Knowledge of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defense (such as spines) or herbivory-related damage; the observation of plant debris in fossilised animal feces; and the structure of herbivore mouthparts.
Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils can show it. As previously discussed, the first land plants emerged around 450 million years ago; however, herbivory, and therefore the need for plant defenses, undoubtedly evolved among aquatic organisms in ancient lakes and oceans. Within 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around , there is evidence that plants were being consumed. Animals fed on the spores of early Devonian plants, and the Rhynie chert also provides evidence that organisms fed on plants using a "pierce and suck" technique. Many plants of this time are preserved with spine-like enations, which may have performed a defensive role before being co-opted to develop into leaves.
During the ensuing 75 million years, plants evolved a range of more complex organs – from roots to seeds. There was a gap of 50 to 100 million years between each organ's evolution and its being eaten. Hole feeding and skeletonization are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.