Résumé
During the Triassic, the global climate was significantly warmer than today and forests were growing in high-latitude regions that are now devoid of tree cover. In what is now Antarctica, paleobotanical and sedimentological data indicate the presence of a diverse vegetation growing under mild climatic conditions deep in South polar latitudes. In recent years, geological exploration in the far north of Victoria Land has led to new discoveries of Middle to J.ate Triassic fossil plants in the Helliwell Hills, in the lower Rennick Glacier area (Bomfleur et al., 2021). This talk focuses on an unusual gymnosperm axis that cooccurred with typical Kyk/oxylon sp. (corystosperms) and Agathoxylon sp. (conifers) wood in the Helliwell Formation.The specimen is a 2-cm-long fragment about 6 x 6.5 cm in diameter. It is decorticated and its external surface shows numerous scars of small shoots. In transverse section, it has a small oval pith (2.7 x 0.9 mm), surrounded by 3-3.5 cm of wood with uniseriate rays, and conspicuous, nearly horizontal vascular traces. In the best-preserved side of the stem the wood shows over 120 growth rings. There is a clear change between the 30 or so inner rings, which are 0.5-0.6 mm in thickness, and the following ones, which only contain a few cells and are typically less than 0.2 mm thick.We compare the specimen with Kykloxylon and Agathoxylon, as well as with Woodworthia, a morphogenus commonly used for Permian-Triassic woods that show numerous small branch scars. We suggest that this unusual specimen represents the trunk of a dwarf gymnosperm tree that grew in a generally favorable regional climate but was subjected to intense local stress