Résumé
Structurally preserved fossils reveal details of the internal organization that can be used to reconstruct the physiology of extinct plants and the evolution of significant functional traits. When looking at well-preserved fossil wood, one important feature that is relatively easy to spot is the presence of tyloses. Tyloses are protoplasmic swellings formed by a parenchyma cell into the lumen of an adjacent conducting cell. They have been reported in a diversity of vascular plants dating back to the Carboniferous. Plants may form tyloses in response to embolism to seal off air-filled conducting cells, but they may also help the plant to attenuate or prevent the spreading of pathogens through the vascular tissues. As a result, tyloses play important roles in the physiology and autecology of vascular plants, and investigating when and how these structures evolved based on the fossil record is critical to understand (1) the evolution of plant defense mechanisms, and (2) the establishment of hydraulic traits, especially strategies to render embolized conducting cells harmless.Here, we report on tyloses in Dameria hueberi, a progymnosperm or gymnosperm from the Tournaisian (lower Mississippian) of Australia. The secondary xylem of the specimens is composed of tracheids and low, uniseriate parenchymatous rays. Pitting consists of one, rarely two, rows of circular pits with a circular aperture. Two of the nine D. hueberi specimens provide evidence of tyloses, which typically occur in tracheids and fill the entire lumen of the cells. Small structures with a circular to irregular shape that also protrude into tracheids here and there are interpreted as early stages in tylosis formation. The trigger for the development of tyloses in D. hueberi remains unknown. There are no clear growth ring boundaries in the wood, suggesting relatively constant growth conditions. There is also no indication of increased fungal activity. Finally, the distribution of the tylose-filled tracheids does not seem to reflect heartwood formation. The discovery of D. hueberi is nevertheless important because this taxon predates the heretofore oldest fossil evidence of tylosis formation in vascular plants by several million years.