A new species of dinosaur is described, on average, every ten days. As many as 31 species have already been reported this year and we can expect a few more before 2016 is over. Of course, figuring out what counts as a distinct species is a tricky problem. Palaeontologists are argumentative by nature, so getting any two of them to agree on a definitive list of species is probably impossible. But by anyone’s count, there were a lot of them – 700 or 800 that we know of, probably thousands in total. So how did the dinosaurs become so diverse?
First we need an idea of just how many dinosaur species there were. One study tried to estimate the total diversity of dinosaurs by using the species-area effect – the idea being that if we know how many species one small part of the Earth can support, we can extrapolate how many must have existed worldwide. These calculations suggest that at the end of the Mesozoic, 66m years ago, the standing diversity of dinosaurs – all the species alive at one point in time – was between 600 and 1,000 species.
This seems to be a reasonable estimate, in that if you counted up all of the living land mammals weighing more than 1kg (the size of the smallest dinosaurs) and then added the extinct species from the past 50,000 years, such as wooly mammoths, ground sloths, and giant kangaroos (correcting for losses to diversity caused by humans) you would end up with a similar figure.
However, this is just the number of species around at one point in time, and the dinosaurs were around for a very, very long time. Over the course of the Mesozoic, dinosaurs constantly evolved and went extinct. Doing some quick and rough estimates, and assuming 1,000 species of dinosaurs lived at any one time, and then that the species turned over every million years – that’s 160 times over the 160m-year reign of the dinosaurs – we end up with 160,000 species. Which is a lot of dinosaurs.
This is, of course, a very rough estimate. It depends on a lot of assumptions, such as how many different species the planet can support, and how quickly they evolve and go extinct. If we assume a lower standing diversity of 500 species and slower turnover, with species lasting 2m years, for example, we end up with around 50,000 species. On the other hand, perhaps standing diversity of 2,000 species is reasonable for the warm, lush, Mesozoic, and perhaps they only lasted just half a million years. That gives us over 500,000 species. So it seems reasonable to guess that there were between 50,000 and 500,000 species of dinosaurs – without including Mesozoic birds, which might double the diversity.
Why so many species, then? It comes down to three things. Dinosaurs were good at specialisation, localisation, and speciation.