Sprecher
Beschreibung
Cycads are the extant gymnosperm clade with the greatest ecological diversity, with species adapted to deserts, mangroves, and wet tropical forests. They display a wide range of growth forms, including subterranean stems, aerial trunks, and even epiphytic habits, as in Zamia pseudoparasitica. Cycads are also exceptional among gymnosperms for their high level of insect pollination. Recent studies suggest that cycad species richness is relatively recent, with most extant clades diversifying during the Mid-Miocene. Interestingly, the ghost lineage represented by the leaves of the Ctenis clade disappeared during the same period. These fossil plants differed markedly from extant cycads by the presence of anastomosing venation. However, the relationship between fossil stems and extant cycads is still unclear. The present study focuses on cycad stem anatomy and related fossil taxa such as Medullosales or Callistophyton.
We analysed the anatomy of fossil stems present in the collections of the Senckenberg Research Institute and Natural History Museum, and performed multiple measurements of the arrangment of the leaf bases, microanatomy, and we constructed 3D models of the specimens for geometric morphometrics. We then revised the literature on fossil cycad stems, finding that several species exhibit unusual anatomical features, including sterome-like tissues, extrafascicular bundles, and vascular cylinders with inverse secondary xylem differentiation. We discuss the impact of the Mid-Miocene transition on extant cycad diversity, particularly within Zamiaceae, in relation to stem anatomy and microanatomy characteristics.