The aerial parts of plants arrange their organs around stems, and this arrangement defines their structure. In most extant plant species, organs occur up to 137.5 degrees from the previous organ. This leads to continuous spiral organs, where a number of clockwise and counterclockwise spirals form consecutive numbers in the Fibonacci sequence. Reconstruction of Early Devonian clubmoss Asteroxylon mackieipaleontologists found another leaf arrangement that suggests Fibonacci-style patterns were not the ancestors of living land plants.

The spirally arranged leaves can be identified on the shoot tip of Asteroxylon mackiei.  Image credit: Turner et al., Doi: 10.1126/science.adg4014.

Spirally arranged blades can be identified on the tip of the bullet Asteroxylon mackiei. Image credit: Turner and more., doi: 10.1126/science.adg4014.

“Spirals are common in plants, with Fibonacci spirals accounting for over 90% of spirals,” said University of Edinburgh paleontologist Alexander Hetherington and colleagues.

“Sunflower heads, pine cones, pineapples and house succulents all contain these unique spirals in their petals, leaves or seeds.

“Why Fibonacci spirals, also known as nature’s secret code, are so common in plants has intrigued scientists for centuries, but their evolutionary origins have been largely overlooked.

“Given their distribution, Fibonacci spirals have long been assumed to be an ancient trait that evolved in the earliest land plants and became highly conserved in plants.

However, the study authors discovered non-Fibonacci spirals in a 407-million-year-old plant fossil.

Using digital reconstruction techniques, they produced the first 3D models of club moss leafy shoots Asteroxylon mackieia member of the oldest group of leafy plants.

The exceptionally well-preserved fossil was found at the famous Rhynie chert fossil site, a Scottish sedimentary deposit near the Aberdeenshire village of Rhynie.

The site contains evidence of some of the planet’s earliest ecosystems – when land plants first evolved and gradually began to cover the Earth’s rocky surface making it habitable.

The results revealed that leaves and reproductive function in Asteroxylon mackieiwere most often arranged in non-Fibonacci spirals that are rare in plants today.

“This changes our understanding of Fibonacci spirals in land plants,” the researchers said.

“It suggests that non-Fibonacci spirals were common in ancient club mosses and that the evolution of leaf spirals split into two distinct pathways.”

The leaves of ancient club mosses had a completely different evolutionary history than other major plant groups today such as ferns, conifers and flowering plants.

“The 3D model of Asteroxylon mackiei allows us to view leaf structure in three dimensions for the first time,” said Dr. Hetherington.

“Our results provide a new perspective on the evolution of Fibonacci spirals in plants.”

“The club moss.” Asteroxylon mackiei is one of the earliest examples of a plant with leaves in the fossil record,” said paleontologist Holly-Anne Turner from University College Cork.

“Using these reconstructions, we have been able to observe individual leaf whorls around the stems of these 407-million-year-old fossil plants.

“Our analysis of leaf structure in Asteroxylon mackiei shows that very early club mosses developed non-Fibonacci spiral patterns.

An article about the results appears in the journal Science.

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Holly-Anne Turner and more. 2023. Leaves and spores evolved in rare non-Fibonacci spirals in early deciduous plants. Science 380 (6650): 1188-1192; doi: 10.1126/science.adg4014

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