A New Pinnate Cycad Leaf from the Upper Triassic Chinle Formation of Arizona

Abstract

A recently discovered Upper Triassic leaf shows many of the characters of the leaves of the living cycads and is assigned to the Cycadales. The leaf, however, does not match any previously described fossil or living cycad leaf in all features and is assigned to a new genus and species, Aricycas paulae. The leaf bears oppositely arranged linear-lanceolate pinnae with pointed apices and contracted bases Pinnae contain prominent midribs and anastomosing lateral veins which end freely near the margins. The ordinary epidermal cells are rectangular with straight to slightly curving side walls and haplocheilic stomata. Pinnae in this leaf are apparently wrinkled and shriveled and in some cases fell before the parent leaves were shed, as is typical of the living cycads. It appears that this character has never been reported in fossil cycad leaves. Since A. paulae is Late Carnian age, it is one of the oldest known cycad leaves and thus supports the suggestion by Delevoryas (1982) that there is as much evidence that plants with compound leaves could have given rise to the Cycadales as plants with entire leaves have.