 Paris models are rigid when their complete theory has a well-founded model

Under the hypothesis of Paris models are well-founded when their complete theory has a well-founded model, a Paris model $M$ is well-founded. Every automorphism of $M$ is then the identity by external epsilon induction: once all members of $x$ are fixed, extensionality gives $\pi (x)=\{\pi (y):y{\in }^{M}x\}=x$. Hence $M$ is a rigid first-order structure.