A multicarpellate fruit from Late Cretaceous sediments of South Bohemia, Czech Republic

The gynoecium of most angiosperms consists of two or more united carpels and syncarpy is generally considered a key innovation. Multicarpellate-syncarpous gynoecia, i.e., with more than five united carpels, are overall rare, probably because of developmental and functional constraints. Based on scanning electron microscopy (SEM) and X-ray computed tomography (microCT), we here describe a new fossil taxon, Covidifructus multicarpellatus, from the Late Cretaceous (Late Turonian-Santonian) of Southern Bohemia (Czech Republic). The new fossil taxon is based on a single syncarpous gynoecium (an immature fruit) consisting of 10 carpels, each containing a single seed; no other floral organs are preserved. The gynoecium morphology of C. multicarpellatus is highly complex and involves an enclosed, undifferentiated floral apex, an empty space in the centre of the ovary formed by the postgenital union of the distal parts of the carpels, an irregular apical closure of the ovary, and possibly an extra-gynoecial compitum. Similar gynoecium morphologies have evolved convergently in various angiosperm lineages and are directly linked with the special developmental (architectural) challenges of forming a functional syncarpous gynoecium with many carpels. Among extant angiosperms, C. multicarpellatus is most similar to Dillenia (Dilleniaceae). However, a systematic assignment based on gynoecium characters alone would involve a high level of uncertainty, even if the gynoecial features of C. multicarpellatus are rare among other angiosperms.