Amniote skulls display diverse architectural patterns including remarkable variations
in the number of temporal arches surrounding the upper and lower temporal fenestrae.
However, the cellular and molecular basis underlying this diversification remains
elusive. Turtles are a useful model to understand skull diversity due to the presence
of secondarily closed temporal fenestrae and different extents of temporal emarginations
(marginal reduction of dermal bones). Here, we analyzed embryos of three turtle species
with varying degrees of temporal emargination and identified shared widespread coexpression
of upstream osteogenic genes Msx2 and Runx2 and species-specific expression of more
downstream osteogenic genes Sp7 and Sparc in the head. Further analysis of representative
amniote embryos revealed differential expression patterns of osteogenic genes in the
temporal region, suggesting that the spatiotemporal regulation of Msx2, Runx2, and
Sp7 distinguishes the temporal skull morphology among amniotes. Moreover, the presence
of Msx2- and/or Runx2-positive temporal mesenchyme with osteogenic potential may have
contributed to their extremely diverse cranial morphology in reptiles.
