Fossil trees lacked trunk rings

The earliest large trees to evolve – around 390 million years ago – had internal structures that looked very different to their distant, modern day descendants, new research shows.

In a paper published in the journal Proceedings of the National Academy of Sciences, a team of researchers from Cardiff University in Wales, Nanjing Institute of Geology and Palaeontology in China, and State University of New York, US, reveal that the trunk of a cladoxlopsid, a group of species critical to the development of the Earth’s forests, looked nothing like that of today’s trees.

The team, led by Hong-He Xu, describe cross-sections of the trunk of a 374-million year old cladoxlopsid fossil found in north-west China.

As every child knows, modern tree trunks in cross-section comprise a series of concentric rings. A new ring grows just beneath the bark every year and is composed of xylem – tissue that transports water from roots to the growing tips.

The regular nature of xylem growth and redundancy is extremely useful to scientists across a range of disciplines, because it forms a calendrical record of changing atmospheric and soil compositions. The study of tree rings is called dendrochronology.

After cutting a cross-section of their cladoxlopsid fossil, however, Xu and his colleagues were faced with a very different arrangement.

Instead of concentric rings, the xylem tissue was dispersed in multiple strands through the outer five centimetres of the truck – appearing as blotches rather than rings.

The strands were interconnected, rather like a complex web of water pipes, allowing nutrients to be efficiently distributed around the whole system.

The distribution of the strands, the researchers suggest, allowed the trees to grow in a robust manner, potentially unrestrained by any mechanically determined maximum size. This would have provided competitive advantage with other ancient trees in the race to reach the canopy of the world’s earliest forests and exploit the sunlight to be found there.

The scientists describe the internal structure of the ancient plant as a “complex tree growth strategy unique in Earth history.”

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