Dinosaur embryo bonebed reveals its secrets

A 190-million-year-old dinosaur bonebed near the city of Lufeng, in Yunnan, China has revealed for the first time how dinosaur embryos grew and developed in their eggs.

Dr Eric Roberts from James Cook University’s School of Earth and Environmental Sciences was part of the international research team that made the discovery, led by University of Toronto Mississauga palaeontologist Robert Reisz.

Scientists from Canada, Australia, Taiwan, the People’s Republic of China, and Germany worked together to excavate and analyse more than 200 bones from individuals at different stages of embryonic development.

The team also investigated the geology of the site, looking for clues to the origin of this unique fossil deposit.

Dr Reisz said that the research, which is published in the April 11 issue of Nature, opened a new window into the lives of dinosaurs.

“This is the first time we’ve been able to track the growth of embryonic dinosaurs as they developed. Our findings will have a major impact on our understanding of the biology of these animals,” he said.

Only about one square metre of the bonebed has been excavated to date, but this small area also yielded pieces of eggshell, the oldest known for any terrestrial vertebrate.

Dr Reisz said this was the first time that even fragments of such delicate dinosaur eggshells, less than 100 microns thick, have been found in good condition.

“A find such as the Lufeng bonebed is extraordinarily rare in the fossil record, and is valuable for both its great age and the opportunity it offers to study dinosaur embryology,” he said.

“It greatly enhances our knowledge of how these remarkable animals from the beginning of the Age of Dinosaurs grew.”

The bones represent about 20 embryonic individuals of the long-necked sauropodomorph Lufengosaurus, the most common dinosaur in the region during the Early Jurassic period. An adult Lufengosaurus was approximately eight metres long.

JCU’s Dr Roberts worked with the team at the site and studied thin sections of the bones and rocks encasing the bones to look for clues to their preservation and to interpret the depositional environment, age, and cause of burial/death of the organisms.

“In this way we were able to interpret that the bone bed was formed by low-energy flooding and slow inundation of a colonial nesting site rather than a just a single nest," Dr Roberts said.

"As eggs in the different nests began to decay and break apart, gentle wave-action acted to concentrate the bones of similar sizes and ultimately bury the embryonic bones and eggshells."

The disarticulated bones probably came from several nests containing dinosaurs at various embryonic stages, giving Dr Reisz’s team the rare opportunity to study ongoing growth patterns.

Dinosaur embryos are more commonly found in single nests or partial nests, which offer only a snapshot of one developmental stage.

To investigate the dinosaurs’ development, the team concentrated on the largest embryonic bone, the femur. This bone showed a consistently rapid growth rate, doubling in length from 12 to 24 mm as the dinosaurs grew inside their eggs.

Dr Reisz says this very fast growth may indicate that sauropodomorphs like Lufengosaurus had a short incubation period.

The team found the femurs were being reshaped even as they were in the egg. Examination of the bones’ anatomy and internal structure showed that as they contracted and pulled on the hard bone tissue, the dinosaurs’ muscles played an active role in changing the shape of the developing femur.

“This suggests that dinosaurs, like modern birds, moved around inside their eggs,” Dr Reisz said. “It represents the first evidence of such movement in a dinosaur.”

Issued: April 11, 2013

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