Reproductive Organs and In Situ Spores of Asteroxylon mackiei Kidston & Lang, the Most Complex Plant from the Lower Devonian Rhynie Chert

Asteroxylon mackiei is the largest and most complex vascular land plant from the Lower Devonian Rhynie Chert. Fertile remains were first reported nearly 40 years after the description of the sterile axes but were not described in greater detail. In this contribution, we document several fertile axes from longitudinal and transverse thin sections. Sporangia, consisting of two kidney-shaped valves, are interspersed among the nonvascularized leaflike appendages and attached to the axis with a short pedicel. The sporangia are curved and lie close to the axis. Sporangia possess a dome-shaped central structure that forms the continuation of the short stalk into the sporangial cavity. The in situ spores are identified as Retusotriletes cf. triangulatus. The sporangia were apparently abscised after sporulation. Fertile zones with leaflike appendages and sporangia alternate with sterile zones with only leaflike appendages within an axis, suggesting a periodicity of growth.