Were Trilobites Herbivores, Omnivores or Carnivores?

Answer 1

There is no conclusive evidence to rule out any of those possible diets. It seems reasonable to use modern-day crustaceans as an analog, which would lead one to believe that they are predator-scavengers, like most modern crustaceans. However, even among modern crustaceans some are filter-feeders ( barnacles), some are planktivores (most larval crustaceans), some are herbivores (many shrimp species), and some are parasites (copepods, isopods), so it is hard to conclude anything about the diet of the trilobite with certainty.

It is even likely that different species of trilobites specialized in different types of diets, with some being omnivores, some being carnivores, some being herbivores, etc. Other possibilities are that they were particle-feeders (likely mixed diet of plant and animal) or filter-feeders (plant, but could have included zooplankton) and even olenimorphs with a symbiotic relationship with sulfur-eating bacteria.

See:
http://www.trilobites.info/feeding.htm

It is not true that some sort of plants did not exist during the time trilobites existed, as cyanobacteria (also called blue-green algae) existed long before trilobites. Cyanobacteria used photosynthesis and predated true algae, but could have been a food source. Red algae (solenopores) also existed in the Cambrian.

If you would like to see a published reference that suggests the possibility of trilobites being herbivores here it is:
Fortey, R.A., & R.M.Owens. 1999. Feeding habits in Trilobites. Palaeontology 42(3):429-65.

Answer 2

Many just ate sediment whole hog the way earthworms do ("nonselective deposit feeder"), or possibly chunks of organic material within the sediment ("selective deposit feeder"). Some, including possibly the lichide trilobites, may have been predators on live animals. On the whole, there is very little evidence to go on. However, I have never seen it suggested that trilobites went after plants or algae. In fact, plants as such didn't even exist for the first 100 million years of trilobite time!

Answer 3

Omnivores I believe.