Analyzing the world's oldest woody plant fossil

Mapping the evolution of life on Earth requires a detailed understanding of the fossil record, and scientists are using synchrotron-based technologies to look back—way, way back—at the cell structure and chemistry of the earliest known woody plant.

A, photograph of Armoricaphyton chateaupannense preserved in 2D as
carbonaceous thin films. B, SEM image of a transverse section of an
axis of a specimen of A. chateaupannense preserved in 3D showing
the radially aligned tracheids [Credit: Canadian Light Source]

Dr. Christine Strullu-Derrien and colleagues used the Canadian Light Source's SM beamline at the University of Saskatchewan to study Armoricaphyton chateaupannense, an extinct woody plant that is about 400 million years old. Their research focused on lignin, an organic compound in the plant tracheids, elongated cells that help transport water and mineral salts. Lignin makes the cells walls rigid and less water permeable, thereby improving the conductivity of their vascular system.

Strullu-Derrien, a scientific associate at the Natural History Museum in London, England and the Natural History Museum in Paris, France, had described A. chateaupannense some years ago and returned to it for this project.

"Studies have been done previously on Devonian plants but they were not woody," she said. "A. chateaupannense is the earliest known woody plant and it's preserved in both 2-D form as flat carbonaceous films and 3-D organo-mineral structures. This allows for comparison to be done between the two types of preservation," she said.

Although the fossils used in the study were collected in the Armorican Massif, a geologically significant region of hills and flatlands in western France, Strullu-Derrien said early Devonian woody plants have also been found in New Brunswick and the Gaspe area in Quebec "although these are 10 million years younger than the French one."

A, part of a reconstructed 3D block of A. chateaupannense preserved in 3D. B, organic structure
of the P-type tracheid cells [Credit: Canadian Light Source]

One of the challenges in this kind of study is that the fossilization process modifies soft plant tissue, which alters or obscures its original biochemical structure and makes it difficult to precisely reconstruct the original chemistry. This study, however, aided by advanced visualization technologies, identified lignified cells in the fossils, suggesting the plant contained decay-resistant lignin compounds.

"Analyses show that both the 2-D and 3-D fossils have the same chemical composition, which is different than modern lignin, but the chemical signal of lignin is not completely lost in the fossilization process," she said. Although the type of preservation of the plant fossils is not unique, "the combination of synchrotron methods used to study the structure and the chemistry of the wood at this level of detail is novel."

Given how ubiquitous and important wood is as a structural component of modern plants, Strullu-Derrien's investigation advances the knowledge around when and how wood first evolved. Yet, questions remain: "Wood first appears in small plants but did it have a different function than it does today in trees, for example?" posed Strullu-Derrien.

To find an answer, she will apply the same techniques used in this study on plants of other geological ages "to follow the evolution of their structure and to be able to find when, or in what condition of preservation, the remaining organic matter has kept a chemical signal of lignin."

"Our study illustrates the capabilities of synchrotrons to investigate the early evolution of tissue systems in plants. It's crucial to have access to these techniques to reach the level of resolution needed for getting chemical signals such as lignin. This represents a developing and promising area for the study of fossils that will complement the morpho-anatomical data and help to interpret the structures," she said.

The researchis published in Palaeontology.

Source: Canadian Light Source [August 28, 2019]