By Azadeh Ansari, CNN
Everyone knows dinosaurs were gigantic, but they grew from tiny embryos just like birds do. What were these extinct reptiles like at this early stage of development?
Scientists have found some new clues that could shed light on this age-old mystery.
In a study published Wednesday in the journal Nature, scientists said they have discovered the oldest known collection of fossilized dinosaur embryos.
"In a way, I think we have set a new standard for dinosaur embryology," said paleontologist Robert Reisz, the lead study author.
Scientists found these dinosaurs grew extremely fast in comparison to present-day living animals and even flexed their muscles while still in the egg.
The bigger the spaces between primary bone cavities, the faster an animal grows. These bone cavities house soft tissue responsible for blood vessel generation.
"In other animals, about 15% to 45% of the embryo bone tissue is made up of these soft tissue cavities; in these dinosaur fossil samples, we found the cavities to make up roughly 60% of the bone tissue," Reisz said.
"Since little is known about the growth patterns of dinosaur embryos, we can't tell you how many days it took for the incubation to be complete, but this is the first time we have been able to study the different stages of dinosaur embryo incubation and examine how the dinosaur embryo grew."
Scientists stumbled upon the embryos in China's Yunnan province in the city of Kunming, roughly 1,242 miles south of Beijing. It is known for being one of the world's oldest embryo preserves and a treasure trove for paleontology. The newly discovered embryonic dinosaur bone bed was nestled in the lowest, most richly fossilized region of the Lufeng Basin.
The remains were not found in one, comprehensive intact skeleton; rather they were from a cross-section of various fossilized embryos.
The bone fragments - some as small as pencil lead - were from sauropodomorphs, a group of dinosaurs known for their gigantic size, with some of these terrestrial vertebrate growing up to 9 meters (29.5 feet) in length.
To understand the significance of these embryonic fossils dating back 190 to 197 million years ago further, scientists from mainland China and Taiwan teamed up and called upon experts across multiple disciples and continents.
"We had paleontologists working hand in hand with chemists, physicists and geologists," Reisz said. "This is a breakthrough in how we approach dinosaur paleontology," adding that "this has never been done before."
Scientists used synchrotron radiation and highly advanced infrared spectroscopy to isolate the main bone crystallizing structure, from which they took uncontaminated samples. They examined the ossification, or bone growth patterns, of more than 200 fossilized embryonic bone fragments from a single species.
Reisz said he and his colleagues were excited about what the findings mean for paleontology but they have much more excavation work ahead to gain a more comprehensive understanding of the origin of dinosaurs.