 The allometric exponent for sail height is similar in magnitude to the scaling of interspecific antler length to shoulder height in cervids . Furthermore , as Bakker ( 1970 ) observed in the context of Dimetrodon , many lizard species raise a dorsal ridge of skin during threat and courtship displays , and positively allometric , sexually dimorphic frills and <unk> are present in extant lizards ( Echelle et al . 1978 ; Christian et al . 1995 ) . There is also evidence of sexual dimorphism both in the robustness of the skeleton and in the relative height of the spines of D. limbatus ( Romer and Price 1940 ) . 
