By Helen Briggs BBC News Online

The fossil of a dinosaur the size of a dog gives an insight into how giant dinosaurs walked the Earth.

Newly discovered skeletons show the tiny dinosaur called Liaoceratops was the distant cousin of giant quadrupeds such as Triceratops.

This animal even though it was small and very primitive probably was a quadruped

Dr Peter Makovicky, Field Museum

The last of the plant-eaters, which had neck frills and horns, died out about 65 million years ago.

The discovery of small, primitive relatives that also walked on four legs suggests the gait was not an adaptation to increasing body size. According to Peter Makovicky of the Field Museum in Chicago, US, it was because the dinosaurs' heads were so large.

"This animal, even though it was small and very primitive, probably was a quadruped," Dr Makovicky told BBC News Online. "It may be that it's in response to this fairly massive skull that horned dinosaurs have.

"Even in these early little guys, the skulls are quite massive," he added. "It just becomes biomechanically advantageous for the animal to move on four legs instead of two."

Body armour

Dinosaurs known as ceratopsians were the last to die out, apart from birds.

A typical ceratopsian dinosaur was about the size of a car and weighed about 10 tonnes.

It had enormous horns and frills which it probably used to fend off predators or tempt a mate.

But two fossils uncovered in northeast China show the snorting, stampeding giants had much smaller relatives.

The bones were found in 124-145-million-year-old rocks from the Early Cretaceous period.

The miniature dinosaurs weighed about three kilograms (seven pounds) and were about 30 centimetres (one foot) tall. They had blunt beaks and a rudimentary frill.

Scientists think the frills may have originally served for supporting large jaw muscles needed to chomp plants.

The work, by a team in China and the United States, is published in the journal Nature.