Arborescent lycopsid stumps are typically fossilized as casts, so that proximal anatomy is rarely available for study. This presents a roadblock for empirically reconstructing their early ontogeny, which requires knowledge of proximal tissue proportions. Here, we describe nine upright, in situ stump casts of Sigillaria from the earliest Permian Wuda Tuff, Inner Mongolia, China that preserve traces of internal anatomy. From these specimens, we interpret thin vasculature and periderm and a broad primary cortex within c. 50 cm of ground level in the living plant. These specimens support recent arguments that periderm production was limited in arborescent lycopsids and that the primary body reached its mature diameter close to ground level rather than distally further up the trunk.

Supplementary material: Additional figures of specimens and data regarding where measurements were made are available at https://doi.org/10.6084/m9.figshare.c.5558620

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