Modern birds have diversified about nine times faster than expected, starting about 103 million years ago, and the fishes that live on coral reefs have diversified about eight times faster than expected, he said.

Crocodiles and alligators are nearly 250 million years old yet have diversified into only 23 species, Alfaro said.

They are diversifying a staggering 1,000 times slower than would have been expected. 'Their species richness is so low, given how old they are,' he said.

The tuatara, which lives in New Zealand and resembles lizards -- although it is actually a distant cousin -- has only two species.

'In the same period of time that produced more than 8,000 species of snakes and lizards, there were only two species of tuatara,' Alfaro said.

'That is one of the big mysteries about biodiversity,' Alfaro said. 'Why these evolutionary losers are still around is a very hard thing to explain,' he said, according to an UCLA release.

Conversely, there are more than 9,000 bird species, more than 5,400 mammal species, approximately 5,500 frog species, some 3,000 snake species and 5,200 lizard species, Alfaro said.

The study, which also shows that new species emerge nearly as often as they die off, was published in the early online edition of Proceedings of the National Academy of Sciences.