New ovulate cupule further informs the relationships among early seed plants and their adaptation to wind pollination.

Abstract

The earliest seed plants occurred in the Late Devonian (Famennian). However, why and how they rapidly diversified remain long-standing mysteries. We investigated the early evolution of seed plants based on a new ovule and evaluated wind pollination performance of the earliest cupulate ovules by using computational fluid dynamics simulations. Zaijunia biloba gen. et sp. nov. is described from the Upper Devonian (Famennian) of South China and shows canonical hydrasperman-type ovules with lobed integuments and a complex nucellar apex. Zaijunia bears ovules in pairs, each ovule within a lateral bilobed cupule. We propose that duplication of the bi-ovulate fertile unit of Zaijunia could produce more complex derivative cupulate ovules, as an evolutionary pathway leading to the early diversification of seeds. We performed computational fluid dynamics simulations of Zaijunia and two other early seed plants (i.e. Pseudosporogonites and Elkinsia), demonstrating that their fertile units improve airborne (pre)pollen capture efficiency. This study sheds additional light on the evolution of cupulate ovules and their potential adaptations to anemophily, as one of the factors driving the earliest radiation of seed plants.