Insect bites in plant fossils reveal that leaves could have closed millions of years ago

252 million years ago, some plants may have curled up their leaves at night for a restful “sleep.”

Fossilized leaves of two now extinct Gigantonoclea Researchers reported on February 15 that the species exhibits markings of nyctinasty, or rhythmic folding at night. Current Biology. The team says this would make these specimens the first known fossil examples of this strange plant behavior.

The two leaf fossils were discovered in a rock layer in southwestern China, dating back between 259 million and 252 million years ago. In both species, the leaves are broad, with serrated edges. But most curiously, they bore strangely symmetric holes.

Paleontologist Zhuo Feng of Yunnan University in Kunming, China, and colleagues said the insects made the holes while feeding on leaves as they folded. Similar patterns of insect damage in leaf fossils, the team says, can be used to distinguish folding behavior from leaves that may have withered as the plant died.

Modern plants, including many in the legume family such as the orchid tree, which fold and unfold their leaves, use specialized cells called plevinus cells, which act somewhat like muscles (SN: 2/3/23). By moving water from one part of the leaf to another, the cells can swell or contract, allowing the leaves to bend or curl.

The team says these cells would be at the base of the leaves, which have not been preserved in fossils, so it is not possible to say whether these ancient plants also contained fibrous cells. Although it is also difficult to prove that this was nocturnal behaviour, the leaves also had to be folded long enough for the insects to be able to chew on them. But the discovery indicates that leaf folding emerged independently in different plant lineages: nearly all modern plants that do it are angiosperms, or flowering plants. but Gigantonoclea The plants were gymnosperms, seed-producing plants such as conifers and ginkgos.