Dinosaur embryos were restless, speedy growers

Dinosaur embryos lived fast and hatched young.

The oldest dino embryos ever discovered — hundreds of miniature bones dating to nearly 200 million years ago — show that some dinosaurs grew rapidly inside their eggs and probably had brief incubation periods, researchers report in the April 11 Nature. The work marks the first time scientists have tracked the development of dinosaur embryos.

“It’s so rare to have a window into the earliest times of dinosaurs’ lives,” says Kristi Curry Rogers, a vertebrate paleontologist at Macalester College in St. Paul, Minn., who wasn’t part of the research team.

The itty-bitty fossils may also retain some of the oldest preserved organic remains found. These compounds may be traces of proteins.

Paleontologist Robert Reisz of the University of Toronto in Mississauga and colleagues unearthed the unhatched dinos in southwestern China’s Yunnan Province. Unlike previous discoveries of dinosaur embryos, the team didn’t recover fully intact skeletons curled up inside fossilized eggs. Instead, they dug up more than 200 isolated jaw, rib, spine, limb and hip bones that once belonged to at least 20 different embryos whose eggs were crushed prior to fossilization, Reisz says. Aspects of the jaws tentatively tie the embryos to the genus Lufengosaurus, which lived during the early Jurassic period more than 190 million years ago and was an early relative of the colossal, long-necked sauropod dinosaurs that included Apatosaurus.

“It was very clear from the beginning these were little bones from embryonic dinosaurs,” Reisz says. In addition to their tiny size — some bones are shorter than the diameter of a dime — the fossils possess telltale signs of being unhatched. These signs include holes in the vertebrae through which a rod of cells called the notochord ran. Before a dinosaur hatched, the notochord would disappear and the hole would close up.

To observe the bones’ internal structure, the researchers sliced thin cross sections from some of the fossils and placed them under a microscope. When a bone fossilizes, it maintains the microscopic structure of cells and tissue, offering a snapshot in time of a developing embryo. By looking for differences inside bones of varying sizes, the scientists could piece together how embryos grew over time.

The thigh bones also revealed that the dinosaurs moved around inside their eggs. A knobby growth where muscles attach to bone got progressively bigger during development. In modern animals, this happens over time as the muscles pull the knob. The researchers think the unhatched dinos must have been kicking their legs.

Another feature of the bones also stood out: They were all highly perforated. In modern animals, a lot of hollow spaces in a bone means it’s growing quickly and allowing plenty of blood vessels to enter and bring nourishment. After dinosaurs hatched, the pockets would fill in with bone, Reisz says. Compared with other known dinosaur embryos, the new ones are more heavily pockmarked. So the embryos probably grew faster than did those of other dinosaurs, and faster than embryos of modern birds, the team suggests. But there’s not enough information to estimate exactly how long the incubation was.

“The high growth rates were necessary for a large [adult] body size,” says Martin Sander, a paleontologist at the University of Bonn in Germany. As a hatchling, Lufengosaurus was probably about 20 centimeters long, the researchers estimate, but eventually grew to be 9 meters. The only way for adults to get that big is to mature fast, starting as an embryo, Sander says.

Other paleontologists are more skeptical of exceptionally speedy growth. “All embryonic dinosaurs grew quite rapidly,” says Gregory Erickson, a paleobiologist at Florida State University in Tallahassee. “Whether these were growing more rapidly than other embryonic dinosaurs I don’t quite buy.”

Among modern birds, he notes, species with a lot of bone perforation can develop at different speeds. So pockmarks may not represent a good way to distinguish growth rates. Rogers suggests that comparing other aspects of the dino embryos with those of birds and crocodiles might help scientists get a better sense of how quickly the embryos matured.

The bones bear witness to more than just developmental patterns. Reisz and colleagues say they’ve found some of the animals’ original organic remains. In the past, researchers have usually searched for ancient proteins by dissolving fossils and looking for organic residues. But critics have questioned whether these samples were contaminated with bacteria or other modern material.

To make a more definitive identification, the researchers studied intact bones, using a technique that measures characteristic patterns of wavelengths of infrared light absorbed by organic compounds. The team reports they found the chemical fingerprint of amino acids, the building blocks of proteins. Reisz says the team plans to compare the compounds with proteins from other animals.

“The authors did a great job incorporating new technologies,” says paleobiologist Mary Schweitzer of North Carolina State University in Raleigh, who has recovered protein from a 68-million-year-old Tyrannosaurus rex specimen (SN: 8/30/08, p. 12). But the new case isn’t a slam dunk, she says. The pattern doesn’t quite match what you’d expect for a protein. It’s possible, she notes, that decaying proteins might have different chemical signals than pristine ones, which could account for the discrepancy.

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Back Story | FOSSIL GAP

With all that’s known about adult dinosaurs, the discovery of at least 20 fossilized embryos of an early sauropod relative in southwestern China illustrates how little information there is regarding how the creatures developed in the egg. Paleontologists have found embryos and eggs of smaller relatives of the enormous, long-necked sauropods in South Africa (SN: 7/30/05, p. 68) dating to about 190 million years ago. Those embryos’ anatomy indicates hatchlings were probably awkward walkers and toothless, signs that the babies would have needed help from mom or dad. The next example of prehatchlings in the sauropod lineage doesn’t come until about 80 million years ago, with the appearance of embryos and eggs of a true sauropod group, the titanosaurs. Scientists think the huge size difference between adults and hatchlings, as well as the large number of eggs in each nest, means these giants didn’t provide much care for their young. How growth, reproduction and parental care evolved in sauropods and their cousins during the more than 100-million-year gap in the embryo fossil record is something of a mystery. The fossils must be somewhere, says Martin Sander of Germany’s University of Bonn, but “people just haven’t looked hard enough.”