The viscoelastic properties of supercooled AsxSe100-x and GexSe100-x (0 ≤ x ≤ 30) liquids are studied using oscillatory parallel plate rheometry. The liquids with average selenium chain segment length L longer than ∼3 to 5 atoms or average coordination number ⟨r⟩ less than ∼2.2 are characterized by the coexistence of a low-frequency bond scission/renewal based relaxation process as well as high-frequency segmental chain dynamics. The latter process disappears for liquids with higher connectivity, thus implying a dynamical rigidity transition. The temporal decoupling of the high-frequency chain mode from that of the bond scission/renewal process and the shear modulus Gs associated with the low-frequency mode are shown to be unique functions of L or ⟨r⟩ and display strong similarity with the corresponding variation in the fragility m and the conformational entropy of the chain segments. When taken together, these results provide direct experimental support to the entropic rigidity argument originally proposed by Phillips but suggest a floppy-to-rigid transition of the structural network at ⟨r⟩ ∼ 2.2, instead of the conventional rigidity percolation threshold value of 2.4.