Silphium certainly seems to have been a member of the (200+ strong) Ferula genus given their physical similarity to the plant depicted on ancient coins from Cyr

Silphium certainly seems to have been a member of the (200+ strong) Ferula genus given their physical similarity to the plant depicted on ancient coins from Cyrene (in modern Libya). However, there seems to be some doubt whether this particular plant, Ferula drudeana, from Turkey, is the same plant or not. The romans, who so valued Silphium, also sometimes used a different plant, Asafoetida as a cheaper alternative, so the properties of Silphium obviously weren't totally unique, and Ferula drudeana may well be very similar if not the exact same plant. The appearance of Silphium on coins does differ a bit from these plants - more fleshy "arms", but maybe that's just artistic interpretation.