"Rock" Containing Agate Turns Out To Be 66-Million-Year-Old Dinosaur Egg

A mineral specimen that's been in the collections for 175 years has turned out to also be a dinosaur egg, collected long before these were first scientifically recognised and potentially one of the first complete eggs ever found.

The egg and where it was dug up also hold clues as to how the biggest dinosaurs ever to exist would have nested.

In 1883 an agate specimen was registered in the Museum's Mineralogy Collection.

The agatized dinosaur egg.
Photo: The Natural History Museum, London

Collected in central India, the mineral measuring around 15 centimetres across was notable for its almost perfectly spherical shape and beautiful light pink and white banded interior. But until recently, the specimen was not thought to hold much other significance.

Because of its aesthetic beauty, the agate was selected to go on display in the Museum's Membership Rooms in 2018, with Robin Hansen, one of the Mineral Curators, helping to prepare it. But it was only a few months later when Robin visited a mineral show in France that she realised the importance of the specimen.

'While I was looking around the show, a dealer showed me an agatised dinosaur egg, which was spherical, had a thin rind, and dark agate in the middle,' recounts Robin. 'That was the lightbulb moment when I thought: "Hang on a minute, that looks a lot like the one we've just put on display in the Museum!"'

Excited by this hunch, Robin took the mineral specimen to Museum dinosaur experts Professor Paul Barrett and Dr Susannah Maidment.

On closer inspection, they both agreed that the specimen was about the right size and shape, and that the thin layer around the agate looked like a shell. There was another tantalising clue in that two other large, spherical objects had once been clustered close to this one.

While the team tried to delve deeper by using a CT scanner, the density of the agate meant that it was simply impossible to see any finer details.

But based on the knowledge of where the specimen was collected, its age at about 60 million years old and its general features, they are pretty certain that it is a dinosaur egg. The size, shape and surface features of the eggshell are consistent with those of titanosaur eggs from China and Argentina. The most common dinosaurs living in India at the time it was laid were titanosaurs, which suggests that it must be a titanosaur egg.

'This specimen is a perfect example of why museum collections are so important,' explains Robin. 'It was identified and catalogued correctly as an agate in 1883 using the scientific knowledge available at the time.'

'It is only now that we have recognized that this specimen has something extra special - the agate has infilled this spherical structure, which turns out to be a dinosaur egg.'

Robin decided to trace back the origins of the specimen. She found that it was collected by a Charles Fraser, who lived in India between 1817 and 1843. These dates are significant. It means that it was collected at least 80 years before dinosaur eggs were first scientifically recognised, and potentially before the word 'dinosaur' even existed.

Dinosaur eggshells have likely been used unknowingly by humans for thousands of years, and as reptiles it was long suspected that dinosaurs laid eggs. But it was not until 1923 that scientists first confirmed this for certain when entire nests were uncovered in Mongolia.

The egg can currently be seen on display in the exhibition Titanosaur: Life as the Biggest Dinosaur, alongside a model that can be touched by visitors, created from a 3D scan of the specimen.

Why were titanosaur eggs so small?

Growing up to 37 metres long and weighing somewhere in the region of 57 tonnes, titanosaurs were truly massive animals. Because of this, they had all sorts of adaptations to support themselves and survive on land.

But one of the most curious features of these gigantic creatures was the size of their eggs. Despite being the largest living land animals ever to exist, they laid surprisingly small eggs.

'It seems really weird because these would've been huge animals,' explains Paul, 'but what they were doing instead is laying a lot of eggs. Many living animals we know use this trade off, in which they either invest in a small number of larger eggs or a large number of smaller eggs.'

'It looks like titanosaurs adopted a strategy of laying large clutches of about 30 or 40 smallish eggs.'

This means that rather than reproducing like a blue whale or an elephant, which give birth to a single large baby after a prolonged pregnancy, the titanosaurs were reproducing more like sea turtles or crocodiles.

In this case, Paul suggests that the titanosaurs - and dinosaurs in general - had advantages over large mammals in that they didn't have to carry young to full term.

'Dinosaurs have the advantage that they just can just lay a lot of eggs in one go very quickly, and could probably lay several of those really large clutches a year,' says Paul. 'It looks like they just laid a lot of eggs and hoped that some of them made it to adulthood, rather than laying one or two that they then had to invest a lot more parental care in when hatched.'

How did the titanosaur egg form?

At the time this egg was laid, the world was a vastly different place. During the Late Cretaceous (100-66 million years ago), India was a massive, continent-sized island drifting across what is now the Indian Ocean on its collision course with Asia.

The island is interesting in that, while it was full of creatures like crocodiles, mammals, turtles and lizards, the diversity of dinosaurs on it was incredibly limited.

'As far as we know, the dinosaur fauna of Cretaceous India was abundant but not particularly diverse,' explains Paul. 'There are lots of titanosaur fossils and there's evidence for a number of different predatory dinosaur groups. But there's no confirmed evidence for any bird-hipped dinosaurs at all - so no ankylosaurs, no horned dinosaurs, no ornithopod dinosaurs.'

'It looks like either they never made it to India before it split off from the rest of Gondwana, or they did make it and became extinct in India for some reason. We don't know why, but they're just not there.'

The environment at the time on the island would likely have been highly seasonal. But the most significant aspect dominating large parts would have been a massive amount of volcanic activity.

'A major area of central western India is called the Deccan Traps,' explains Robin. 'This is a massive basaltic flood plain where there was lots of volcanic activity with huge lava flows that covered the whole area.'

'In terms of minerals, there are many that are found in geodes in the Deccan Traps. This is where there was a gas bubble in the lava that then - as the rock solidified around it - became a cavity. These cavities were later infilled with different minerals, including agate.'

Intriguingly the fossils of titanosaurs have been found in between these layers of basaltic rock. This suggests a cycle, in which after the volcanoes erupted and spewed lava over the landscape, the titanosaurs returned and recolonised the area, before more volcanic activity covered the land once more.

It's been suggested there was a reason that the dinosaurs continuously returned to this area of volcanic activity time and again. Once more, the egg agate could be a clue.

'My understanding is that palaeontologists assume that the ground would've been warm, which would've been perfect for laying eggs in,' says Robin.

This would tally with the way in which these massive dinosaurs likely incubated their eggs. Despite laying lots of eggs, it would have been impossible for the animals to actually sit on them to keep them warm. Instead, it seems they were relying on the toasty volcanic soils.

This would also help explain how the egg agate formed. It's possible that shortly after a titanosaur laid its eggs in the warm sands, a nearby volcano erupted. The debris and lava it ejected would have smothered the landscape, including the unfortunate nest.

This volcanic rock would have then solidified, with the egg remaining intact within. After all the internal structures and embryo rotted away, silica-rich water must have repeatedly percolated through the rock and the shell of the egg. This filled the void, creating the banded agate specimen that was eventually dug up tens of millions of years later.

The above post is reprinted from the Natural History Museum.