Anaspida

Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period. They were classically regarded as the ancestors of lampreys. Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.
Anatomy
Compared to other prehistoric agnathan groups, such as the Heterostraci and Osteostraci, 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 the birkeniids, the body was covered in rows of tile-like scales made of aspidine, an acellular bony tissue. Anaspids all had prominent, laterally placed eyes with no sclerotic ring, with the gills opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The major synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.
Taxonomy
Now that Jamoytius and its close cohorts, i.e., Euphanerops, have been moved to Jamoytiiformes, Class Anaspida now consists of two orders, the monogeneric Lasaniida, which contains the genus Lasanius and represents a basal anaspid group, and Birkeniida, which contains all other recognized anaspid taxa. 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. Two recently described genera, Kerreralepis and Cowielepis, are considered to be Birkeniida incertae sedis.
Some authors have suggested that anaspids are stem-cyclostomes, related to hagfish and lampreys.
A newer taxonomy based on the work of Mikko's Phylogeny Archive, Nelson, Grande and Wilson 2016 and van der Laan 2018.
Class †Anaspida Janvier 1996 non Williston 1917
Order †Endeiolepidiformes Berg 1940
Family †Endeiolepididae Stensiö 1939 corrig.
Genus †Endeiolepis Stensiö 1939
Order †Birkeniiformes Berg 1940
Genus †Cowielepis Blom 2008
Genus †Hoburgilepis Blom, Märss & Miller 2002
Genus †Kerreralepis Blom 2012
Genus †Maurylepis Blom, Märss & Miller 2002
Genus †Rytidolepis Pander 1856
Genus †Schidiosteus Pander 1856
Genus †Silmalepis Blom, Märss & Miller 2002
Genus †Vesikulepis Blom, Märss & Miller 2002
Family †Pharyngolepididae Kiær 1924 corrig.
Genus †Pharyngolepis Kiaer 1911
Family †Pterygolepididae Obručhev 1964 corrig.
Genus †Pterygolepis Cossmann 1920 [Pterolepis Kiaer 1911 non Rambur 1838 non De Candolle ex Miquel 1840; Pterolepidops Fowler 1947]
Family †Rhyncholepididae Kiær 1924 corrig.
Genus †Rhyncholepis Kiær 1911 non Miquel 1843 non Nuttall 1841
Family †Tahulalepididae Blom, Märss & Miller 2002
Genus †Tahulalepis Blom, Märss & Miller 2002
Genus †Trimpleylepis Miller, Märss & Blom 2004
Family †Lasaniidae Goodrich 1909
Genus †Lasanius Traquair 1898
Family †Ramsaasalepididae Blom, Märss & Miller 2003
Genus †Ramsaasalepis Blom, Märss & Miller 2003
Family †Birkeniidae Traquair 1899
Genus ?†Vilkitskilepis Märss 2002
Genus †Ctenopleuron Matthew 1907
Genus †Saarolepis Robertson 1945 [Anaspis Robertson 1941 non Geoffroy 1762 non Thomson 1893]
Genus †Birkenia Traquair 1898
Family †Septentrioniidae Blom, Märss & Miller 2002
Genus †Liivilepis Blom, Märss & Miller 2002
Genus †Manbrookia Blom, Märss & Miller 2002
Genus †Ruhnulepis Blom, Märss & Miller 2002
Genus †Spokoinolepis Blom, Märss & Miller 2002
Genus †Septentrionia Blom, Märss & Miller 2002
Notes
External links
Prehistoric fish classes
Paleozoic jawless fish
Llandovery first appearances
Early Devonian extinctions