Biogeographic and evolutionary history of Crotonoideae based on pollen evidence from Indian Late Cretaceous and Paleogene sediments

Euphorbiaceae Juss. is a globally distributed angiosperm family with considerable diversity. It is divided into five subfamilies, each of which possesses pollen of distinct morphology. The subfamily Crotonoideae, occurring in evergreen, moist deciduous as well as dry deciduous forests of the world, shows a disjunct distribution pattern in Gondwana and Southeast Asia. This subfamily underwent multiple speciation and extinction events during the northward voyage of the Indian Plate during the Late Cretaceous-early Paleogene. We studied Crotonoideae fossil pollen from the Late Cretaceous Deccan Intertrappeans (Central India) and early Paleogene sediments from the western Indian lignite mines, and as a result, report 10 pollen fossils showing similarity with the extant genera Blachia, Croton, Endospermum, Jatropha, Klaineanthus, and Tetrorchidium, representing 4 tribes of Crotonoideae viz. Adenoclineae, Codiaeae, Crotoneae, and Jatropheae. Fossil evidences from the present study suggest a Gondwanan origin for Crotonoideae. Based on the extant and fossil species, Africa could be the place of origin for most of the Crotonoideae lineages that later dispersed to India via the Kohistan-Ladakh Island Arc during the late Maastrichtian-Paleocene time. The former distribution of Crotonoideae in mid-high latitudinal regions is due to the dispersal of the subfamily from Africa to mid latitudinal regions of northern hemisphere during the early Paleogene via the boreotropical dispersal route. This study also corroborates the "Out of India" hypothesis for the dispersal of Crotonoideae from India to Southeast Asia. The present deep-time fossil records from India discussed in this study substantiate the evolutionary and biogeographic history of subfamily Crotonoideae.