We study semilinear third‐order (in time) evolution equations with fractional Laplacian
and power nonlinearity
, which was proposed by Bezerra–Carvalho–Santos (J. Evol. Equ. 2022) recently. In this manuscript, we obtain a new critical exponent
for
. Precisely, the global (in time) existence of small data Sobolev solutions is proved for the supercritical case
, and energy solutions blow up in finite time even for small data if
. Furthermore, to more accurately describe the blow‐up time, we derive new and sharp upper bound and lower bound estimates for the lifespan in the subcritical case and the critical case.