Agnatha

Agnatha Temporal range: Cambrian Stage 3–Present, [1] PreꞒ Ꞓ O S D C P T J K Pg N
Lampetra fluviatilis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Agnatha Cope, 1889
Groups included
† Myllokunmingiida †Metaspriggina? † Haikouellidae † Conodonta Ostracodermi †Pteraspidomorphi † Thelodontomorphi †Thelodonti [2] †Anaspidomorphi †Cephalaspidomorphi Cyclostomi †Palaeospondylus?
Cladistically included but traditionally excluded taxa
Gnathostomata (from Cephalaspidomorphi)

Agnatha (

In modern agnathans, the body is covered in skin, with neither dermal or epidermal scales. The skin of hagfish has copious slime glands, the slime constituting their defense mechanism. The slime can sometimes clog up enemy fishes' gills, causing them to die. In direct contrast, many extinct agnathans sported extensive exoskeletons composed of either massive, heavy dermal armour or small mineralized scales.

Appendages

Almost all agnathans, including all

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.[14] There is very little of the larval stage that characterizes the lamprey. Lamprey are only able to reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. Lampreys reproduce in freshwater riverbeds, 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.

Evolution

Although a minor element of modern marine

Approximately 500 million years ago, two types of recombinatorial adaptive immune systems (AISs) arose in vertebrates. The jawed vertebrates diversify their repertoire of immunoglobulin domain-based T and B cell antigen receptors mainly through the rearrangement of V(D)J gene segments and somatic hypermutation, but none of the fundamental AIS recognition elements in jawed vertebrates have been found in jawless vertebrates. Instead, the AIS of jawless vertebrates is based on variable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences.^[19] Three VLR genes (VLRA, VLRB, and VLRC) have been identified in lampreys and hagfish, and are expressed on three distinct lymphocytes lineages. VLRA+ cells and VLRC+ cells are T-cell-like and develop in a thymus-like lympho-epithelial structure, termed thymoids. VLRB+ cells are B-cell-like, develop in hematopoietic organs, and differentiate into "VLRB antibody"-secreting plasma cells.^[20]

Classification

Subgroups of jawless fish

Subgroup		Example	Comments
Cyclostomes	Myxini	hagfish	craniata to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme.[9]
	Hyperoartia	lamprey	monophyletic lineage Jamoytiiformes that may in fact be very close to the ancestral jawed vertebrates .
Myllokunmingiida	† Myllokunmingiidae (extinct)	Haikouichthys	The Zhongjianichthys.[22][23]
Conodonta	† Conodont (extinct)	Panderodus	Conodont elements". It wasn't 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.[24][25][26][27][28][29]
Ostracoderms	†Pteraspidomorphi (extinct)	Astraspis	†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)		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,[31] although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.[32]: 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.[33][34] 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.[35]
	†Anaspida (extinct)		Early Silurian, and flourished until the Late Devonian extinction,[38] where most species, save for lampreys , became extinct due to the environmental upheaval during that time.
	†Cephalaspido- morphi (extinct)	Thyestes	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.[39] Undisputed subgroups traditionally contained with Cephaloaspidomorphi, also called "Monorhina", include the classes Osteostraci, Galeaspida, and Pituriaspida

Groups

Phylogeny based on the work of Mikko Haaramo and Delsuc et al.^[40][41]

While the "agnatha" Conodonta was indeed jawless, if it would have continued to live, its descendants would still be closer related to e.g. humans than to lampreys, and also contempory it was closer related to the ancestor of humans. Due to such considerations, Agnatha can not be consolidated into a coherent grouping without either removing any non-cyclostomata, or by including all vertebrata thus rendering it into a junior synonym of vertebrata.

The new phylogeny from Miyashita et al. (2019) is considered compatible with both morphological and molecular evidence.^[42]

Vertebrata /

†Myllokunmingiidae Agnatha †Anaspidomorphi † Euphanerida †Jamoytiiformes †Anaspida †Endeiolepis †Birkeniiformes † Conodonta †Paraconodontida †Proconodontida † Euconodonta †Protopanderodontida †Panderontida †Prioniodontida †Paracordylodus †Balognathidae †Prioniodinida †Ozarkodinida Cyclostomi Hagfish Lampreys †Pteraspidomorphi †Anatolepis †Arandaspida † Heterostracomorphi †Astraspida †Astraspidiformes †Tesakoviaspidida †Eriptychiida †Heterostraci †Tesseraspidiformes † Cardipeltiformes †Lepidaspidiformes †Corvaspidida †Traquairaspidiformes †Tolypelepidida †Cyathaspidiformes †Pteraspidiformes †Thelodonti †Thelodontiformes †Archipelepidiformes †Furcacaudiformes †Galeaspida †Eugaleaspidiformes †Polybranchiaspidida †Huananaspidiformes †Polybranchiaspidiformes †Pituriaspida †Osteostraci †Ateleaspididae †Cephalaspidida † Zenaspida †Benneviaspidida †Thyestiida Gnathostomes

Craniata

See also

• Gnathostomata
• Amphirhina, an alternate name for the above parallel, or sister, classification
• Cyclostomata

References

Wikispecies has information related to Agnatha.