One of evolution's greatest success stories is that of the Incredible Shrinking Dinosaur, scientists say.
The reptiles that ruled the world for almost 200 million years never went away. At least some of them just got smaller and turned into birds.
Now researchers have shown that shrinking was key to survival for this group, which became one of the most diverse and abundant families of animals alive today.
Only those dinosaurs destined to be birds broke the lower body weight limit of one kilogram seen in their relatives.
Lead scientist Dr Roger Benson, from the Department of Earth Sciences at Oxford University, said: "Dinosaurs aren't extinct; there are about 10,000 species alive today in the form of birds. We wanted to understand the evolutionary links between this exceptional living group, and their Mesozoic relatives, including well-known extinct species like T rex, Triceratops, and Stegosaurus.
"We found exceptional body mass variation in the dinosaur line leading to birds, especially in the feathered dinosaurs called maniraptorans. These include Jurassic Park's Velociraptor, birds, and a huge range of other forms, weighing anything from 15 grams to three tonnes, and eating meat, plants, and more omnivorous diets."
Small body size may have been a vital difference that helped the ancestors of modern birds remain on Earth after other dinosaurs were wiped out 65 million years ago, say the researchers.
A huge asteroid impact off the coast of Mexico is generally thought to have brought the dinosaurs' long reign to an end. However, many experts believe dinosaurs were already in decline when the meteor delivered the coup de grace that finished them off.
Together with Canadian colleagues from the Royal Ontario Museum, Dr Benson's team estimated the body mass of 426 dinosaur species by measuring the thickness of their leg bones.
The scientists found that dinosaurs underwent rapid changes in body size soon after they first appeared around 220 million years ago.
Thereafter only the evolutionary line leading to birds continued to change size at such a fast rate, and did so for a further 170 million years.
The study, published in the online journal Public Library of Science Biology, revealed that dinosaurs ranged in size from the 90 tonne Argentinosaurus - the largest creature ever to walk on land - to the bird-like Qiliania, which weighed 15 grams and was the size of a sparrow.
The team worked on the basis that if members of a family of related animals are similar in size, their evolution is likely to have been slow. On the other hand having close relatives that are very different in size implies fast rate of evolution.
"What we found was striking," said co-author Dr David Evans, from the Royal Ontario Museum.
"Dinosaur body size evolved very rapidly in early forms, likely associated with the invasion of new ecological niches. In general, rates slowed down as these lineages continued to diversify.
"But it's the sustained high rates of evolution in the feathered maniraptoran dinosaur lineage that led to birds - the second great evolutionary radiation of dinosaurs."
The bird ancestors kept experimenting with different and often radically smaller body sizes, allowing them to adopt new designs and adaptations more quickly than larger dinosaurs.
Other dinosaur groups became locked into narrow ecological niches from which they could not escape, say the researchers. This may have ultimately contributed to their extinction.
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