PRIMARY TISSUES DOMINATED PROXIMAL TRUNK THICKNESS IN SIGILLARIA: EVIDENCE FROM THE EARLY PERMIAN WUDA TUFF, INNER MONGOLIA

Casts and molds of fossil plants usually do not preserve internal anatomy due to their mode of formation: a plant axis becomes surrounded by sediment and the subsequent decay of that plant leaves a mold; that void is then infilled with additional sediment forming the cast. Arborescent lycopsid stumps are typically preserved as casts, so their proximal anatomy can rarely be seen. This presents a roadblock for reconstructing their early ontogeny, which requires knowledge of proximal tissue proportions. Recent research has suggested minimal secondary tissues (i.e., wood and bark) at the tree base, implying a broad primary body proximally, but the lack of fossils would seem to bar further investigation. Here, we describe 9 upright, in situ stump casts of the arborescent lycopsid Sigillaria from the earliest Permian Wuda Tuff, Inner Mongolia, China that preserve the transitions between different tissue types and traces of tissue anatomy. From these extraordinary specimens, we are able to interpret the proportions of vasculature, primary cortex, and secondary cortex (i.e., periderm, or bark) at the bases of the trees. Overall, the vasculature is thin (7-39% of stem diameter) and bark, only present in the largest of three species, is minimal (0-10% of stem diameter), whereas the primary cortex is universally broad (61-93% of stem diameter). This supports the recent argument that the arborescent lycopsid primary body reached its mature diameter very close to ground level. The Wuda Tuff Flora represents a peat swamp that was entombed by a volcanic ash fall, providing a spectacular view into the catastrophic termination of the ecosystem. The preservation of these lycopsid specimens with both internal tissue transitions and external form offers an important and unique view into the organismal paleobiology of the preserved trees and highlights the potential of the Wuda Tuff and similar late Paleozoic ash-fall floras for coupling plant anatomy and gross morphology.