The morphology and development of flowers and pseudanthia of Calycopeplus paucifolius are described in detail in the context of recent molecular phylogenies of the tribe Euphorbieae and a recent comparative developmental analysis of other taxa within this tribe. Calycopeplus resembles subtribes Neoguillauminiinae and Anthosteminae in some respects (dichasial formation of male flowers within male partial inflorescences, late formation of a constriction in male and female flowers and early formation of a female perianth), but resembles Dichostemma (subtribe Anthosteminae) in possessing only four male partial inflorescences. Calycopeplus and all other Euphorbieae possess only three carpels, except Dichostemma, which has four carpels per female flower. The studied species differs from the closely related Neoguillauminia cleopatra (subtribe Neoguillauminiinae) in that only four nectaries are formed, situated on the rim of the cuplike involucre (in Neoguillauminia 8–10 nectaries arise directly from the base of the pseudanthium). In contrast to all other studied Euphorbieae with trimerous gynoecia, the unpaired carpel of C. paucifolius is oriented in an upper/adaxial position (it lies in the lower/abaxial position in all other studied taxa). On the basis of these results we discuss possible pathways of cyathium evolution and the role of the cyathium as a possible key innovation within Euphorbieae.

‘Calycopeplus is as perfect an example of a connecting link as a morphologist may wish for.’ (Croizat 1937, p. 404)