Rare egg-in-egg fossil offers insight on dinosaur reproduction

The evolution of dinosaurs, whose pieces are spread across the world and raked over by scientists for many decades, is an enormous puzzle. A recent discovery in the Dhar district of Madhya Pradesh in India offers a new thread on dinosaurs’ reproductive ecosystem.

Researchers from Delhi University (DU) recovered a egg belonging to Titanosaur, a herbivorous dinosaur of the sauropod group, a rare example of an egg within an egg — known as ovum-in-ovo.

The abnormal fossil, dating back to the late Cretaceous period, was found in a nest alongside nine regular eggs, near the village of Padlya, close to Bagh town, in Dhar,

The find was embedded between two adjoining sandy limestone rocks, which had eggshell fragments on their surface, signposting the possibility of a nest.

“The probability of this egg being ovum-in-ovo appeared small, since no such pathological egg had been reported in any dinosaur,” said Harsha Dhiman, the study’s lead author. But, by comparing the other eggs in the clutch, the researchers were able to elucidate its characterizing features.

The fossil of the ovum-in-ovo displays two continuous and circular eggshell layers separated by a small gap — similar to bird pathology. In birds, an antiperistalsis movement pushes one smaller egg within a larger egg, set apart by albumen or yolk.

Researchers believe their discovery is a step forward in understanding the evolutionary link between reptilian and avian species. Birds are closely related to dinosaurs.

The study¹ suggests that dinosaurs’ reproductive biology was similar to their immediate cousins, archosaurs (crocodiles and birds), rather than turtles or lizards.

It was assumed that dinosaurs had an unsegmented oviduct tract — a passageway from an ovary in animals. This reproductive pathology has also been found in other reptiles. Birds and crocodiles, however, have segmented oviducts.

The discovery opens the possibility that Titanosaur had an oviduct morphology similar to birds or crocodiles, and evolved to adopt the egg-laying feature associated with birds. Birds lay eggs one at a time, while crocodiles release all eggs together.

The ovum-in-ovo condition is rare even among birds. The authors say this abnormality could be a result of some external stress such as overcrowding, illness, food crisis, lack of suitable nesting habitat, sudden climatic changes, or other catastrophic events, like floods, volcanism or drought.

Some parts of Madhya Pradesh, including Manawar, Bagh, and Kukshi areas in the Dhar district, feature marine and terrestrial rock belts known as the Bagh and Lameta Beds. Scientists have previously uncovered shark and marine mollusc remains, sauropod bones, dinosaur nests and tree fossils from the Dhar region.

Titanosaurus indicus eggs were also found in Jabalpur, Madhya Pradesh,

Titanosaurs were globally distributed giants, and the last surviving group of the sauropods during the Cretaceous Period. On a field trip in 2017, this team explored the Upper Cretaceous Lameta Formation. “During the course of the work we identified about 92 sauropod egg clutches comprising 256 eggs,” said Guntupalli V. R. Prasad, a paleontologist at DU’s department of geology, who co-authored this study.