Figure 1

The evolution of cranial dermal bones and the developmental mechanisms. (A–C) The skull roof of Eusthenopteron(A), Acanthostega(B), Ichthyostega(C). (D) The developmental mechanisms of the frontal bone in mice. The molecules that initiate dermal differentiation from the ectoderm to the mesenchyme have not been identified yet, though the WNT signaling pathway is suggested to be involved (Goodnough et al., 2014). ano, anocleithrum; esl, lateral extrascapular; esm, medial extrascapular; f, frontal; ip, interparietal; n, nasal; o, opercular; oc, occipital; p, parietal; pf, postfrontal; po, postorbital; pop, preopercular; pot, post-temporal; pp, postparietal; prf, prefrontal; st, supratemporal; sq, squamosal; sclei, supracleithrum; t, tabular. Illustrations in (A–D) are redrawn with permissions from Clack (2012), Jarvik (1980), and White et al. (2016).