A new species of Gymnostoma (Casuarinaceae) present during the Neogene aridification of Southern Australia

Abstract

The Miocene to Pliocene silcrete sediments at Stuart Creek in South Australia have been reported for about 50 years, but no species have been formally described. One of the most common macrofossils has been reported in varying ways as belonging to the family Casuarinaceae. New techniques allow for detailed investigation and identification of the genus and a new species. The macrofossils represent vegetative branchlets, female infructescences and probable male inflorescences of a single species of Gymnostoma, here named G. stuartii. This is the sixth fossil species of Gymnostoma described and is by far the most recent. This species is distinguished from other described living and fossil species by a combination of the shape and angle of the leaf tip, the shape of the branchlet in cross section, the number of nodes in the female infructescence, the shape of the bracts and the protrusion of the bracteoles in the female infructescence. This new fossil species, together with the other two fossil Gymnostoma species already known from Australia, suggests that Gymnostoma may have been quite diverse in Australia during the Cenozoic, and also that their extinction from southern Australia occurred after a long persistence to near the end of the Cenozoic. This extinction was possibly as part of the general transition of the vegetation from scleromorphic to xeromorphic.