| Summary: | A new species of the osmundalean fossil morphogenus Millerocaulis Tidwell emend. Vera, Millerocaulis tekelili sp. nov. is defined, based on several permineralized stems recovered from exposures of the Lower Cretaceous Cerro Negro Formation on Livingston Island, South Shetland Islands, Antarctica. This new species is characterized by the presence of an ectophloic-dictyoxylic siphonostele, inner parenchymatic and outer sclerotic cortices, heterogeneous sclerotic ring in the petiole bases, absence of sclerenchyma associated with the petiolar xylem trace, petiolar inner cortex with sclerenchyma strands and stipular wings having a large sclerenchyma bundle and several smaller ones. The presence of non-homogeneous sclerotic rings in the petiole bases allows this new species to be clearly distinguished from other Antarctic Millerocaulis, and suggests that it may represent an intermediate form in the evolutionary lineage leading from Millerocaulis to subgenus Claytosmunda of Osmunda. © 2012 Copyright Taylor and Francis Group, LLC. Fil:Vera, E.I. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
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