Among the extant vertebrates, only the bony fishes (Osteichthyes) possess bone
(a) Why, then, do we make the assumption that the ancestors of cartilaginous fishes must have had bone that was subsequently lost? (b) Why don't we think this is true for the lampreys and hagfishes?
a. We can deduce the presence of bone in ancestors of the Chondrichthyes from vertebrate phylogeny when the fossils are included. In particular, the ostracoderms are less derived than any gnathostome, but they do possess dermal bone (and some perichondral bone).
b. Hagfishes and lampreys are phylogenetically less derived than ostracoderms. Thus the absence of hard tissues in these lineages is assumed to be primary rather than secondary.
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