ALBUQUERQUE, NEW MEXICO — A US team of scientists say they've found the reason why meat-eating dinosaurs tended to be either small or very large, with very few medium-sized species in-between. <br /><br />The scientists say their findings also explain why there are more large than small species of dinosaurs — and why, compared to other fossil species, dinosaur species are not very diverse. <br /><br />The study was published in the journal called Science. It studied the sizes of some 550 dinosaur species from around the world. <br /><br />Researchers found that the gap between small and big species appeared when giant carnivorous dinosaurs were present in a specific region. <br /><br />The populations of these giant carnivores would usually include high numbers of pre-adult youngsters, and these smaller, hyperactive young dinosaurs would form hunting packs that would out-compete all mid-sized dinosaurs, thus wiping out the mid-sized species. <br /><br />In this way, the youths would form their own temporary sub-species that eliminated all mid-sized species.
