What was the population density of dinosaurs?

Let me rephrase this a bit to help explain the nature of your question: "If I went back in time to the Human Era, how long until I encounter a mammal?" It would depend strongly on what kind of biome you went to and what kind of animal you count. read more

Generally, the smaller the creature, the higher population density of them can be supported in a given habitat, with some habitats being richer than others. Warm wet floodplains full of lush plant life were very densely populated with dinosaurs (and everything else, from insects and fish through early mammals and beyond). read more

] Of course, these figures are based on estimates of what we know: There may be more types of dinosaurs to discover. read more

That said, here's one paper that estimated the population density of herbivorous dinosaurs based on estimates of body mass and dietary requirements. For the western United States in the late Jurassic period (~150 million years ago), the authors estimated a few hundred animals per square kilometer, of which a few tens were large adults and sub-adults. read more