The 190-million-year-old fossils unearthed in China on an "embryonic bone bed" belonged to Lufengosaurus, a long-necked, plant-eating dinosaur known for its gigantic size, with adults growing up to nine metres long.
An international team working in the southwestern Chinese province of Yunan was able to analyze what are now the oldest known embryos of any land-dwelling animal to study how the creatures developed.
"This is the only case that we've been able to do this," team leader and University of Toronto paleontologist Robert Reisz told The Canadian Press.
"Usually what you get is a single glimpse...here we have an extended range of size so we can actually track how particular bones changed through time in the embryonic life."
A detailed look at more than 200 bones and fragmented egg shells from 20 individual animals at various stages of development revealed the creatures grew much more rapidly inside the egg than other dinosaurs and flexed their muscles in much the same way as birds and humans.
Details of the discovery were published in Thursday's issue of the journal Nature.
The cache of bones was uncovered three years ago, but it has taken this long to analyze them — not an unusual lag time for dinosaur finds.
The extracted embryo fragments remain in China, where scientists hope to analyze them further and add to the collection through further excavation.
What's particularly interesting about the latest find, said Reisz, is the organic matter that has been detected in the embryonic remains.
"This is something that is totally unexpected," he said, adding that Taiwanese scientists collaborating on the project were able to explore the possibility of the material existing in the bones that were found.
"We think they are collagen fibres that represent the framework on which the bone was built. We have very good evidence that this is native dinosaur tissue."
Scientists hope to be able to extract and study the collagen to compare it to collagen in living animals.
The embryos found by Reisz's team were the same age as a separate set of fossils he reported about in 2005 from South Africa. The two types of dinosaurs, which roamed during the early Jurassic age, were close relatives.
In the earlier discovery, however, the embryos were curled up inside the eggs and scientists were not allowed to remove the skeletons. The new collection contained bones that were scattered, allowing researchers examine them in finer detail.
The latest embryos were not in as pristine condition as the previous find, noted University of Maryland paleontologist Thomas Holtz, who was not part of the discovery team.
But they have allowed scientists to chart dinosaur growth, which wasn't possible before, Holtz said.
— with files from the Associated Press