Science & Technology

‘Herbivorous dinosaurs were closer to birds than reptiles’

Down To Earth speaks to GVR Prasad, palaeontologist and professor in the Department of Geology at the University of Delhi, on his team’s discovery of an abnormal dinosaur egg in Madhya Pradesh

 
By Varun Kheria
Published: Monday 25 July 2022

Couple of Brachiosaurus altithorax and a flock of Pterosaurs in a scenic Late Jurassic landscape. Photo: iStockCouple of Brachiosaurus altithorax and a flock of Pterosaurs in a scenic late Jurassic landscape. Photo: iStock

The journal Scientific Reports recently published a research article about the discovery of an abnormal dinosaur egg in the Upper Cretaceous (67 million years old) Lameta Formation of Padlya of Dhar district, Madhya Pradesh.

The team found the abnormal titanosaur egg which exhibited an ovum-in-ovo pathology.

Ovum-in-ovo pathology refers to a situation where an egg forms outside an already existing egg, essentially resulting in an egg within an egg.

This discovery suggests that sauropods (the group of dinosaurs the titanosaurids belongs to) might have had a reproductive biology similar to that of birds today, which opens up the possibility for sequential laying of eggs as it is the case in birds.

Down To Earth spoke to GVR Prasad, an accomplished palaeontologist and a professor in the Department of Geology at the University of Delhi regarding the importance and significance of this discovery. Edited excerpts:

Varun Kheria: There are a lot of places in your article where you have referred to the ovum-in-ovo feature as a pathology. Do you think it does not occur commonly?

 

GVR Prasad: Actually, it is a kind of abnormality that is not found in normal eggs. It happens only when the animal is distressed.

Suppose if there’s overcrowding leading to competition for space and food resources, lack of suitable sites for egg laying, extreme events like floods or droughts, high or low temperatures, anything that puts the animal under stress.

It can also be related to sickness or in capacitation to old age. So, during these stressed conditions, the animals develop abnormal eggs. That is what happens even today with most animals.

For example, chickens normally lay eggs every day. But someday, if they undergo stress, they do not lay eggs for one or two or several days.

VK: What does sequential laying of eggs mean?

GVRP: Eggs are formed in one place (anterior part of oviduct) and then they go to the uterus from where they are released.

The uterus is where eggs get their outer calcareous shell over the organic shell membrane that was formed before.

In the case of reptiles like turtles and lizards, the membrane formation and shell formation take place simultaneously for all the eggs in the uterus and they are then released together in a clutch.

In case of birds, the different layers of the eggs are made in different regions of the oviduct and are laid one at a time. That is sequential laying of eggs, one at a time.

VK: How distinct was the fossil where you discovered this pathology?

GVRP: They were nearly complete. As you can see in the image, the entire shell is intact, except for small fragmentations. You can see two distinct shell layers, one inside another, with a gap in between the two shell layers.

This gap possibly represents the space for the yolk. This means the inner egg formed first and another egg formed around it.

We took a thin section of the shell from the fossil to see whether it is a dinosaur egg or not. We found that the structure is very similar to what we have found on other eggs of the nest.

It clearly shows that this (the egg with the ovum-in-ovo pathology) is a titanosaurid dinosaur egg.

VK: Do you think dinosaurs other than the sauropod type could exhibit this type of pathology or behaviour when they are in stressful conditions too?

GVRP: We have two groups of dinosaurs, the herbivores and the carnivores. We have evidence from Mongolia which tells us that some carnivorous dinosaurs used to incubate their eggs.

Their skeletons were found sitting over their eggs in the nest primarily to incubate the eggs. It has been interpreted that they were doing the same thing that birds do today, brooding the eggs in their nests.

Thus, it is said that the carnivorous dinosaurs had similar reproductive function as present-day birds. And now we have evidence for sauropods having biology similar to that of present-day birds as well.

VK: Initially, people had the perception that dinosaurs were closer to reptiles than to birds. Does this discovery add to the evidence that the opposite was probably true?

GVRP: You can still consider dinosaurs to be reptiles because they were part of the reptilian group. But if you look at the evolutionary tree, you see how they evolved.

Birds had actually evolved from dinosaurs, specifically from carnivorous dinosaurs. Until now, it was accepted that carnivorous dinosaurs were closer to birds.

Herbivorous dinosaurs were thought to have reproductive biology similar to turtles, lizards and crocodiles. But this evidence shows that their biology is closer to that of birds as well.

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