The Earliest Ascidiate Carpel and Its Implications for Angiosperm Evolution

摘要

According to the classic doctrine of angiosperm evolution, the archetype carpel in angiosperms is of plicate form. However, this concept is facing increasing challenges from the systematics based on molecular data, which now takes ascidiate carpel as ancestral. This reorientation in evolutionary thinking cannot be fully achieved unless there is supporting fossil evidence. Here a fruit, named Liaoningfructus, is reported from the famous Yixian Formation (Early Cretaceous, 125 Ma), which yielded several pioneer angiosperms in the past decades. The configuration of the fruit suggests that the carpel giving rise to the fruit is most likely ascidiate, favoring the above reorientation. There are two seeds in the fruit, and at least one of them is served by a vascular bundle arising from the bottom of the fruit, implying a basal placentation in the former carpel, which again is different from the classic thinking. This discovery not only enhances the diversity of angiosperms in the Yixian Formation, but also accelerates the on-going switching of thinking on angiosperm evolution.