Complex sentence: Although herbivory was long thought to be a Mesozoic phenomenon, fossils have shown that within less than 20 million years after the first land plants evolved, plants were being consumed by arthropods.
Split into simpler sentences: Although herbivory was long thought to be a Mesozoic phenomenon, evidence of it is found as soon as fossils which could show it. Within less than 20 million years after the first land plants evolved, plants were being consumed by arthropods.