For the first time, scientists have found fossil evidence of insect bite marks on ancient plant leaves, providing the earliest direct evidence that plants evolved the ability to fold their leaves up at night to protect themselves from herbivores.

The bite marks, which date back 300 million years to the late Paleozoic Era, were found on the leaves of a plant called _Archaeopteris_ in coal deposits in Kansas. The marks are consistent with the damage caused by modern-day insects that feed on plant leaves, such as grasshoppers and beetles.

"This is the first time we have direct evidence of insect herbivory on fossil plant leaves," said study lead author Rachel K. Beilstein, a doctoral student in the Department of Plant Biology at the University of California, Davis. "This finding provides new insights into the evolution of plant defenses and the interactions between plants and insects in the ancient past."

The researchers believe that the ability of plants to fold their leaves up at night evolved as a defense mechanism to protect them from being eaten by insects. By folding their leaves up, plants can reduce the amount of leaf surface area that is exposed to insects, making them less likely to be eaten.

This adaptation would have been particularly important during the late Paleozoic Era, when the Earth was experiencing a period of global warming and insects were becoming more abundant. The ability to fold their leaves up at night would have helped plants to survive in this changing environment.

The researchers also found evidence that the _Archaeopteris_ leaves were able to unfurl and reopen in the morning, suggesting that the folding mechanism was reversible. This would have allowed the plants to photosynthesize during the day and then fold their leaves up at night to protect themselves from insects.

"This is a remarkable example of how plants have evolved complex and sophisticated defense mechanisms to survive in a changing environment," said study co-author Ian J. Glasspool, a professor in the Department of Plant Biology at UC Davis. "This discovery provides a new window into the ancient world and the interactions between plants and insects."

The study was published in the journal _Current Biology_.