Early eudicot reproductive structure: Fruit and flower morphology of Ranunculaecarpus Samyl. from the Early Cretaceous of eastern Siberia

Abstract Floral and fruit morphology of the early eudicot Ranunculaecarpus quinquecarpellatus Samyl. is described based on details from sectioning and microscopy of the permineralized type material from the Albian Buor-Kemyus Formation of the Zyryanka coal basin. Serial sections confirmed most of the originally described characters but revealed additional information, including hypogynous perianth and several stamens with in situ pollen. Each fruit consists of five free follicles inserted on a short receptacle. Follicles are elongate, with a dorsal keel, ventral suture and an attenuate apex, and are thin-walled, with two rows of small seeds in marginal placentation. The seeds are anatropous, ovoid, 1.3–1.7 in length, with an exotesta of cells that are rounded-hexagonal in surface view. The hypogynous perianth is composed of several free tepals. The stamens are short, with tetrasporangiate, dithecal anthers dehiscing by longitudinal slits. Pollen in situ is 18–20 mm long, 13–15 mm in equatorial diameter, with uncertain aperture configuration and a loose reticulum supported by narrow, widely spaced columellae. The combination of macromorphological characters support possible affinity to extant Ranunculaceae. However, Ranunculaecarpus is distinguished from modern members of the family by the persistence of the perianth in fruit, a smaller number of stamens (ca 10) than is typical, and pollen that is unlike that of any extant genera. Given that there are also similarities with Saxifragales, the systematic affinities of Ranunculaecarpus remain uncertain.