| Anaspida | |
|---|---|
 | |
| Cowielepis | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Infraphylum: | Agnatha |
| Superclass: | †Anaspidomorphi |
| Class: | †Anaspida Traquair, 1899 |
| Type species | |
| †Birkenia elegans Traquair 1898 | |
| Orders | |
| Synonyms | |
| |
Anaspida ("shieldless ones") is an extinct group ofjawless fish that existed from the earlySilurian period to the lateDevonian period.[2] They were classically regarded as the ancestors oflampreys,[3] but it is denied in recentphylogenetic analysis,[4] although some analysis show these group would be at least related.[5] Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctivelyhypocercal.
Compared to many otherostracoderms, such as theHeterostraci andOsteostraci, anaspids did not possess a bony shield or armor, hence their name. The anaspid head and body are instead covered in an array of small, weakly mineralized scales, with a row of massive scutes running down the back, and, at least confirmed among thebirkeniids, the body was covered in rows of tile-like scales made of aspidine, an acellular bony tissue.[6] Anaspids all had prominent, laterally placed eyes with nosclerotic ring, with thegills opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The majorsynapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.[1]
Now thatJamoytius and its close cohorts, i.e.,Euphanerops, have been moved toJamoytiiformes,[7]Class Anaspida now consists of twoorders, themonogeneric Lasaniida, which contains the genusLasanius and represents a basal anaspid group,[8] andBirkeniida, which contains all other recognized anaspid taxa.[9] Birkeniida is further divided into several families, including Birkeniidae, Pterygolepididae, Rhyncholepididae and Pharyngolepididae, which contain those taxa known from whole body fossils (in addition to several taxa known only from scales) and the family Septentrioniidae, whose subtaxa are known exclusively from scales.[9] Two recently described genera,Kerreralepis[8] andCowielepis,[10] are considered to be Birkeniidaincertae sedis.
Some recent studies have suggested that anaspids are stem-cyclostomes, more closely related tohagfish andlampreys than tojawed fish.[5]
A newer taxonomy based on the work of Mikko's Phylogeny Archive,[11] Nelson, Grande and Wilson 2016[12] and van der Laan 2018.[13]
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