Hemitrapa Miki (Lythraceae) from the earliest Oligocene of southeastern Qinghai-Tibetan Plateau and its phytogeographic implications

Hemitrapa Miki (Lythraceae) is an extinct aquatic genus that shows the close morphological affinity to water chestnut (Trapa), it has rich fossil records ranging from the late Paleocene to early Pliocene of the Northern Hemisphere. Recently, we found numerous Hemitrapa fruit fossils and pollen grains of Hemitrapa from the earliest Oligocene of southeastern Qinghai-Tibetan Plateau, comprising its southmost fossil record worldwide and the earliest fossil record in Asia. We studied the morphology of fruit fossils and pollen grains of Hemitrapa, and compared them with previous fossil records. Two pairs of arms and parallel ridges along the main body of the fruit, as well as pollen grains with three distinctive crests and of a smaller size than living Trapa natans, clearly assign these fossils to Hemitrapa. We described here a new species, namely Hemitrapa alpina T. Su et Z.K. Zhou sp. nov. The finding of H. alpina could not only contribute to plant diversity in the geological past of the Qinghai-Tibetan Plateau, but also indicate that Hemitrapa grew on the plateau by the earliest Oligocene. Meanwhile, H. alpina greatly expands the geographic distribution of Hemitrapa during the Paleogene of the Northern Hemisphere, the disappearance of Hemitrapa might be due to dramatic paleoenvironmental changes on the plateau since the Paleogene.