Anaspida

[5] Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.

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.

[6] 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.

Some recent studies have suggested that anaspids are stem-cyclostomes, more closely related to hagfish and lampreys than to jawed 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.

Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribbon-shaped, ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes . – Philippe Janvier [ 1 ]
Life reconstruction of Lasanius problematicus