Studying the dynamics of urban form means questioning the processes of evolution of the form in general. The discussion on the architecture of buildings and urban spaces has drawn the concept of adaptation from the theories of natural evolution. These notes propose a reflection on the opposite concept of exaptation proposed by the biologist and paleontologist Stephen Jay Gould in 1982. Through some examples (some different transformations of the Roman amphitheaters of the imperial age and the metamorphoses that occurred to the Chinese urban fabrics, originally made by courtyard houses, in the contemporary city), it is possible to extend to urban form the idea of casual co-optation for new uses of organs and anatomical parts developed for other reasons. This kind of reflections opens up innovative considerations on the potential of transitional urban analysis and its repercussions on evolutive urban transformation processes.