Figure 9

Potential developmental basis that generates morphological diversity in the temporal region of the reptilian skull. All extant reptilian lineages are considered to be derived from ancestor with diapsid skull. In crocodiles that have both upper and lower temporal bars like the stem Diapsida (e.g., Petrolacosaurus), osteogenic mesenchymal precursor cells which express Runx2 and/or Msx2 are distributed at the domain where future temporal bars are formed in the head of early stage embryo (top of the middle column). Through ontogeny (black arrow), these osteoblast precursors may differentiate into the dermatocranial elements including upper and lower temporal bars (bottom of the middle column). Between these bony bars, both upper and lower temporal fenestrae are clearly recognized. In turtles (left white arrow), distribution of osteogenic mesenchymal precursor cells is broadened in a dorsal-ventral direction, filling the whole lateral portion of the head of early embryos (top of the left column). Through ontogeny, these osteoblast precursors may differentiate into the dermatocranial elements roofing the temporal region of the head (bottom of the left column). In snakes that have modified diapsid skull where temporal bars are absent, osteogenic mesenchymal precursor cells do not fill lateral domain of the head of embryo. Rather, these cells are mainly distributed in the vicinity of the brain (top of the right column). Through ontogeny, these osteoblast precursors may differentiate into the dermatocranial elements accompanying the braincase, without forming bony temporal bars (bottom of the right column). A condensed mesenchymal layer that differentiates into the braincase in later stages is highlighted by dotted line in the head of embryos.