Fig. 9
From: Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system
A model of evolution for the regulatory system involved in odontode morphogenesis in the course of vertebrate diversification. The phylogenetic framework is from [56] and the odontode structures (lateral views) are from [55]. The relative expression domains of Shh and Bmps are indicated in green and orange respectively. The orientation of growth, following the putative Shh signal, is indicated by a green arrow. Odontodes are an ancestral character for vertebrates that first occurred as outer mineralized structures (A). In particular, thelodonts display both radially symmetrical and bilaterally symmetrical scales. A strict coupling of the Shh and Bmp pathways involved in cusp growth is currently described only in mammals (B) but may be proposed as a mechanism for cuspid growth in placoderms