Anatomy, Biology and Ecology of a silicified Claytosmunda (Osmundaceae) Rhizome from Antarctica

Osmundaceae is the only extant family of the Osmundales, the most basal order of leptosporangiate ferns. Claytosmunda beardmorensis from the Upper Triassic of Antarctica, one of the earliest representatives of the crown group Osmundaceae was first described in 1978. However, the unusual growth habit and ecology of the species have never been described in detail and some important anatomical characters were left unknown. We present new information on the species based on a detailed study of a second, better-preserved silicified rhizome collected from the type locality at Fremouw Peak.

Stem and petiole anatomy, growth habit and associated remains were analysed through acetate peels of serial section surfaces. A digital three-dimensional model showing the arrangement and growth trajectory of stems within the specimen was created from vectorized section surfaces using 3d modelling software. A morphological phylogenetic analysis of the crown-group Osmundaceae was conducted with the new anatomical information.

Unknown morphological characters were clarified and inaccurate information emended. The digital 3d model shows numerous branching, semi-erect stems within the rhizome that are arranged in a funnel-shape. The phylogenetic analysis shows that C. beardmorensis occupies a basal position within Claytosmunda and is more closely related to the genus Osmundastrum than previously assumed.

Several autochthonous and allochthonous remains of other organisms were found within the specimen. Represented groups include arthropods, fungi and several different seed plants. Noteworthy are locally abundant sporangial remains with patch-shaped annuli, presumably belonging to the host plant, as well as arbuscular mycorrhizal fungi in the host plant roots.