As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia . [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Osteostraci Zenaspida Benneviaspidida Thyestiida The class Osteostraci (meaning "bony shells")
221-420: 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 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,
238-461: The known range of the group from the Silurian and Devonian periods to the present day . They are the closest relatives of jawed fishes , who may have emerged from within them; if this is true, they would survive if the jawed fish are included. Cephalaspidomorphs were, like most contemporary fishes, very well armored. The head shield was particularly well developed, protecting the head, gills and
255-423: 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
272-1105: 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
289-404: The relations among jawless vertebrates have varied. Most workers have come to regard Agnatha as paraphyletic , having given rise to the jawed fishes . Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to Cephalaspidomorphi. Many biologists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and
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