Dinosaurs were a large subgroup of the reptile class; they dominated the terrestrial environments of our planet Earth for 160 million years, up until approximately 65 million years ago. They all belonged to the taxonomic superorder of Dinosauria, which contained two specific reptile orders: Ornithischia and Saurischia.
Most, if not all, of these species lived and thrived on land masses that formed continents that very few people would readily recognize in comparison to today’s geography. They shared their world with many other reptilian species, such as the pterosaurs in the skies and the pliosaurs in the seas. The pliosaurs almost certainly dominated not only their plesiosaur cousins, but the gigantic sharks and crocodillians of that period as well. But these other reptiles, while sharing the high level taxonomic class Reptilia, were not dinosaurs.
While most of us tend to think of dinosaurs as enormous animals, only a small percentage of dinosaur species were actually that gigantic. Most dinosaur species were of a relatively moderate size, comparable to some of today’s larger mammals, and many were considerably smaller, down to the size of a farmyard hen.
Of the two taxonomic orders of dinosaurs, all of the species that were Ornithischians were herbivorous, plant eaters, although many of the larger species could be technically classified as “omnivorous by accident”. Nearly as many millions of species of insect as live today, lived during the time of the dinosaurs, millions of trillions of individual insects; making it virtually unavoidable for the larger, plant-eating dinosaurs to not consume large numbers of insects simply residing on their vegetable diet.
The vegetation consumed by the herbivorous and omnivorous dinosaurs was very different to that today’s herbivores eat. Most modern herbivores are classified as browsers or grazers, either eating leaves from bushes and trees or grasses respectively. Most of those bushes and trees, some 235,000 species, are flowering plants (angiosperms), but both flowering plants and grasses evolved after the demise of the dinosaurs.
Smaller herbivorous dinosaurs would have eaten mosses, lichens and ferns, while the larger would have eaten bush and tree-sized fern species and non-flowering bushes and trees (gymnosperms) similar to the cycads and conifers of today. Fruit is produced as a result of flower fertilization on angiosperms; at the time of the dinosaurs there would have been no fruit, so there were no fruit eating dinosaurs.
While many of the Saurischian dinosaurs were also herbivorous, a small number of species were deliberately omnivorous and a larger percentage were decidedly carnivorous. As with many omnivorous species today, the relatively small number of omnivorous dinosaur species would almost certainly have been predominantly scavengers, predating only on sick or injured live animals. Many of the carnivore species probably obtained the majority of their meat diet through scavenging as well, but a number of species were primary carnivores, needing fresh kill and therefore actively hunting and killing their prey.
The larger dinosaur predators, such as the Tyrannosaurus rex, and the socialized pack hunters, such as the velociraptors, would have eaten large herbivorous dinosaur kills in the main. Small dinosaur predators are likely to have been insectivores. There was a multitude of large insect species sharing the dinosaurs world that would make the many large butterflies and beetles of south-east Asia and the Giant weka of New Zealand seem average to small. Middle-sized carnivores, unless pack-hunters, would have eaten the smaller herbivorous dinosaurs, some of the larger insect species and those bothersome, small furry burrowing creatures; the proto-marsupials that eventually evolved into today’s mammals.