Fig. 1

Schematic representation of the carpel and phylogenetic relationships of seed plants. A The carpel consists of an ovary which produces the ovules, the stigma which receives the pollen and the style which guides the pollen tube to the ovules. One or more carpels can exist per flower and are assembled in the gynoecium. B Relationships within the crown groups of Angiosperms and Gymnosperms and the position of fossil seed plants (in grey writing) are based on Doyle [7]. The evolutionary transition from the precursor of the carpel to the ancestral carpel, marked as “origin of the carpel”, remains difficult to describe and accurately place in time. Two approaches have been prescribed to help pinpoint it: in a “top-down” approach reconstruction of character states in the ancestral carpel is achieved based on character states in extant angiosperms, particularly the early diverging lineages of the ANA grade (this hypothetical ancestral carpel sits somewhere between the origin of the carpel and the root of the crown angiosperms, represented with a black dot); in a “bottom-up” approach character states in the ancestral, or precursor of the carpel are reconstructed based on the closest known relatives of the angiosperms (this hypothetical carpel precursor sits somewhere between the closest known fossil relatives of angiosperms and the time point of the origin of the carpel, represented with a grey dot) [10]. Representative morphologies of female reproductive structures are show above the tree with the different tissues that have been hypothesized as homologs or precursors to the carpel in green. From left to right: conifer cone in longitudinal section, Ginkgo reproductive shoot, Cycas megasporophyll, Glossopteris cupule-bract unit, Caytonia cupules on reproductive axis, ascidiate carpel in longitudinal section, plicate carpel in transverse section