A fossil assigned to this genus was found at Montceau-les-Mines, France, in ironstone concretion deposits of Late Carboniferous (Stephanian) age, about 305–299 million years old. Montceau fossils are generally preserved in such a way that fine details can be observed and three-dimensional analysis is possible. In the case of Idmonarachne, computerized tomography was used to construct a "virtual fossil".