New seed fossil sheds light on wind dispersal of plants

Scientists have discovered one of the first examples of winged seeds, gaining insight into the origin and early evolution of wind dispersal strategies in plants.

The study, published in eLifedetails the second oldest known winged seed, Alasemenia, dating from the late Devonian, approximately 360 to 385 million years ago. The authors use what the editors call sound mathematical analysis to demonstrate that Alasmenia’s three-winged seeds are more adapted to wind dispersal than one-, two-, or four-winged seeds.

Wind dispersal of plant seeds is a natural mechanism that allows plants to spread their seeds through the air to new areas. This helps reduce competition for resources, thereby increasing the plant’s chances of survival. Examples of wind dispersal strategies include tumbleweeds, parachutes like dandelions and milkweeds, and winged seeds like those of maple, often called helicopter seeds.

The oldest known plant seeds date back to the late Devonian.

“This period marks an important evolutionary step in the history of plants, as they moved from spore-based reproduction, as with ferns and mosses, to seed-based reproduction,” explains the lead author. Deming Wang, professor at the Key Laboratory of Orogenic Belts and Crustal Evolution, Department of Geology, Peking University, Beijing, China. “However, little is known about seed dispersal by wind during this period, as most fossils lack wings and are usually surrounded by a protective cup.”

Cupules are cup-shaped structures that partly enclose the seeds, much like in acorns or chestnuts (although Devonian cupules do not share the same origin with these modern cupules), and could be associated with d other methods of dispersion, such as transport by water.

To better understand the early mechanism of wind dispersal, Wang and his colleagues studied several Late Devonian fossil seeds from the Jianchuan Mine in Xinhang City, Anhui Province, China. From there, they identified a new fossil seed, Alasemenia.

They first described the characteristics of Alasamenia by carefully analyzing the fossil samples, including making slices to visualize the internal structures of the seed. They found that Alasamenia seeds are approximately 25 to 33 mm long and clearly lack a cup, unlike most other seeds from this period.

In fact, this is one of the oldest known observations of a seed without a cup, 40 million years earlier than previously thought. Each seed is covered by a layer of seed coat, or seed coat, which radiates outward to form three wing-like lobes. These wings taper toward the ends and curve outward, creating broad, flattened structures that would have helped the seeds catch the wind.

The team then compared Alasmenia to other winged seeds known from the Upper Devonian: Warstenia and Guazia. These two seeds have four wings: that of Guazia is wide and flat and that of Warstenia is short and straight.

The researchers performed a quantitative mathematical analysis to determine which seed had the most efficient wind dispersal. This revealed that having an odd number of wings, as in Alasmenia, confers a more stable and higher rotation speed as the seeds descend from their branches, allowing them to catch the wind more effectively and therefore disperse further of the mother plant.

“Our discovery of Alasemenia adds to our knowledge of the origins of aeolian dispersal strategies in early land plants,” says lead author Pu Huang, an assistant researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences. , Nanjing, China.

“In combination with our previous knowledge of Guazia and Warsteinia, we conclude that winged seeds resulting from seed coat outgrowth emerged as the first form of wind dispersal strategy during the Late Devonian, before other methods such than parachutes or plumes.”

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