The most detailed look inside the world's oldest dinosaur eggs

In 1976 a clutch of dinosaur eggs was unearthed in South Africa.

They were laid by a species known as Massospondylus roughly 190 million years ago, making them a good contender for the oldest dinosaur eggs in the world.

The embryos' size suggested that the dinosaurs were likely close to hatching before they were rapidly buried and fossilised.

A new study has looked again at these embryos in unprecedented levels of detail to reveal that this was unlikely to be the case and that they were far earlier in development.

Dr Vincent Fernandez, the micro-CT scanning manager at the Museum, worked with South African colleagues Dr Kimberley Chapelle and Dr Jonah Choiniere to conduct the scanning and imaging of the specimens.

They were able to scan the embryos using high powered X-ray beams, before Kimberley painstakingly pieced the images back together to create one of the best 3D models of a dinosaur embryo skull seen to date.

'The biggest surprise was that these embryos were so young,' says Vincent. 'It was originally thought that they were nearly about to hatch because the bones look quite well developed, but we now estimate that they were only about 60% through their incubation period.'

Not only that, but the detail in the scanning has also revealed for the first time in dinosaurs that the embryos produced two different types of teeth while still developing in the egg, before one set was lost just before hatching.

An ancient dinosaur nesting ground

Well known from the rock formations of Southern Africa, Massospondylus was one of the first large species of dinosaurs to evolve.

Measuring around six metres and weighing in at about one tonne, they were barrel chested bipedal dinosaurs with a long slender neck and small boxy head. Appearing towards the end of the Triassic around 200 million years ago, they are part of a group known as sauropodomorphs, the early ancestors to the gigantic sauropods of the Cretaceous.

Their fossil bones are common in South Africa, but one site in particular has preserved even more.

The Golden Gate Highlands National Park is located in the Free State, just north of Lesotho. It was here that the first clutch of Massospondylus eggs was discovered in the 1970s. Since then, at least 10 other nests of the dinosaur have been found containing up to 34 eggs in each clutch, from different layers in the rock. 

This indicates that this region may once have been a well-used nesting ground for Massospondylus parents over multiple generations.

Studies of the embryos found within the dinosaur eggs show that while the adults were bipedal, the young had vastly different body proportions and probably started off on all fours. As the animals grew, their proportions changed and they shifted into more of a two-legged stance.

What was happening inside the eggs as the embryos developed, however, is understandably harder to ascertain.

'It is difficult to scan embryos because their bones are not very well ossified so their density is very similar to the surrounding material,' explains Vincent.

By taking the fossil to the European Synchrotron Radiation Facility in Grenoble, France, the team were able to get scans of the embryos down to level of individual bone cells.

Growing teeth inside dinosaur eggs

These scans have given the researchers an unprecedented view of the developing dinosaur embryos.

In order to figure out just how close to hatching they may have been, Kimberley compared them to bird eggs, as well as embryos of crocodiles, lizards and turtles. It was from doing this that she could work out that the dinosaurs in the eggs were far younger than previously thought.

But the scans also revealed something else.

'Dinosaur embryo skulls appear to develop in the same order as those of today's crocodiles, chickens, turtles and lizards,' says Kimberley. 'I was really surprised to find that these embryos not only had teeth, but they had two types of teeth.

'The teeth are so tiny, smaller than the tip of a toothpick.'

The dinosaur embryos had something known as 'null-generation' teeth. Seen in crocodiles and lizards such as geckos, this is where the first set of teeth grown by an embryo are shed while still within the egg. 

'Adult crocodiles and lizards replace their teeth all the time, and so did dinosaurs,' explains Vincent. 'But they even do this when they are developing. So when a crocodile hatches it has already replaced its teeth maybe twice while still inside the egg.

'We were able to see in the dinosaur embryos two types of teeth, ones that look like those of the adult and another present in the embryo that were lost before hatching.'

This is the first time that it has been shown that dinosaurs produced these primary sets of teeth while still in the egg before reabsorbing them and replacing them with their usual set.

It means that these dinosaurs were developing inside their eggs in a very similar way to their reptilian relatives, a process which hadn't changed for hundreds of millions of years. It suggests that this may have therefore been true for other species of dinosaurs.

The next steps will be to look at the Massospondylus embryos in more detail, as well as extending the research to different dinosaur species.

'We're hoping to look at some other dinosaur embryos and compare their development,' says Kimberly. 'We're also looking at the rest of the Massospondylus embryo skeletons and trying to determine the incubation period so we know how long they were in the eggs for.'

The research has been published in Scientific Reports.