The stensioellids and the pseudopetalichthyids are only known from a few poorly preserved specimens that were classified as placoderms because they appear to have had dermal plates covering both their skull-roof and shoulder girdle. However so far no trace of their median dorsal plates have been found and they may not in fact belong in the Class Placodermi. The stensioellids, at least, may in fact be early holocephalans akin to the 'bradyodonts'.
The 'bradyodont' holocephalan Menaspis, the name for this family of chondrichthyans comes from their supposedly slow tooth replacement.
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