  Two residual-type error estimators for the mortar staggered discontinuous
Galerkin discretizations of second order elliptic equations are developed. Both
error estimators are proved to be reliable and efficient. Key to the derivation
of the error estimator in potential $L^2$ error is the duality argument. On the
other hand, an auxiliary function is defined, making it capable of decomposing
the energy error into conforming part and nonconforming part, which can be
combined with the well-known Scott-Zhang local quasi-interpolation operator and
the mortar discrete formulation yields an error estimator in energy error.
Importantly, our analysis for both error estimators does not require any
saturation assumptions which are often needed in the literature. Several
numerical experiments are presented to confirm our proposed theories.
