One of the planet’s most “bizarre group of dinosaurs” has left behind a 67-million-year-old legacy that is now allowing an international team of scientists a peek inside the animal’s mysterious biological history.

The nesting behavior of oviraptorids is poorly understood, despite the dinosaur having left behind the “most abundant record of fossil dinosaur eggs and clutches.” The two-footed dinosaurs exhibit a “peculiar” nesting pattern, arranging up to 30 paired eggs in three to four rings. To understand where in the egg-laying evolutionary tree they reside, paleontologists from the University of Bonn sought out to determine whether hatchlings emerged from their eggs simultaneously or at different times.

Teaming up with neuron radiography scientists at the Technical University of Munich, researchers looked at the development of embryos in three Cretaceous-period oviraptorid egg fossils from the Ganzhou Basin in China. Neutron radiography is similar to X-ray radiography but can image light elements like water or carbon, penetrate heavy elements like lead or titanium, and distinguish between different isotopes, according to The McClellan Nuclear Research Center. To estimate whether they would have hatched at the same time, scientists looked at the developmental stage of the embryos and the length of their bones – more bones with higher connectivity meant the eggs were more developed.

The length and position of the embryo bones suggest that a single egg was laid at an earlier time than another pair of eggs in the same clutch. An analysis of the eggshell thickness confirmed this.

It was previously believed that the reproductive biology of oviraptorids placed them between modern crocodiles and birds. Crocodiles, for example, lay and bury their eggs at the same time, which leads to their offspring hatching at the same time. On the other hand, birds lay their eggs at different times and their hatchlings follow suit by breaking out of their shells at different times. This  “hatching asynchrony” was previously thought unique to modern birds.

“In conclusion, oviraptorid dinosaurs exhibited peculiar and unique nesting strategies that are not analogous to those of any modern animal. The peculiar oviraptorid reproductive biology renders a simple dichotomy between a ‘bird model’ and a ‘crocodile model’ to infer behaviors of extinct animals problematic,” write the authors in Integrative Organismal Biology.

The authors conclude that their findings put paleontology one step closer to understanding the life history of these enigmatic animals.