In an ideal world, defining what a species is would be as easy as "a group of organisms that can successfully interbreed". But guess what? It isn't that easy.
Some organisms, such as certain types of starfish, reproduce asexually (they "clone" themselves) so they don't really "interbreed" at all. And on top of that, this definition can't be applied to dinosaurs or any other extinct critter as it's impossible to test whether two fossils could or would mate.
When paleontologists are asked "what is a species?" they point to what is known as the "morphological species concept" based on observations of modern animals and define a species thus; "a group of organisms that share a certain degree of physical similarity".
Some organisms, such as certain types of starfish, reproduce asexually (they "clone" themselves) so they don't really "interbreed" at all. And on top of that, this definition can't be applied to dinosaurs or any other extinct critter as it's impossible to test whether two fossils could or would mate.
When paleontologists are asked "what is a species?" they point to what is known as the "morphological species concept" based on observations of modern animals and define a species thus; "a group of organisms that share a certain degree of physical similarity".
Further reading
