The systematic and palaeoecological value of foliage anatomy in Late Palaeozoic medullosalean seed-plants

This paper reviews the anatomical evidence of medullosalean foliage known from cuticles and coal balls, and discusses what it tells us about the systematics and palaeoecology of the parent plants. Medullosalean foliage shows more diversity in epidermal anatomy than in any other group of gymnospermous seed-plants, especially in the configuration of subsidiary cells around the stomata, the detailed structure of the hairs, and the shapes of the cells. Diagnostic for the order are the fronds typically having dorsiventral pinnules supported by rachises with a dissected stele, resin canals in both rachises and pinnules, and distinctive epidermal hairs consisting of a single row of many cells. Foliage anatomy is of little value for distinguishing medullosalean families but is useful for distinguishing genera and species. Anatomy also provides insights into the autecology of the plants. A number of features previously regarded as xeromorphic are re-interpreted as adaptations to high-humidity conditions (stomata sunken or protected by papillae, papillae on lower surface of leaf). Different species could grow in either shaded or well-illuminated positions, indicated by the absence or presence of a hypodermis below the upper epidermis. Few species show evidence of defensive strategies against herbivory except possibly in juvenile fronds. Evidence from stomatal distribution and structure for determining variation in atmospheric CO2 is equivocal but may indicate a rise in CO2 levels in early Stephanian (latest Moscovian) times.