A eudicot from the Early Cretaceous of China

摘要

The current molecular systematics of angiosperms~1 recognizes the basal angiosperms and five major angiosperm lineages: the Chloranthaceae, the magnoliids, the monocots, Ceratophyllum and the eudicots, which consist of the basal eudicots and the core eudicots~2. The eudicots form the majority of the angiosperms in the world today. The flowering plants are of exceptional evolutionary interest because of their diversity of over 250,000 species and their abundance as the dominant vegetation in most terrestrial ecosystems, but little is known of their very early history. In this report we document an early presence of eudicots during the Early Cretaceous Period. Diagnostic characters of the eudicot fossil Leefructus gen. nov. include simple and deeply trilobate leaves clustered at the nodes in threes or fours, basal palinactinodromous primary venation, pinnate secondary venation, and a long axillary reproductive axis terminating in a flattened receptacle bearing five long, narrow pseudo-syncarpous carpels. These morphological characters suggest that its affinities are with the Ranunculaceae, a basal eudicot family. The fossil co-occurs with Archaefructus sinensis~3 and Hyrcantha decussata~4 whereas Archaefructus liaoningensis~5 comes from more ancient sediments. Multiple radiometric dates of the Lower Cretaceous Yixian Formation place the bed yielding this fossil at 122.6-125.8 million years old~(6-8). The earliest fossil records of eudicots are 127 to 125 million years old, on the basis of pollen~(9,10). Thus, Leefructus gen. nov. suggests that the basal eudicots were already present and diverse by the latest Barremian and earliest Aptian.