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Semitransparent flesh reconstruction of embryonic dinosaur inside egg, with skeleton (artwork by D. Mazierski)

Oldest dinosaur bonebed reveals embryo development

Opens a new window into the lives of dinosaurs

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.

The great age of the embryos is unusual because almost all known dinosaur embryos are from the Cretaceous Period. The Cretaceous ended some 125 million years after the bones at the Lufeng site were buried and fossilized.

Led by University of Toronto Mississauga paleontologist Robert Reisz, an international team of scientists from Canada, Taiwan, the People鈥檚 Republic of China, Australia and Germany excavated and analyzed over 200 bones from individuals at different stages of embryonic development.

鈥淲e are opening a new window into the lives of dinosaurs,鈥 says Reisz. 鈥淭his is the first time we鈥檝e 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.鈥

Skeleton of Lufengosaurus in a museumThe bones represent about 20 embryonic individuals of the long-necked sauropodomorph Lufengosaurus (pictured left), the most common dinosaur in the region during the Early Jurassic period. An adult Lufengosaurus was approximately eight metres long.

The disarticulated bones probably came from several nests containing dinosaurs at various embryonic stages, giving Reisz鈥檚 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鈥 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. Reisz says this very fast growth may indicate that sauropodomorphs like Lufengosaurus had a short incubation period.

Reisz鈥檚 team found that the femurs were being reshaped even as they were in the egg. Examination of the bones鈥 anatomy and internal structure showed that as they contracted and pulled on the hard bone tissue, the dinosaurs鈥 muscles played an active role in changing the shape of the developing femur. 鈥淭his suggests that dinosaurs, like modern birds, moved around inside their eggs,鈥 says Reisz. 鈥淚t represents the first evidence of such movement in a dinosaur.鈥

The Taiwanese members of the team also discovered organic material inside the embryonic bones. Using precisely targeted infrared spectroscopy, they conducted chemical analyses of the dinosaur bone and found evidence of what Reisz says may be collagen fibres. Collagen is a protein characteristically found in bone.

鈥淭he bones of ancient animals are transformed to rock during the fossilization process,鈥 says Reisz. 鈥淭o find remnants of proteins in the embryos is really remarkable, particularly since these specimens are over 100 million years older than other fossils containing similar organic material.鈥

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

鈥淎 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,鈥 says Reisz. 鈥淚t greatly enhances our knowledge of how these remarkable animals from the beginning of the Age of Dinosaurs grew.鈥

The research is published in the April 11 issue of Nature.

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