Shoot apical meristem and initial vascular development of a late Palaeozoic spermatophyte (order Medullosales)

The medullosans are ancient spermatophytes (order Medullosales) with an unusual stem anatomy and have been studied extensively from the late Palaeozoic of North America and Europe. Among the extensive fossil material of Medullosa, a single apex has been discovered from the early Permian Chemnitz Fossil Forest. By setting it in an updated functional and developmental context, we aim to provide new knowledge and understanding on the evolution of the vascular development in ancient seed plants.

We re-examine historical sections of a stem apex of Medullosa stellata Cotta, 1832 emend. Luthardt et al., 2021 with state-of-the-art stereomicroscopic analysis techniques and digital drawings.

The apex exhibits the following unusual combination of features in comparison to modern spermatophytes: (1) an uncommon type of shoot apical meristem with a zone of multiple cell initials only; (2) various coiled leaf primordia completely covering the apex; (3) a flat morphology of the apex indicating considerable primary stem thickening; (4) a primary vascular system of mainly tangentially and horizontally oriented vascular strands; (5) secondary tissues developing centripetally and centrifugally around the primary vascular tissues; and (6) numerous spherical mucilage cavities distributed throughout the sections.

Medullosans exhibited a peculiar stem development not comparable to that of most extant seed plants. Only extant cycads show several specific functional analogies (e.g. a 'stem-girdling' vasculature) that might refer to arborescent growth by a pachycaulous stem. This configuration requires sophisticated hydraulic support at an early developmental stage, including vertical and horizontal transportation of water. Our study demonstrates that the stelar evolutionary path of the medullosans was complex and characterized by various adaptive modifications and transitional stages.