Fruits and seeds of Craigia bronnii (Malvaceae – Tilioideae) and associated flower buds from the late Miocene Inden Formation, Lower Rhine Basin, Germany

Abstract

Abundant, well-preserved fruits of Craigia bronnii (Unger) Z. Kvaček, Bůžek et Manchester in various ontogenic stages have been found together with associated dispersed seeds and flower buds from a fossiliferous layer of late Miocene age within the Inden Formation in the Hambach open-cast lignite mine near Düren, the Lower Rhine Basin. Detailed morphological, anatomical features, along with characteristics of the in situ pollen provide new information on this arboreal element, which was widely distributed in the European Tertiary. The flower buds, previously described as Tilia gieskei Pingen et Gregor, are similar both to Tilia and Craigia, which are closely related extant genera. We interpret them to belong to the same species as the fossil fruits because of co-occurrence, and shared type of pubescence. The pollen is tricolporate with microreticulate, heterobrochate sculpture, thin endexine, and a thick footlayer. We caution that it could be difficult to distinguish Craigia and Tilia pollen without the combination of LM and SEM features and it is likely that many reports of Tilia, and Tilia-like pollen from the Tertiary of Europe may actually correspond to Craigia. Although the fruit morphology of these fossils conforms precisely to that of extant Craigia, this fossil plant differs in some respects (pollen sculpture, position of hilum) from the extant Craigia yunnanensis W.W. Smith et W.E. Evans, a relict of southern China. The associations of taxa, in which C. bronnii often occurs, including Taxodiaceae and hygrophilic broad-leaved deciduous trees and herbs, support the interpretation that it thrived in wetland habitats such as the Inden vegetation. Elsewhere in the European Tertiary, it entered also into the non-swampy parts of mesophytic forests. Hence its ecological tolerance may have been wider than that of its extant relatives.