Fossil reveals how plants use winged seeds to expand their habitats

Scientists have uncovered one of the earliest examples of a winged seed, shedding light on the origins and early evolution of wind dispersal strategies in plants. 

The study, published in the journal eLife, details the second-oldest known winged seed — Alasemenia – from the Late Devonian epoch, which occurred roughly 360 to 385 million years ago. 

Through a solid mathematical analysis, the authors demonstrate that the three-winged seeds of Alasemenia were better adapted to wind dispersal compared to seeds with one, two, or four wings.

Wind dispersal of plant seeds

Wind dispersal is a natural mechanism by which plants spread their seeds to new areas using the air, helping to reduce competition for resources and improve survival chances. 

Some common wind dispersal strategies include parachutes, like those used by dandelions and milkweeds, as well as winged seeds, such as the “helicopter” seeds from maple trees.

The earliest plant seeds date back to the Late Devonian epoch, a time marking a significant evolutionary shift. 

“This period marks a significant evolutionary milestone in plant history, as they transitioned from spore-based reproduction, as with ferns and mosses, to seed-based reproduction,” said lead author Deming Wang, a professor at Peking University‘s Department of Geology. 

Seed wind dispersal mechanisms 

However, little is known about wind dispersal mechanisms in seeds from this era, as many fossils are encased in a cupule, a protective cup-shaped structure that may suggest different dispersal methods, such as water transport.

To better understand how early plants used wind to disperse their seeds, Wang and colleagues examined seed fossils from the Late Devonian, sourced from the Jianchuan mine in China’s Anhui Province. Among these, they identified a new fossil species, Alasemenia.

Helping seeds catch the wind

The team carefully analyzed the Alasemenia fossils, slicing them to study their internal structure. 

The experts found that these seeds are about 25 to 33 millimeters long and lacked the cupule that was common in most seeds of the period, making Alasemenia one of the earliest known examples of a seed without such a structure — 40 million years earlier than previously thought. 

Instead, Alasemenia had a seed coat, or integument, that extended outward to form three wing-like lobes. These wings, with their outward-curving, broad, flattened structures, would have helped the seeds catch the wind.

Best wing design for wind dispersal 

Next, the researchers compared Alasemenia to two other known winged seeds from the Late Devonian: Warstenia and Guazia, both of which had four wings. While Guazia‘s wings were broad and flat, Warstenia‘s were short and straight. 

To determine which design was best suited for wind dispersal, the team conducted a quantitative mathematical analysis. Their results showed that Alasemenia’s odd number of wings provided a more stable and faster spin rate as the seed fell, allowing it to catch the wind more effectively and disperse farther from the parent plant.

“Our discovery of Alasemenia adds to our knowledge of the origins of wind dispersal strategies in early land plants,” said senior author Pu Huang, an assistant research fellow at the Nanjing Institute of Geology and Paleontology. 

“Combined with our previous knowledge of Guazia and Warsteinia, we conclude that winged seeds as a result of integument outgrowth emerged as the first form of wind dispersal strategy during the Late Devonian, before other methods such as parachutes or plumes.”

Evolutionary progression of seeds 

According to Wang, the evolutionary progression of winged seeds didn’t stop there. 

“The three-winged seeds seen in Alasemenia during the Late Devonian would have subsequently been followed by two-winged seeds during the Carboniferous period, and then single-winged seeds during the Permian.”

This research offers key insights into the evolutionary timeline of wind dispersal strategies in plants, marking Alasemenia as a critical piece in understanding how plants first took advantage of the wind to spread their seeds.

“These findings suggest that during the Famennian [belonging to the late Devonian] the outer layer of some seeds that lacked cupules evolved wings to help the seeds disperse in the wind,” noted the study authors.

“It also indicates that seeds with four or three wings evolved first, followed by other groups of seed plants with fewer seed wings.”

According to the researchers, future studies may find more winged seeds and further our understanding of their evolutionary roles in the early history of seed plants.

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