Agnatha

Agnatha, or jawless fish, are a group of animals that belong to the family of vertebrates, which includes animals with backbones. The name "Agnatha" comes from ancient Greek words meaning "without jaws," and these animals are special because they do not have jaws like most fish and other animals do.

This group includes both living animals, such as hagfishes and lampreys, and many extinct animals that lived long ago. Scientists believe that jawed fish, called gnathostomes, evolved from jawless ancestors during a time called the Silurian period, by changing the structure of their gill arches.

Today, there are about 120 species of living jawless fish. Hagfishes are interesting because they lost their vertebrae secondarily, even though they are still considered part of the vertebrates. These ancient creatures first appeared in the fossil record during the Cambrian period, showing just how old and important they are in the history of life on Earth.

Metabolism

Agnathans are ectothermic, which means they do not control their own body temperature. Their metabolism slows down in cold water, so they don't need to eat much. They don't have a distinct stomach but have a long gut that is mostly the same all along.

Lampreys eat dead animals, other fish, and marine mammals, but some do not eat meat. Hagfish mostly eat dead animals, and sometimes they have been seen hunting live ones. They use sharp teeth to break down their food. Since their teeth can't move up and down, the types of food they can eat are limited.

Morphology

Modern agnathans, or jawless fish, lack jaws and paired fins. They have a notochord, which they keep from their larval stage into adulthood, and seven or more paired gill pouches. Lampreys have a light-sensitive pineal eye, similar to the pineal gland in mammals. Most agnathans, both living and extinct, do not have a clear stomach or paired appendages. They reproduce externally without parental care, are ectothermic or cold-blooded, and have a cartilaginous skeleton with a heart that has 2 chambers.

In modern agnathans, the body is covered in skin without scales. Hagfish have many slime glands in their skin, which help protect them by releasing slime that can block the gills of other fish. In contrast, many extinct agnathans had tough outer coverings made of heavy armour or small mineralized scales.

Almost all agnathans, including all living ones, do not have paired appendages, though many have a dorsal or caudal fin. Some ancient agnathans, like osteostracans and pituriaspids, did have paired fins, a feature that later appeared in their jawed descendants.

Reproduction

Lampreys have a special way of having babies. They do this outside their bodies in freshwater riverbeds. They work together in pairs to build a nest and lay their eggs about an inch under the sand. After about four years, the tiny babies grow and become adults.

For hagfishes, scientists don't really know much about how they have babies. It is thought that they might only have about 30 eggs in their whole lives. There doesn't seem to be any care from parents after the eggs are laid.

Evolution

See also: Evolution of fish

Agnathans, or jawless fish, were very important in the early days of fish evolution during the early Paleozoic period. Scientists have found fossils of early creatures with fins, muscles like vertebrates, and gills from the Cambrian period in China. These include Haikouichthys and Myllokunmingia, which might be early agnathans.

Conodonts, a group of agnathans from the early Cambrian, survived until the Triassic period. Their teeth, often the only part that fossilized, are used by scientists to date rocks from that time.

Many agnathans from the Ordovician, Silurian, and Devonian periods had heavy, bony plates on their bodies. These armored agnathans, called ostracoderms, were precursors to bony fish and eventually to tetrapods like humans. By the Late Silurian, agnathans had reached their peak in evolution. However, their numbers dropped in the Devonian and they never returned to their earlier levels.

Around 500 million years ago, early vertebrates developed special systems to fight off diseases. Jawed vertebrates use a certain method involving genes to create these defenses, but jawless vertebrates like lampreys and hagfish use a different system based on variable lymphocyte receptors (VLRs). These VLRs help the immune system recognize and fight off harmful invaders in a unique way.

Classification

Subgroups of jawless fish

Subgroup		Example	Comments
Cyclostomi	Myxini	Eptatretus hexatrema (sixgill hagfish)	Myxini (hagfish) are eel-shaped slime-producing marine animals (occasionally called slime eels). They are the only known living animals that have a skull but not a vertebral column. The group has gone through the most extensive gene loss of all vertebrates, with 1,386 missing gene families. Along with lampreys, hagfish are jawless and are living fossils; hagfish are basal to vertebrates, and living hagfish remain similar to hagfish 300 million years ago. The classification of hagfish has been controversial. The issue is whether the hagfish is itself a degenerate type of vertebrate-fish (most closely related to lampreys), or else may represent a stage which precedes the evolution of the vertebral column (as do lancelets). The original scheme groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving clade of vertebrates alongside gnathostomes (the now-ubiquitous jawed-vertebrates). An alternative scheme proposed that jawed-vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduces the category Craniata to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme.
	Petromyzontida	Entosphenus tridentatus (Pacific lamprey)	Petromyzontida, also called Hyperoartia, is a disputed group of vertebrates that includes the modern lampreys and their fossil relatives. Examples of hyperoartians from early in their fossil record are Endeiolepis and Euphanerops, fish-like animals with hypocercal tails that lived during the Late Devonian Period. Some paleontologists still place these forms among the "ostracoderms" (jawless armored "fishes") of the class Anaspida, but this is increasingly considered an artificial arrangement based on ancestral traits. Placement of this group among the jawless vertebrates is a matter of dispute. While today enough fossil diversity is known to make a close relationship among the "ostracoderms" unlikely, this has muddied the issue of the Hyperoartia's closest relatives. Traditionally, the group was placed in a superclass Cyclostomata together with the Myxini (hagfishes). More recently, it has been proposed that the Myxini are more basal among the skull-bearing chordates, while the Hyperoartia are retained among vertebrates. But even though this may be correct, the lampreys represent one of the oldest divergences of the vertebrate lineage, and whether they are better united with some "ostracoderms" in the Cephalaspidomorphi, or not closer to these than to e.g. to other "ostracoderms" of the Pteraspidomorphi, or even the long-extinct conodonts, is still to be resolved. Even the very existence of the Hyperoartia is disputed, with some analyses favoring a treatment of the "basal Hyperoartia" as a monophyletic lineage Jamoytiiformes that may in fact be very close to the ancestral jawed vertebrates.
Myllokunmingiida	† Myllokunmingiidae (extinct)	Haikouichthys ercaicunensis	The myllokunmingiids were a primitive order of agnathans that were endemic to the Cambrian aged Maotianshan Shales lagerstätte in China. These creatures are the earliest known group of craniates (chordates with a skull of hard bone or cartilage). Currently the group includes 3 known genera, Haikouichthys, Myllokunmingia, and Zhongjianichthys.
Conodonta	† Conodonti and Cavidonti (extinct)	Panderodus unicostatus	Conodonts were eel like agnathans that lived from the Cambrian up until the beginning of the Jurassic period. They were very diverse in terms of lifestyles, with some species being filter feeders and others being macropredators. For over a century, these animals were only known because of their microscopic, phosphatic tooth structures called "conodont elements". It was not until the mid-1980s that body fossils of conodonts were found in Scotland and Wisconsin, showing these animals true appearance. Their teeth make great index fossils, as many species lived and died out in a relatively short period of time. These fish reached their peak in diversity during the middle of the Ordovician, but were hit hard by the Ordovician-Silurian extinction event. They then reached another spike in diversity in the mid-late Devonian before again declining in the Carboniferous. They were relatively rare in the Permian, but dramatically increased in numbers in the early Triassic. Despite this, they went extinct during the lower Jurassic period, with some of the last surviving populations being in Japan. They possibly survived longer there due to the relative remoteness of the area. Originally, it was thought that they were wiped out by the large extinction at the end of the Triassic. Instead, it is now thought that they were out competed by newer Mesozoic taxa.
"Ostracodermi"	†Pteraspidomorphi (extinct)	Astraspis desiderata	†Pteraspidomorphi is an extinct group of early jawless fish. The fossils show extensive shielding of the head. Many had hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water. The taxon contains the subgroups Heterostraci, Astraspida, Arandaspida.
	†Thelodonti (extinct)		Thelodonti (nipple teeth) are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armour. There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish. Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.: 206  "Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time. The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They occupied a large variety of ecological niches, with a large amount of species preferring reef ecosystems, where their flexible bodies were more at ease than the heavily armoured bulks of other jawless fish.
	†Anaspida (extinct)		Anaspida (without shield) is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods. They are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
	†Cephalaspido- morphi (extinct)	Thyestes verrucosus	Cephalaspidomorphi is a broad group of extinct armored agnathans found in Silurian and Devonian strata of North America, Europe, and China, and is named in reference to the osteostracan genus Cephalaspis. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, as lampreys are sometimes thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the early Silurian period through the Mesozoic, and into the present day. Cephalaspidomorphi were, like most contemporary fish, very well armoured. Particularly, the head shield was well developed, protecting the head, gills and the anterior section of the innards. 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 organ. The eyes were rather small and placed on the top of the head. There was no proper jaw. The mouth opening was surrounded by small plates making the lips flexible, but without any ability to bite. Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina", include the classes Osteostraci, Galeaspida, and Pituriaspida.

Phylogeny

See also: Placodermi § Cladogram

The study of how jawless fish are related to other animals shows interesting patterns. Even though some jawless fish, called Conodonta, lived long ago, scientists think if they had survived, they would be closer related to humans than to lampreys. This makes it tricky to group all jawless fish together without changing what we mean by the whole group of animals with backbones. Recent research from 2019 helps explain these relationships better, matching both physical traits and genetic evidence.