Leaf Architecture of Extant Species of Rosa L. and the Paleogene Species Rosa lignitum Heer (Rosaceae)

Leaf fossils of Rosa lignitum Heer have often been documented as an accessory element in mixed mesophytic forests from the Oligocene and Miocene of central Europe. Its relationship to extant rose species is not yet firmly understood because leaf morphology contributes only marginally to the current taxonomy of the genus Rosa L. In this study, we investigate which extant rose species have structural leaf characters most similar to those of R. lignitum and whether the distribution and ecology of these extant species fit with the conditions hypothesized for the paleovegetation containing R. lignitum. Therefore, we examined the leaf characters of 32 extant rose species for their diagnostic value in taxonomy and compared these data with characters observed in R. lignitum from five Paleogene floras in central Europe. Most remarkably, we detected semicraspedodromous venation in R. lignitum. This venation pattern was found only in a few species of Rosa distributed in Southeast Asia, whereas the majority of extant rose species developed strictly craspedodromous venation. The distribution of extant species with semicraspedodromous venation in mixed broad-leaved deciduous and evergreen forests in subtropical China clearly corresponds to the hypothesized mixed mesophytic forests in the European paleovegetation. Thus, this study of detailed leaf morphology of Rosa again supports the previous assumption that the zonal vegetation of the central European Oligocene is partly comparable to extant forest types in Southeast Asia.