| Cephalaspidomorphs | |
|---|---|
 | |
| Life restoration ofCephalaspis lyelli. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Infraphylum: | Agnatha |
| Class: | †Cephalaspidomorphi |
| Type species | |
| †Cephalaspis lyelli Agassiz, 1835 | |
| Subgroups[3] | |
†Cephalaspidiformes | |
Cephalaspidomorphi (alternatively calledMonirhina, or simplycephalaspids)[4] is aclass ofjawless fishes that is presently regarded as uniting theosteostracans,galeaspids andpituriaspids. Mostbiologists regard thistaxon as extinct, but the name is still sometimes used in the classification oflampreys because they were once thought to be descended. If lampreys are included they would extend the known range of the group from theSilurian andDevonian periods, when they are traditionally assumed to have lived, to thepresent day. Modern works typically assume the cephalaspidimorphs to be the closest relatives ofjawed fishes, who may have emerged from within them; if this is true, they would survive if thejawed fish are included. The cephalaspidomorphs possessed armored head-shields, aheterocercal tail fin, and in some groups paired pectoral fins.
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 theviscera. The body was in most forms well armored as well. The head shield had a series of grooves over the whole surface, forming an extensivelateral line organ. The eyes were rather small and placed on the top of the head. There was nojaw, and instead the mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.[5]
No internal skeleton is known, outside of the head shield. If they had avertebral column at all, it would have beencartilage rather than bone. Likely, theaxial skeleton consisted of an unsegmentednotochord. A fleshy appendage emerged laterally on each side, behind the head shield, functioning aspectoral 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 aswim 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.[5] The whole group were likely algae- or filter-feeders, combing the bottom for small animals, much like the modern armored bottom feeders, such asLoricariidae orHoplosternum catfish.[6]
In the 1920s, the biologistsJohan Kiær andErik 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.[7]
Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regardAgnatha asparaphyletic, having given rise to thejawed 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 behindPalaeos.com have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such asGaleaspida and potentially thePituriaspida.[8] Also following the latter definition, the class (and itsmonotypic superclass Osteostracomorphi) is assumed to be themonophyletic sister taxon to Gnathostomata by the 2016 edition ofFishes of the World.[4]
Some reference works and databases have regarded Cephalaspidomorphi as aLinnean class whose sole living representatives are thelampreys.[9] Evidence now suggests that lampreys acquired the characters they share with cephalaspids byconvergent evolution.[10][11]As such, many newer works about fishes classify lampreys in a separate group calledPetromyzontida or Hyperoartia.[12]
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