New insights into the anatomy, development, and affinities of corystosperm trees from the Triassic of Antarctica

• We describe the anatomy of corystosperm trunks of Triassic age from Antarctica.
• The specimens provide new information on secondary growth in corystosperm.
• They are assigned to Kykloxylon, a genus based on young stems with attached leaves.
• Kykloxylon and Jeffersonioxylon from the in situ Gordon Valley forest are congeneric.
• The study supports the existence of 2 morphological types of Dicroidium-bearing stems.

Anatomically preserved trunks and young stems of corystosperm seed ferns are described from the Triassic of Fremouw Peak, Beardmore Glacier area, Antarctica. Based on characters of the primary and secondary vascular system, these new specimens are assigned to Kykloxylon, a genus that was established based on young stems with attached Dicroidium leaf bases. The largest specimens illustrate how some secondary growth characters, such as unequal cambial activity, appeared during later development, which enables a better comparison of Kykloxylon with trunks assigned to other corystosperm genera. Jeffersonioxylon from the Gordon Valley, Antarctica, and Cuneumxylon from South America show strong similarities with the newly described larger Kykloxylon trunks from Fremouw Peak, and might be considered congeneric. Our results provide further support for the presence of two anatomically and morphologically distinct kinds of Dicroidium-bearing trees in the Triassic vegetation of Gondwana, one with a palm-like habit and Rhexoxylon stems and the other with a more Ginkgo-like habit and Kykloxylon/Cuneumxylon-type stems.