Dinosaur communities were separated by both time and geography. The 'age of dinosaurs' (the Mesozoic Era) included three consecutive geologic time periods (the Triassic, Jurassic, and Cretaceous Periods).
Approximately 700 species have been named. However, a recent scientific review suggests that only about half of these are based on fairly complete specimens that can be shown to be unique and separate species.
Figuring out ways to estimate animal population sizes and their trends involves a lot of research time at the USGS Patuxent Wildlife Research Center.
Bird banding data are useful in both research and management projects.
The term "historical" as used in the GNIS means specifically and only that the feature no longer exists on the landscape. It has no reference to age, size, condition, extent of habitation, type of use, or any other factor.