Cephalaspidomorphs are a group of

jawless fishes Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostome ...

named for '' Cephalaspis'' of the osteostracans. Most biologists regard this

taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

as extinct, but the name is sometimes used in the classification of

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

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 The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

and

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

periods to the present day. They are the closest relatives of jawed fishes, who emerged from within them and they would survive if the jawed fish are included.

Biology and morphology

Cephalaspidomorphi were, like most contemporary fishes, very well armoured. The head shield was particularly well developed, protecting the head,

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s and the anterior section of the

viscera In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

. The body was in most forms well armoured as well. The head shield had a series of grooves over the whole surface, forming an extensive

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

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 is known, outside of the head shield. If they had a

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

at all, it would have been

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...

rather than bone. Likely, the

axial skeleton The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk of a vertebrate. In the human skeleton, it consists of 80 bones and is composed of six parts; the skull (22 bones), also the ossicles of the middle ...

consisted of an unsegmented

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...

. A fleshy appendage emerged laterally on each side, behind the head shield, functioning as

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

s. The tail had a single, wrap-around tail-fin. Modern fishes with such a tail are rarely quick swimmers, and the cephalaspidomorphi were not likely very active animals. They probably spent much of their time semi-submerged in the mud. They also lacked a

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...

, and would not have been able to keep afloat without actively swimming. The head shield provided some lift though, and would have made the cephalaspidomorphi 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 armoured bottom feeders, such as

Loricariidae The Loricariidae is the largest family of catfish (order Siluriformes), with 92 genera and just over 680 species. Loricariids originate from freshwater habitats of Costa Rica, Panama, and tropical and subtropical South America. These fish are not ...

or ''

Hoplosternum ''Hoplosternum'' is a small genus of freshwater catfish in the Callichthyinae subfamily of the armored catfish family. A fossil catfish has been identified as ''Hoplosternum sp.'' from the middle Miocene in the La Venta formation, Magdalena ...

'' catfish.

Classification

In the 1920s, the biologists Johan Kiær and

Erik Stensiö Prof Erik Helge Osvald Stensiö HFRSE (2 October 1891 – 11 January 1984) was a Swedish paleozoologist. He later took his new surname from his place of origin and is occasionally referred to with both names (as Erik Andersson Stensiö, Erik ...

first recognized the Cephalaspidomorphi as including the osteostracans,

anaspid Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and became extinct soon after the start of the Devonian. They were classically regarded as the ancestors of lampre ...

s, 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 the

agnatha Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

, 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 biologists 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.

Lampreys

Some reference works and databases have regarded Cephalaspidomorphi as a Linnean

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

whose sole living representatives are the

s. Evidence now suggests that lampreys acquired the characters they share with cephalaspids by

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

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

References

*Janvier, Philippe. ''Early Vertebrates''. Oxford, New York: Oxford University Press, 1998.

External links

The Tree of Life discusses relations among jawless fish
{{Taxonbar, from=Q857497 Paleozoic jawless fish Silurian first appearances Devonian extinctions