Volcaniclastic silcrete floras in eastern Australia provide new fossil evidence of the fern genus Pteridium (bracken).

Andrew C. Rozefelds (Queensland Museum Network).

Pteridium is a cosmopolitan genus occurring in all continents, except Antarctica, and is among the most widespread vascular plant species known; and yet the fossil record world-wide is poor. This is because the fronds disintegrate while they are still on the plant, and the potential for fossilization is therefore limited. At four fossil sites in eastern Australia, however, fern foliage referable to Pteridium is identified. Characters traits useful in identification of Pteridium include vein arrangement, shallowly grooved stipe and costa, recurved margins of the pinnules, free laminal lobes on rachis, large frond size (several times pinnate), and a polycyclic siphonostele rhizome. All sites occur with Oligocene-Miocene volcanism and the ferns are preserved in situ in volcaniclastic sediments (palaeosols) - time capsules of the vegetation growing at each site. Pteridium today is associated with dry Eucalyptus forest, disturbed pastures and occurs along forest edges. Disturbance due to regional volcanism in Australia would have allowed Pteridium and other ferns to colonize these new volcanic terrains and they are therefore interpreted as early successional floras. The character traits in fossil Pteridium indicates closer links to P. esculentum than to P. aquilinum, which suggests that species divergence had occurred earlier than previously suggested.