Revealing the transformation routes among existing fullerene isomers is key to understanding the formation mechanism of fullerenes which is still unclear now because of the absence of typical key links. Herein, we have crystallographically identified four new fullerene cages, namely, C₂(27)-C₈₈, C₁(7)-C₈₆, C₂(13)-C₈₄ and C₂(11)-C₈₄, in the form of Eu@C_2n, which are important links to complete a transformation map that contains as many as 98% (176 compounds in total) of the reported metallofullerenes with clear cage structures (C_2n, 2n = 86–74). Importantly, the mutual transformations between the metallofullerene isomers included in the map require only one or two well-established steps (Stone–Wales transformation and/or C₂ insertion/extrusion). Moreover, structural analysis demonstrates that the unique C₂(27)-C₈₈ cage may serve as a key point in the map and is directly transformable from a graphene fragment. Thus, our work provides important insights into the formation mechanism of fullerenes.