A new specimen of Millerocaulis (Osmundales: Osmundaceae) from the Cerro Negro Formation (Lower Cretaceous), Antarctica

Resumen

A new petrified stem of Millerocaulis Erasmus ex Tidwell emend. Vera, collected in the Aptian Cerro
Negro Formation outcrops of Livingston Island (Antarctica), is described in detail. This specimen is probably
conspecific with Millerocaulis australis (Vera), from the same unit, but it is smaller than the holotype and only
known specimen of the latter species. Furthermore, differences between several characteristics of both specimens
are observed. A discussion based on biological aspects of this group of plants is given, taking into account
the ontogenetic variability. A conservative referral of the new specimen to M. australis is preferred over the
proposal of a new morphotaxon for the inclusion of the new materials.