The genus Mezoneuron (Caesalpinioideae) of the legume family is distributed throughout the tropical and subtropical Old World, with 23 extant species documented worldwide. It is a typical group for indicating tropical environments. Three species occur in southwest China, but no fossil occurrences have been reported from Asia or any other region of its modern distribution range.

During a field investigations in southern Yunnan, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences found a unique winged fruit fossil from the Lower Miocene Sanhaogou Formation of the Jinggu Basin. After careful morphological studies and comparison with extant species, the researchers confirmed it as a new Mezoneuron pod fossil, and represented the first fossil occurrence of this genus in Asia and the only one within its current distribution range.

The new Mezoneuron pod fossil was named as Mezoneuron zhekunii in honor of Prof. ZHOU Zhekun, a distinguished palaeobotanist who is deeply committed to Cenozoic paleobotanical research in China. Related results were published in Review of Palaeobotany and Palynology.

Mezoneuron zhekunii presents a vascularized wing along the placental suture and with poorly developed, irregular reticulate venation, which are typical pod characters in Mezoneuron. It has a similar morphology to the single-seeded North American fossil pods, including M. claibornensis and M. spokanensis . However, it is much larger and has finer central veins than these two fossil species. It has fine wrinkles on the pod surface, especially noticeable around the seed chamber, a feature never documented in previous fossil records.

The discovery of M. zhekunii suggests that Mezoneuron reached its present distribution by the early Miocene. Mezoneuron appeared in tropical Asia at least in the early Miocene, and its specialized pods help it dispersed by wind in open areas such as tropical forest edges, so it continues to evolve and differentiate in this region.

"Mezoneuron attained relatively low latitudes in Asia no later than the early Miocene, with our fossil representing the lowest latitudinal record," said HUANG Jian of XTBG.

Pod fossil of Mezoneuron zhekunii. (Image by HUANG Jian)

Extant Mezoneuron cucullatum. (Image by HUANG Jian)