Fossil fruits and pollen grains of Trapa from the Upper Pliocene of the Sanying Formation (Yunnan, China)

Abstract

Trapa (water chestnut or water caltrop) is a genus of annual free-floating plants mostly native to Africa and Eurasia and included in the family Lythraceae. The genus only contains 10 species today but has a large number of species in the Neogene record, mainly based on fruits but also on the occurrence of pollen grains. In China, several species have previously been recorded from the Neogene, but few have a detailed description. Here we describe a new species of fruit from the Pliocene of the Sanying Formation, Yangjie coalmine, western Yunnan, namely Trapa sanyingensis sp. nov., based on the following characters: the size of the fruit, the relatively long upper horns, the well-developed and triangular lower horns, the well-developed tubercles between the lower and upper horns, and a long and wide neck bearing hairs without a corona. In addition, pollen grains belonging to Sporotrapoidites cf. weiheensis were uncovered from the same sediment and have the following characters: tricolpate, obtuse triangular shape in polar view, and three meridional crests covering the apertures. T. sanyingensis sp. nov. shows a close morphological similarity to the Miocene species Trapa chengsenii from the Yunnan and to the modern species Trapa natans. The pollen grains also closely resemble those of Trapa natans. The occurrence of T. sanyingensis sp. nov. suggests that the sedimentary site corresponds to open and shallow water with a water temperature above 20 °C during the growing season.