Herbivores would have needed appropriate digestive abilities to digest any available foliage currently available.
The reason that anything needs digestive systems is to bring food and nutrients to the cells. Single cells just take the nutrients from the environment, either taking in simple molecules or taking in more complex things (even other cells) and breaking them down.
Digestive systems are needed when the bodies become too large for the individual cells to take in the nutrients from the environment. So the first multicellular organisms were small until they could evolve simple transport systems where a few cells will take in the food and send it to the other -- look at sponges or jellyfish.
Then as the creature becomes more complex, so will the digestive system. They will become complex together.
Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
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M. Alan Kazlev
™ Version 4.2