Early plant predators...
 Well, the fossil record of things like slugs and snails is not outstanding, but there is no question that they are very successful terrestrial plant predators. Particularly, the snails; they bring a nice little coiled shell containing their marine environment with them. This allows them to operate in a relatively dry environment. So, we suspect that the first terrestrial community included things, not necessarily precisely like the slug, but close relatives of slugs and snails. The first terrestrial plant predators (I regard herbivores as plant predators) the first terrestrial plant predators were probably things like slugs and snails.
The other things that we find very commonly in this early terrestrial fossil record are things like millipedes and centipede-type creatures. Judging from the habits of their living descendants, these organisms did not eat the plant directly but rather ate partially decayed plant parts that had been pre-digested by bacteria. Such organisms are detritivores, organisms that feed upon detritus. It appears that much early consumption of plants occurred in this manner, after they had begun to decompose.
It appears that these early plant communities supported few herbivores; probably largely gastropods and a fair number of detritivores. This is completely unlike the present day, where one normally thinks of a food pyramid, building up from plants up through vertebrate & insect herbivores to the carnivores. In the Devonian we are dealing with a food pyramid in which the majority of plant material is probably consumed by detritivores, with few invertebrate herbivores. Much plant material is never eaten!.
I am of course, in the temple of Larson so I would like to acknowledge Professor Larson, who has an especially good sense of evolution. While insects and slugs are setting up relations with plants, our ancestral vertebrate friends are in the water. Some of these are increasingly becoming adapted to the point that they can come out on land. I suggest that what they should be staring at is not a baseball, but rather a plant. For, as these organisms,
here shown splashing and having a good time in the water, are diversifying, the early plant community become more and more complex. We started off with the Silurian with a low, two-dimensional vegetation. In the Early Devonian, the vegetation is still pretty much low. By the Middle and later Devonian we begin to see the appearance of three dimensional communities involving trees. These communities are still tied to water, and are of limited extent, but they have an increasing productivity. They are going to have an increasing influence on the terrestrial environment. There is more food. There are more different places to live.
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