Cephalaspidomorphi
Encyclopedia
Cephalaspidomorphs are a group of jawless fishes
named for the cephalaspids, a group of osteostracans
. Most biologists regard this taxon
as extinct, but the name is sometimes used in the classification of lamprey
s because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian
and Devonian
periods to the present day.
Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard the 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 the Cephalaspidomorphi. Many biolgists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and 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.
whose sole living representatives are the lamprey
s.
Evidence now suggests that lampreys acquired 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
.
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
named for the cephalaspids, a group of osteostracans
Osteostraci
The class Osteostraci was a group 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....
. Most biologists regard this taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
as extinct, but the name is sometimes used in the classification of lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...
s because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
periods to the present day.
Classification
In the 1920s, the biologists Johan Kiær and 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 the relations among jawless vertebrates have varied. Most workers have come to regard the agnatha
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
as paraphyletic, having given rise to the jawed fishes
Gnathostomata
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...
. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to the Cephalaspidomorphi. Many biolgists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and 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
Galeaspida
Galeaspida is an extinct taxon of jawless marine and freshwater fish. Their name is derived from a Latin word for helmet, galea, and refers to their massive bone shield on the head...
and Pituriaspida
Pituriaspida
The Pituriaspida are a small group of extinct armored jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia...
, that were largely unknown in the 1920s.
Lampreys
Some reference works and databases have regarded Cephalaspidomorphi as a Linnean classClass (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
whose sole living representatives are the lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...
s.
Evidence now suggests that lampreys acquired the characters they share with cephalaspids by convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia
Hyperoartia
Hyperoartia or Petromyzontida is a group of jawless fish that includes the modern lampreys and their fossil relatives, the jawless fish of the class Anaspida. Examples of hyperoartians from early in their fossil record are Endeiolepis and Euphanerops, fishes with hypocercal tails that lived during...
.