Preparation protocols for SEM visualization of charred fossil plants: the case of Weichselia reticulata pinnule anatomy

Abstract

The cosmopolite Mesozoic fern Weichselia reticulata presents many peculiarities in the anatomy of its stem as well as in the architecture of its leaves. Previous analyses of the ultrastructure of their pinnule laminas provided some insight on xeromorphic traits. In this work new protocols for the preparation of charred compression foliage specimens embedded in a limestone matrix for their observation under SEM are presented, which enhance the observation of different plant tissues. The preparation with HCl allows for the visualization of the whole lamina, from the upper to the lower epidermis, which had not been done to date. Additionally, collodion, latex, or wax coverings of the fossils before dissolving the matrix with HCl enhance the observation of different structures and prevent the fragmentation of the specimen. These techniques have permitted a better description of the vascular tissues, and a redescription of the stomata, which do not have subsidiary cells as previously described. Moreover, the traits observed suggest a closer systematic relationship of Weichselia reticulata with the Marattiales and that this fern lived in areas with fluctuating water availability