Conifer seed fossils caught learning helicopter flight

18th Mar 2015

Leonardo da Vinci is often credited with producing an early helicopter design – but trees came up with rotating flying machines at least 270 million years before him. The strategy wasn't an instant evolutionary success, though.

We think of plants as spending their lives rooted to the ground, yet they were actually one of the earliest life forms to develop wings. Seeds with pairs of wings, allowing them to glide, date back at least 370 million years.

About 100 million years later, conifer trees evolved a new way to fly: the helicopter spin. Their seeds began to feature a single wing, which sends them into a tailspin as they fall. This slows the seeds' flight and increases their chances of catching a breeze and travelling some way from the parent tree, meaning the seedlings are more likely to develop without having to compete with the parent for light and nutrients.

But the ploy got off to an unpromising start. Last year, Cindy Looy and Robert Stevenson at the University of California in Berkeley noticed that the seeds of the earliest known helicoptering conifer – an extinct species called Manifera talaris – typically didn't carry just one wing. Like some other early conifer seeds, they had two.

In nearly 80 per cent of the 120 seeds they studied, the second wing was small and stunted, but in 13 per cent both wings were essentially the same length, forming a symmetrical pair. Fewer than 10 per cent had the familiar single wing.

Now, Looy and Stevenson, and their colleague Dennis Evangelista, have discovered that those second wings were bad news for the first helicopters. The researchers made models of the seeds using materials chosen to match the properties of modern conifer seeds. Then they dropped them in a special breezeless chamber originally designed to study small winged animals, and recorded what happened using high-speed cameras.

The results suggest that the double-winged seeds were all poor flyers – particularly those with symmetrical wings. In fact, almost half them simply plummeted to the ground when the models had a mass of 6.5 milligrams – similar to that of modern helicoptering conifer seeds. All of the single-winged seed models of that mass helicoptered gracefully downwards instead.

What's more, the few symmetrical double-winged seed that did rotate during their descent still fell roughly twice as fast as single-winged seeds. The seeds with one large and one small wing were in between – they descended more slowly than the symmetrical double-winged seeds, but more quickly than the single-winged seeds.

Based on this, Looy thinks Manifera offers a snapshot of conifers perfecting the art of helicopter flying. Clearly, seeds with a single well-developed wing "fly" better, but early in their evolution, conifers produced a large number of poor flyers too. Why was that the case, and what changed later on?

Looy thinks it comes down to the conifer cones on which the seeds form. A few dozen of the individual scales – the individual seed-bearing structures – from Manifera cones survive as fossils – some, remarkably, with seeds still intact. "It's sheer luck that we found those specimens," says Looy.

These are not your typical simple pine cone scales, though: each looks rather like a hand with five or more finger-like projections. Three of these, the seed-bearing ones, are called sporophylls.

The researchers found that the single-winged seeds, and the seeds with one large and one small wing, seemed to develop only on the outermost sporophylls – equivalent to the thumb and little finger. The central sporophylls – equivalent to the middle finger – were developmentally programmed to produce seeds that were more symmetrical, with two well-developed wings.

Looy suspects that it was the cone rather than the seeds themselves that changed. Younger relatives of Manifera that evolved to produce only single-winged seeds lost the central sporophyll – probably to avoid investing resources in the poorly performing double-winged seeds. "It's a slightly speculative idea because the number of fossils we have to work on is small," says Looy. "But it's logical."

"I think it's a great paper," says David Lentink, a flight mechanics researcher at Stanford University in California. "It's really nice to see a functional study of how these different seeds perform."

William DiMichele, an authority on early plant fossils at the Smithsonian Institution in Washington DC, says that the work provides a great example of evolution in action. "In effect we get to see Nature working the bugs out."

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