Background: Although Trapa is a well-defined genus of distinctive freshwater plants with accumulations of extensive morphological and embryological autapomorphies, its phylogenetic relationships have long been unclear. Formerly placed in the monotypic family Trapaceae, Trapa is now recognized as sister to Sonneratia within Lythraceae s.l., although both genera lack morphological synapomorphies. Thus, a split between the two taxa must have occurred in deep evolutionary time, which raises the possibility of finding transitional forms in the fossil record.

Results: Here we describe a new genus and species, Primotrapa weichangensis Y. Li et C.-S. Li (Lythraceae s.l.: Trapoideae), based on three-dimensionally preserved floral cups, fruits, and seeds from the early Miocene of Weichang County, Hebei Province, China. Primotrapa is characterized by a shallow, saucer-shaped floral cup, four distally barbellate sepals, four intersepal appendages alternating with the sepals at the rim of cup, a superior to basally inferior ovary, a fusiform or ovoid, one-seeded fruit with a ribbed surface, and a long persistent peduncle. Two fossil species of Hemitrapa are proposed as new combinations of Primotrapa, namely P. alpina (T. Su et Z.-K. Zhou) Y. Li et C.-S. Li comb. nov. and P. pomelii (Boulay) Y. Li et C.-S. Li comb. nov. Our phylogenetic analysis based on fifteen flower and fruit characters supports the placement of Primotrapa, Hemitrapa and Trapa in a monophyletic clade, which comprise subfamily Trapoideae. The phylogenetic analysis places Primotrapa at the base of Trapoideae.

Conclusions: In view of its superior ovary, which is a plesiomorphic character of Lythraceae s.l., the newly recognized genus Primotrapa and its three species likely represent transitional forms that bridge the evolutionary gap between the basal taxa of Lythraceae s.l., i.e. Lythrum, and the highly derived taxon Trapa.