Ordovician-Devonian lichen canopies before evolution of woody trees

Gregory J. Retallack

Summarized by Saraiyh Newton, an undergraduate geology student at University of South Florida.

What data were used? Nematophyte fossil data was collected from the Silurian-age Bloomsburg and Ordovician-age Juniata formation (in Pennsylvania and Tennessee, USA respectively); ancient layers of soil called paleosols, that encases the fossil nematophytes, were also used in this study. Nematophytes are a loosely defined grouping of organisms such as algae and lichen found in the Silurian to the Devonian period that were able to form large canopy- like structures (similar to tall trees in modern forests); in this study, the nematophytes are the lichen canopies the title alludes to. The researcher used various data from modern plants to calculate estimated heights of the fossil lichen canopies. 

Methods: This study illustrates the presence of these nematophytes that created canopies, which has components that could have nurtured early land plants that grew under it, which would eventually allow woody trees to thrive. First, the study identifies where the nematophytes are through various methods, like visual surveys of the paleosols for features such as extensive fungi-like root traces.

After finding the nematophyte remnants, the study author calculated the possible height that the nematophytes might have grown to when they were alive. Plant height is calculated by using an allometric growth equation, which is the relative change in proportion of a part compared to its body, based on 670 modern species of trees. This equation uses the trunk’s diameter at about 1.4 meters above the trunk’s base, or breast height, to calculate the height of the tree. With the calculated height, Retallack calculated the possible density of plants per square meter to create a picture of what forests may have look like with these nematophytes. 

Results: The calculated density and spacing between the trees (which is also found within the paleosols) illustrates a dense cluster of the nematophytes within ancient forests. Since these nematophytes were most likely densely clustered all over forest during the time, these organisms might have nurtured and created an environment where vascular plants, like woody trees, would be able to thrive in the future (later Devonian to present, when trees became extremely widespread). This could have happened because the nematophyte fungi-like roots had an extensive reach in the soil and this fungi network could have nourished environments to the point where plants would have thrived.

Why is this study important? This topic is quite interesting because when we look at modern lichen, they are mostly flat, low growing organisms. They usually do not grow very tall, but they can spread over different surfaces in the forest. So, knowing this about modern lichen species and learning that there was once lichen species that were big enough to have canopies could show how forest ecosystem and species changes over time. 

The big picture: These nematophytes can illustrate how a dominating species in a region can create an environment that allows newer species to thrive in the future. These nematophytes made a good environment for woody trees to thrive in and woody trees eventually became a dominate plant grouping, so we may be able to study how woody trees might be facilitating the evolution of other organisms, too. Learning about how species affect the world around them can improve our knowledge on how ecosystems can change over time. 

Citation: Retallack, G., 2022. Ordovician-Devonian lichen canopies before evolution of woody trees. Gondwana Research, 106, pp.211-223.