Cycads from the Triassic of Antarctica: Permineralized Cycad Leaves

Abstract

Permineralized cycad petioles and/or rachides with associated pinnae are described from two Triassic localities in the Queen Alexandria Range, central Transantarctic Mountains, Antarctica. Petiole‐rachides display an inverted‐omega‐shaped arrangement of vascular bundles typical of most genera of extant Cycadales and exarch primary xylem that link them to the modern order. Pinnae associated with the Antarctic petiole‐rachides are thin, with regularly spaced vascular bundles. They are similar to those of extant Zamia and most other genera of extant Cycadales, whose pinnae lack midribs. Other Mesozoic fossil cycads (e.g., Charmorgia, Lyssoxylon, Lioxylon) have endarch petiole vascular bundles that in some cases were previously considered more similar to those of Bennettitales than those of Cycadales. We suggest, however, that the endarch xylem of these taxa is typical of Cycadales because in extant cycads, the protoxylem changes from endarch to exarch within the base of the petiole. Evolution of cycad leaf form is reviewed based on evidence from the fossil record.