An egg inside an egg, known as ovum-in-ovo, is something that’s sometimes seen in birds such as chickens whose specialized uteruses are more prone to the magic trick compared to those of reptiles. However, in a first for dinosaurs – and, in fact, reptiles in general – researchers have uncovered a fossilized egg within a fossilized egg: a truly eggceptional find.

The ovum-in-ovo comes from a titanosaurid dinosaur, revealing clues into their oviduct morphology, which was probably more similar to that of birds which are well adapted for the sequential laying of eggs. This could indicate that this group of sauropod dinosaurs were also capable of popping out egg after egg, like their extant relatives.

The “rare and important find” comes from the Upper Cretaceous Lameta Formation in Madhya Pradesh, western Central India, and is detailed in a paper published in the journal Scientific Reports. Researchers on the paper uncovered the egg in Dinosaur Fossils National Park Bagh, where they found 52 nests of titanosaur sauropods near Padlya village.

One of the sauropod dinosaur nests contained 10 eggs including our egg-in-egg superstar, which was found to contain two layers of shell. The significance of this ovum-in-ovo is what it infers about these dinosaurs’ reproductive strategies and the morphology of the associated organs.

Reptiles like sea turtles lay their eggs in one fell swoop from a generalized uterus, depositing up to hundreds of eggs in a single session. Conversely, birds have a specialized uterus that lays one egg before getting to work on the next, which is believed to be connected to the formation of these deformed double eggs.

Crocodiles have specialized, segmented uteruses more similar to those of birds but stick to the reptilian method of making and releasing all the eggs in one go. So, what does our fossilized ovum-in-ovo tell us about sauropods?

“The discovery of ovum-in-ovo egg from a titanosaurid dinosaur nest suggests that their oviduct morphology was similar to that of birds opening up the possibility for sequential laying of eggs in this group of sauropod dinosaurs,” concluded the study authors.

“This new find underscores that the ovum-in-ovo pathology is not unique to birds and sauropods share a reproductive behavior very similar to that of other archosaurs.”

While the authors urge that further research is needed to draw firm conclusions, the surprise egg-in-egg discovery demonstrates that this oological quirk is not unique to birds. It also suggests that the sauropods had more in common with birds and crocodiles than they did the non-archosaurian reptiles, making it a golden nugget for evolutionary biology as well as a world-first find.

“The present discovery of an ovum-in-ovo pathological egg in a titanosaurid dinosaur nest is the first of its kind in dinosaurs and demonstrates its presence in reptiles as well,” wrote the authors.