Fossil bug bites reveal plant leaf movements

Many plants open and close their leaves and petals on a daily basis, but how and when did this evolve? Incredibly, insect bite marks in fossilised leaf remains have helped to shed some light on this. Scientists have stumbled on leaf fossils with holes in them where presumably a big hungry cretaceous caterpillar has made a meal of them; but these holes are symmetrical about the leaf’s midline. And the scientists who’ve found them think this is because when the animal bit into it, the leaf was closed; when it opened again, the hole was in both surfaces; it’s a bit like when children make symmetrical paper doily cutouts by folding a piece of paper in half and trimming the folded surface. Steve Mcloughlin, from the Swedish History Museum in Stockholm, told James Tytko what they think they’ve found…

Steve - So I had a Chinese colleague, Zhuo Feng, but he noticed back in 2013 that some of these sleeping plants have a particular symmetrical line of damage down each side of the leaf. When they fold up their leaves at night, the insects bite through one side of the leaf and come out the other side. So they have symmetrical rows of damage on the leaf. And then he started to look out for this damage in the fossil record. And then in 2016, he found a fossil leaf from an extinct 255 million year old plant called a Gigantopterid from China. And it had exactly the same type of damage down each side of the midline of the leaf. Now one specimen is not always convincing to the reviewers, so he kept on looking and last year he found another specimen with exactly the same type of symmetrical damage. We think that these are mature leaves and that it represents the first real evidence, in the fossil record, of leaves that folded up in the past into this very distinctive sleeping behavior.

James - Could you go through again, just what the giveaway clues were in those particular samples that this leaf folding behaviour was occurring all those millions of years ago?

Steve - It's the symmetrical nature of the holes. So we have these broad leaves. They look a little bit like an oak leaf. The fossils that we found have these regular holes sort of parallel to the mid vein. And it seems unlikely that two insects created exactly the same chain of holes down either side of the leaf. So the more sensible explanation is that the leaf folded up along the midline and that an insect chewed through the leaf.

James - So that insect got two for the price of one when it took a bite in that leaf, it got through to two layers. Why is it that these leaves actually do this in the first place? What does the plant gain from folding up at night in this daily cycle?

Steve - Well that's a very good question, and it's intrigued botanists for a very long time. In fact, Charles Darwin and his son Francis, wrote a book covering this topic back in 1880. It was called the Power of Movement in Plants. And it sold out very quickly because people were really intrigued by these sorts of movements. It occurs a lot in tropical plants, but not all tropical plants. And it also occurs in temperate plants. So if you go into a local temperate woodland or meadow, you'll find things like clover and wood sorrels that also fold up at night. It clearly must have some benefits to the plants because many plants do it. People have suggested it's temperature regulation. If you close the leaves up, you are less likely to be frosted. But that seems to be very minimal benefit for temperature. It might only be a fraction of a degree. Some have suggested it's improved shedding of water, but there are other adaptations that leaves do that would be much better for shedding water. And then there are the more trophic or multi trophic suggestions where, if you rotate your leaves upright or fold your leaves up, it makes the surface less accessible to insects that will chew on the leaf. Or it might also be that the herbivorous insects that land on a vertical leaf become easier to see for carnivorous insects or other animals that are patrolling the night forests. So we really don't know. It still remains a field of open research, but there are many potential reasons that plants might do it.

James - Fascinating. So this is your sort of mission to get to the bottom of this mystery. Are you waiting essentially for some new fossils to blow the case wide open?

Steve - Yeah. Well, as we were doing a literature survey for this project, we came across some illustrations of fossils that might also have had this behavior. At least they have, these are mostly legume fossils, and they have what's called a pulvinus, a thickening at the base of the leaf. And it's in this thickening that, in modern plants, the calcium and potassium ions activated and helped change the turgor that moves the leaf. So I think there is probably more evidence out there for this behavior, but people just haven't looked for it.