Agnatha
Encyclopedia
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes.
The agnathans as a whole are paraphyletic, because most extinct agnathans belong to the stem group of gnathostomes. Recent molecular data, both from rRNA and from mtDNA strongly supports the theory that living agnathans, known as cyclostome
s, are monophyletic.
The oldest fossil agnathans appeared in the Cambrian
, and two groups still survive today: the lamprey
s and the hagfish
, with about 100 species
in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata
was created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fin
s; the presence of a notochord
both in larvae and adults; and seven or more paired gill
pouches. There is a light sensitive pineal eye (homologous to the pineal gland
in mammal
s). All living and most extinct Agnatha do not have an identifiable stomach
or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic or cold blooded, with a cartilaginous
skeleton
, and the heart
contains 2 chambers.
While a few scientists still regard the living agnaths as only superficially similar, and argue that many of these similarities are probably shared basal
characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.
. The skin of hagfish
has copious slime glands, the slime constituting their defence mechanism, the slime can sometimes clog up enemy fish's gills, causing them to die. Many extinct agnathans sported heavy dermal armour
or small mineralized scales (see below).
and a caudal fin. Some fossil agnathans, such as osteostracans, did have paired fins, a trait inherited in their jawed descendants
.
. Two types of Early Cambrian
animal apparently having fins, vertebrate
musculature, and gills are known from the early Cambrian Maotianshan shales
of China
: Haikouichthys
and Myllokunmingia
. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella
. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale
of British Columbia
.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the Ostracoderm
s, precursors to the bony fish and hence to the tetrapods (including human
s)—are known from the middle Ordovician
, and by the Late Silurian
the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts. Agnathans declined in the Devonian
and never recovered.
The agnathans as a whole are paraphyletic, because most extinct agnathans belong to the stem group of gnathostomes. Recent molecular data, both from rRNA and from mtDNA strongly supports the theory that living agnathans, known as cyclostome
Cyclostome
Cyclostome is a biological term used in a few different senses:* for the taxon Cyclostomata, which comprises the extant jawless fishes: the hagfish and the lampreys . This was thought for a time to be a paraphyletic group and this usage of the term was deprecated by some...
s, are monophyletic.
The oldest fossil agnathans appeared in the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, and two groups still survive today: 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 and the hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...
, with about 100 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata
Craniata
Craniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives...
was created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fin
Fin
A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...
s; the presence of a notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
both in larvae and adults; and seven or more paired gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
pouches. There is a light sensitive pineal eye (homologous to the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...
in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s). All living and most extinct Agnatha do not have an identifiable stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic or cold blooded, with a cartilaginous
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
, and the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
contains 2 chambers.
While a few scientists still regard the living agnaths as only superficially similar, and argue that many of these similarities are probably shared basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.
Metabolism
Agnathans are ectothermic, meaning they do not regulate their own body temperature. Agnathan metabolism is slow in cold water, and therefore they do not have to eat very much. They have no distinct stomach, but rather a long gut, more or less homogenous throughout its length. Lampreys are parasitic, feeding off other fish and mammals. They rely on a row of sharp teeth to shred their host. Fluids preventing clotting are injected into the host, causing the host to yield more blood. Hagfish are decomposers, eating mostly dead animals. They also use a sharp set of teeth to break down the animal. The fact that all Agnathan teeth are not able to move up and down limit their possible food types.Body covering
The only modern Agnathan body covering is skin, with neither dermal or epidermal scalesScale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
. The skin of hagfish
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...
has copious slime glands, the slime constituting their defence mechanism, the slime can sometimes clog up enemy fish's gills, causing them to die. Many extinct agnathans sported heavy dermal armour
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
or small mineralized scales (see below).
Appendages
Most agnathans, including all those living today have no paired appendages, although they do have a tailTail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
and a caudal fin. Some fossil agnathans, such as osteostracans, did have paired fins, a trait inherited in their jawed descendants
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...
.
Reproduction
Fertilization in lampreys is external. Mode of fertilization in hagfishes is not known. Development in both groups probably is external. There is no known parental care. Not much is known about the hagfish reproductive process. It is believed that hagfish only have 30 eggs over a lifetime. Most species are hermaphrodites. There is very little of the larval stage that characterizes the lamprey. Lampreys can only reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. Lampreys reproduce in freshwater river beds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment. The resulting hatchlings go through four years of larval development before becoming adults. They also have a certain unusual form of reproduction.Fossil agnathans
Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early PaleozoicPaleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
. Two types of Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
animal apparently having fins, vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
musculature, and gills are known from the early Cambrian Maotianshan shales
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
of China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
: Haikouichthys
Haikouichthys
Haikouichthys is an extinct genus of craniate believed to have lived c. 530 million years ago, during the Cambrian explosion of multicellular life...
and Myllokunmingia
Myllokunmingia
Myllokunmingia is a chordate from the Lower Cambrian Maotianshan shales of China, thought to be a vertebrate, although this is not conclusively proven. It is 28 mm long and 6 mm high....
. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella
Haikouella
Haikouella is a probable chordate from the Lower Cambrian Maotianshan shales of Chengjiang County in Yunnan Province, China.It is similar to the form Yunnanozoon, which is possibly a hemichordate. Still, there are anatomical differences from Yunnanozoon, including a larger stomach and smaller ...
. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the Ostracoderm
Ostracoderm
Ostracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe...
s, precursors to the bony fish and hence to the tetrapods (including human
Human
Humans are the only living species in the Homo genus...
s)—are known from the middle Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, and by the Late 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...
the agnathans had reached the high point of their evolution. Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts. Agnathans declined in the 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...
and never recovered.
Groups
- CyclostomesCyclostomataCyclostomata is a group of chordates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have round mouths that lack jaws but have retractable horny teeth...
- Myxini (hagfish)
- HyperoartiaHyperoartiaHyperoartia 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...
(Petromyzontida)- Petromyzontidae (lampreyLampreyLampreys 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)
- Petromyzontidae (lamprey
- OstracodermOstracodermOstracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe...
s- †PteraspidomorphiPteraspidomorphiPteraspida is an extinct class of early jawless fish. The fossils show extensive shielding of the head. Some species may have lived in fresh water.The taxon contains the subgruops Heterostraci, Astraspida, Arandaspida....
- †ThelodontiThelodontiThelodonts are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armor....
- †AnaspidaAnaspidaThe Anaspida are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked scales and paired fins, but have a striking highly hypocercal tail...
- CephalaspidomorphiCephalaspidomorphiCephalaspidomorphs 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 lampreys because lampreys were once thought to be related to cephalaspids...
- †GaleaspidaGaleaspidaGaleaspida 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...
- †PituriaspidaPituriaspidaThe 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...
- †OsteostraciOsteostraciThe 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....
- †Galeaspida
- †Pteraspidomorphi
See also
- GnathostomataGnathostomataGnathostomata 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...
- AmphirhinaAmphirhinaAmphirhina are animals, a phylogenetic classification within the subphylum vertebrata. They are more commonly known as the Branch Gnathostomata, and are described as having double nasal chambers, or nostrils, and jaws. The parallel branch in this naming system is Monorhina , which possess a single...
, an alternate name for the above parallel, or sister, classification