Aerial parts of plants arrange their organs around stems, and this arrangement defines their structure. In most existing plant species, organs emerge at 137.5 degrees from the previous organ. This results in continuous spirals of organs, with the number of clockwise and anticlockwise spirals forming consecutive numbers in a Fibonacci sequence. Reconstructing the Early Devonian clubmoss Asteroxylon mackiei, paleontologists found alternative leaf arrangements suggesting that Fibonacci-style patterning was not ancestral to living land plants.
Spiral arranged leaves can be identified at the shoot tip of Asteroxylon mackiei. Image credit: Turner et al., doi: 10.1126/science.adg4014.
“Spirals are common in plants, with Fibonacci spirals making up over 90% of the spirals,” said University of Edinburgh paleontologist Alexander Hetherington and colleagues.
“Sunflower heads, pinecones, pineapples and succulent houseplants all include these distinctive spirals in their flower petals, leaves or seeds.”
“Why Fibonacci spirals, also known as nature’s secret code, are so common in plants has perplexed scientists for centuries, but their evolutionary origin has been largely overlooked.”
“Based on their widespread distribution it has long been assumed that Fibonacci spirals were an ancient feature 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 leafy shoots in the clubmoss Asteroxylon mackiei, a member of the earliest group of leafy plants.
The exceptionally preserved fossil was found in the famous fossil site the Rhynie chert, 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 started to cover the earth’s rocky surface making it habitable.
The findings revealed that leaves and reproductive structures in Asteroxylon mackiei, were most commonly arranged in non-Fibonacci spirals that are rare in plants today.
“This transforms our understanding of Fibonacci spirals in land plants,” the researchers said.
“It indicates that non-Fibonacci spirals were common in ancient clubmosses and that the evolution of leaf spirals diverged into two separate paths.”
The leaves of ancient clubmosses had an entirely distinct evolutionary history to the other major groups of plants today such as ferns, conifers and flowering plants.
“The 3D model of Asteroxylon mackiei lets us examine leaf arrangement in 3D for the first time,” Dr. Hetherington said.
“Our findings give a new perspective on the evolution of Fibonacci spirals in plants.”
“The clubmoss Asteroxylon mackiei is one of the earliest examples of a plant with leaves in the fossil record,” said University College Cork paleontologist Holly-Anne Turner.
“Using these reconstructions we have been able to track individual spirals of leaves around the stems of these 407 million year old fossil plants.”
“Our analysis of leaf arrangement in Asteroxylon mackiei shows that very early clubmosses developed non-Fibonacci spiral patterns.”
A paper on the findings appears in the journal Science.
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Holly-Anne Turner et al. 2023. Leaves and sporangia developed in rare non-Fibonacci spirals in early leafy plants. Science 380 (6650): 1188-1192; doi: 10.1126/science.adg4014
