In arXiv:1008.3825, Totaro gave examples of a K3 surface such that its automorphism group is not commensurable with an arithmetic group, answering a question of Mazur. We give examples of rational surfaces with the same property. Our examples $Y$ are log Calabi--Yau surfaces, i.e., there is a reduced normal crossing divisor $D \subset Y$ such that $K_{Y} + D = 0$.