Anatomically preserved Dicroidium foliage from the central transantarctic mountains

Abstract

The first anatomically preserved species of Dicroidium, D. fremouwensis Piggs sp. nov. is described from the Triassic of the central Transantarctic Mountains, Antarctica. Dicroidium fremouwensis has pinnules that ranges from entire-margined to highly lobed morphologies. Entire-margined pinnules occur basally and pinnules become progressively more dissected at more distal levels of the frond. Dicroidium fremouwensis is most similar to the compression-impression forms D. dubium var. dubium (Feistmantel) Retallack and D. dubium var. tasmaniense (Anderson et Anderson) Retallack. Fronds also bear some resemblance to D. sp cf. D. feistmantelii sensu Lele, D. dubium (Feistmantel) Gothan and D. feistmantelii (Johnston) Gothan. Distal to the frond bifurcation, pinna rachides have a dorsiventral vascular anatomy composed of an abaxial ring of six to eight bundles and an adaxial linear group of five to eight bundles. Bundles of the adaxial line produce lateral strands that vascularize laminar pinnules, while the abaxial bundles remain in a ring that diminishes to three bundles distally. Pinnules have odontopterid venation, differentiated mesophyll, resinous cells and transfusin tracheids. The vascular organization and other anatomical features of D. fremouwensis are similar to those of the fronds of some extant cycads. This information provides a new level of resolution in our knowledge of Dicroidium and documents how permineralized fossils promise to refine our understanding of complex Mesozoic gymnosperms.