MATERIALS AND METHODS

The specimens are preserved in silicified peat collected from the Skaar Ridge site (Beardmore Glacier area, Queen Alexandra Range) in the central Transantarctic Mountains (84°49'15.8" S, 163°20'18.9" E, 2289 m altitude, Buckley Island Quadrangle, Barrett and Elliot 1973). The site is included in the Buckley Formation of the Beacon Supergroup and is considered Late Permian (~250 Ma) based on palynological data (Farabee et al. 1991). These deposits are thought to have accumulated in a rapidly subsiding foreland basin formed along the ProtoPacific margin of Antarctica as a result of the tectonic activity that affected this region during the Late Permian to the Early Triassic (Taylor et al. 1989). The landscape has been interpreted as one dominated by fluvial deposition, either through means of a meandering river or a braided system traversing a wetland environment, that was increasingly subjected to volcanic activity (Collinson and Isbell 1986; Taylor et al. 1989). Peat accumulated in ponded settings associated with other fluvial deposits, and its petrification is considered to have happened rapidly, favored by the ingression of large loads of volcanic sediments into the basin (Taylor et al. 1989). The presence of organically connected plant organs and in some cases the fine preservation of anatomical details (i.e., the fungus described here) supports the rapid deposition and the autochthony of the assemblage.

The majority of the plant fragments observed show signs of fungal infection, in particular most plants showed the presence of at least a few encysted zoospores. Infected are remains of different plant organs of the dominant Glossopteridales including leaves, stems, and roots (called Vertebraria when not found organically connected to the main plant), probable fragments of the fern Skaaripteris, as well as other unidentifiable plant fragments (see e.g., Pigg 1990; Pigg and Taylor 1993; Galtier and Taylor 1994). However, zoospores usually occur either as a group of a few individuals or singly, only possibly representing encystment on the available substrate at that moment and not necessarily encystment on a specific host. Additionally, based on the morphology of the zoospores alone, it is not possible to determine unequivocally whether the same or a different fungus than the one described here is responsible for the infection of the plant tissue. Zoospores of extant endoparasites are known to swarm around for some time searching for a potential substrate after which they lose motility and encyst on any substrate available at that moment, although the infection only proceeds when encystment occurs on the preferred host(s) (Karling 1964; Alexopoulos et al. 1996; Carlile et al. 2001; Webster and Weber 2007). Thin- and thick-walled sporangia as well as zoospores in different states of development are most common and abundant in highly distorted oblique and cross sections of roots, leaves, and stems that cannot be identified and assigned to any of the fossil plants known from the Permian assemblage because of their advanced state of decay. Additionally, in some of these plant remains it is possible to discern hypertrophied cells. The description and reconstruction of the life cycle of the fungus described here is made based on these highly distorted plants, which clearly indicate that they were at least one of the hosts that the fungus preferentially infected.

The silicified peat was studied using the acetate peel technique, where individual peels that represent sections of approximately 75 µm thick of individual peat boulders were obtained (Galtier and Phillips 1999). Peels of several rocks of silicified peat were surveyed and pieces of those containing the plant fragments infected with the fungus described here were mounted on microscopic slides, observed, and photographed in transmitted light. Selected peel specimens 11665 C-top, 11653 C-top, 11654 G-bot, 11654 F-top, 11657 E-top, and 11657 D-bot and slides with accession numbers 21639, and 21643-21648 obtained from these peels are housed in the Division of Paleobotany, Natural History Museum and Biodiversity Research Center at the University of Kansas.