Cephalaspidomorphi (alternatively called Monirhina, or simply cephalaspids) is a

class Class, Classes, 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 d ...

of jawless fishes that is presently regarded as uniting the osteostracans, galeaspids and pituriaspids. Most

biologist A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...

s regard this

taxon In biology, a taxon (back-formation from ''taxonomy''; : 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 name and ...

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

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s because they were once thought to be descended. 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 23.5 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 third and shortest period of t ...

and

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

periods, when they are traditionally assumed to have lived, to the present day. Modern works typically assume the cephalaspidimorphs to be the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the

jawed fish Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant taxon, extant vertebrates, including all living bony fishes (bot ...

are included. The cephalaspidomorphs possessed armored head-shields, a

heterocercal Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

tail fin, and in some groups paired pectoral fins.

Biology and morphology

Cephalaspidomorphs possessed armored plates on the exterior of their bodies. The head shield was particularly well developed, protecting the head,

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, 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 r ...

s and the anterior section of the

viscera In a multicellular organism, 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 a ...

. 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 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 epithelia ...

organ. The eyes were rather small and placed on the top of the head. There was no

jaw The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth ...

, and instead 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 spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

at all, it would have been

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

rather than bone. Likely, the

axial skeleton The axial skeleton is the core part of the endoskeleton made of the bones of the head and trunk of vertebrates. In the human skeleton, it consists of 80 bones and is composed of the skull (28 bones, including the cranium, mandible and the midd ...

consisted of an unsegmented

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

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

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

s. 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 The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

, 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 Loricariidae is the largest family (biology), family of catfish (order Siluriformes), with over 90 genus, genera and just over 680 species. Loricariids originate from freshwater habitats of Costa Rica, Panama, and tropical and subtropical South A ...

or '' Hoplosternum'' catfish.

Classification

In the 1920s, the biologists Johan Kiær and

Erik Stensiö Prof Erik Helge Osvald Stensiö HFRSE (2 October 1891 – 11 January 1984), né Andersson, was an influential Swedish paleozoologist and founder of the so-called "Stockholm School" of vertebrate paleontology. He later took his new surnam ...

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 Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, 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 Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from ''galea'', the Latin word for ''helmet'', and refers to their massive bone shield on the head. Galeaspida lived in shallo ...

and potentially the Pituriaspida. Also following the latter definition, the class (and its

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

superclass Osteostracomorphi) is assumed to be the

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

sister taxon to Gnathostomata by the 2016 edition of

Fishes of the World ''Fishes of the World'' is a standard reference for the systematics of fishes. It was first written in 1976 by the American ichthyologist Joseph S. Nelson (1937–2011). Now in its fifth edition (2016), the work is a comprehensive overview of t ...

Lampreys

Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class 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 comm ...

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

References

External links

The Tree of Life discusses relations among jawless fish
{{Authority control Paleozoic jawless fish Silurian first appearances Devonian extinctions

Biology and morphology

Classification

Lampreys

References

Further reading

External links