cephalaspidomorphi

File:Cephalaspis Lyellii.jpg lyelli]]

Cephalaspidomorphs possessed armored plates on the exterior of their bodies. The head shield was particularly well developed, protecting the head, gills and 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, and instead the mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.{{cite book|last=Morales|first=Edwin H. Colbert, Michael|title=Evolution of the vertebrates : a history of the backboned animals through time|year=1991|publisher=Wiley-Liss|location=New York|isbn=978-0-471-85074-8|edition=4th}}

No internal skeleton 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 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 the bottom for small animals, much like the modern armored bottom feeders, such as Loricariidae or Hoplosternum catfish.{{cite book|last=Lucas|first=F.A.|title=Animals of the past : an account of some of the creatures of the ancient world|year=1922|publisher=American Museum of Natural History|location=New York}}

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.Stensiö, E.A. (1927): The Devonian and Downtonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. Skrifter om Svalbard og Ishavet, no. 12, pp. 1–391.

Since then, opinions on 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. Some biologists regard the name Cephalaspidomorphi as obsolete because relations among Osteostraci and Anaspida are unclear, and the relation of lampreys to these groups is no longer supported. Others, such as the authors behind Palaeos.com have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and potentially the Pituriaspida.{{Cite web

| last = White

| first = Toby

| title = Thelodonti: Cephalaspidomorphi

| work = Palaeos

| url = http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.400.html#Cephalaspidomorphi

| access-date = 2009-10-27

| archive-url = https://web.archive.org/web/20090306221641/http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.400.html#Cephalaspidomorphi

| archive-date = 2009-03-06

| url-status = dead

}} Also following the latter definition, the class (and its monotypic superclass Osteostracomorphi) is assumed to be the monophyletic sister taxon to Gnathostomata by the 2016 edition of Fishes of the World.

Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class whose sole living representatives are the lampreys.

{{cite book

| author = Nelson, Joseph S.

| year = 1994

| title = Fishes of the World

| edition = Third

| publisher = John Wiley and Sons

| isbn = 0-471-54713-1}} Evidence now suggests that lampreys acquired the characters they share with cephalaspids by convergent evolution.

{{Cite book

| author = Forey, Peter

| author2 = Janvier, Philippe

| name-list-style = amp

| year = 2012

| contribution = Agnathans and the origin of jawed vertebrates

| title = Shaking the tree: readings from Nature in the history of life

| editor-last = Gee

| editor-first = Henry

| editor-link = Henry Gee

| pages = 251–266

| place = USA

| publisher = University of Chicago Press; Nature/Macmillan Magazines

| isbn = 978-0-226-28497-2

}}

{{cite journal

| author = Janvier, Philippe

| year = 2008

| title = Early Jawless Vertebrates and Cyclostome Origins

| journal = Zoological Science

| pmid = 19267641

| volume = 25

| issue = 10 | pages = 1045–1056

| doi = 10.2108/zsj.25.1045

| doi-access = free

}}

As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia.

{{cite book

| author = Nelson, J. S.

| year = 2006

| title = Fishes of the World

| publisher = John Wiley and Sons, Inc.

| location = New York

| edition = 4th

| pages = 601 pp

| isbn = 0-471-25031-7

}}

{{Reflist}}

• Janvier, Philippe. Early Vertebrates. Oxford, New York: Oxford University Press, 1996. {{ISBN|0-19-854047-7}}

• [http://tolweb.org/Vertebrata/14829 The Tree of Life discusses relations among jawless fish]

{{Evolution of fish|state=collapsed}}

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Category:Paleozoic jawless fish

Category:Silurian first appearances

Category:Devonian extinctions