Fossilized dinosaur eggs in the Museum of National History in Ulaanbaatar.

Fossilized dinosaur eggs in the Museum of National History in Ulaanbaatar. These are from the Gobi Desert in Mongolia. Image via Wikimedia Commons.

Dinosaur diaries: The first dinosaur eggs were soft

A new study has found that the eggs of the earliest dinosaurs had soft shells, and others are examining how these ancient animals evolved.

Discover more with the Museum's team of dinosaur experts.

Dinosaur eggs have been found all over the world in deposits ranging widely in age. However, most dinosaur egg fossils are relatively young (geologically speaking).

It has long been thought that dinosaurs laid eggs with hard and rigid shells, like crocodiles, birds and some turtles and geckos. By contrast, lizards, snakes, tuataras and some turtles lay eggs with soft or flexible shells, which are less likely to preserve in the fossil record.

Mark Norell at the American Museum of Natural History and colleagues have examined the chemical composition of many different eggshells, from a range of living and extinct reptiles. Surprisingly, they found for the first time that the eggs of two dinosaurs, Protoceratops and Mussaurus, were soft-shelled.

The team also examined how dinosaur eggs evolved through time, and believe that early dinosaurs laid soft-shelled eggs, then a change to hard-shelled eggs evolved later and independently in each of the three main dinosaur groups.

A challenge when looking at any dinosaur egg is to deduce who laid it. We can only be sure of the parent if an embryo is preserved inside that can be positively identified. In this case the Protoceratops and Mussaurus eggs both had preserved embryos. 

A model of a nest and dinosaur hatchlings.

Maiasaura is one of the most famous examples of dinosaur nests and parental behaviour. This is a Museum model of what a nest with eggs and hatchlings might have looked like. 

 

However, scientists aren't usually that lucky. Fossils are usually small fragments of eggshell and identifying the potential parent is difficult. This lack of fossils makes research difficult, as there just are not enough eggs with embryos preserved, and time will tell if these new results on dinosaur eggshell evolution hold up.

Eggshell evolution aside, the fact that dinosaurs laid soft-shelled eggs is interesting in itself. Soft-shelled eggs do not offer the same level of protection as a hard-shelled egg. They can't be laid out in the open as they would dry out or could be crushed by their parent. This gives an indication that some dinosaurs probably buried their eggs and left them to get on with it. This new work might also explain why the early part of the dinosaur fossil record is poor when it comes to eggshell - soft-shelled eggs just don't preserve as well as hard-shelled eggs.

  • Read the paper in the journal Nature.
This painting by Donald E. Davis shows an asteroid slamming into tropical, shallow seas.

This painting by Donald E. Davis shows an asteroid slamming into tropical, shallow seas. The aftermath of this immense asteroid collision is believed to have caused the extinction of the dinosaurs and many other species on Earth. Image via Wikimedia Commons.

 

Were dinosaurs already decreasing in diversity before the end of the Cretaceous?

A massive extinction event 66 million years ago killed the dinosaurs and a variety of other animals that lived alongside them. But some studies have suggested that dinosaurs were already in decline by then, and the meteorite that hit Earth only served to finish them off.

Measuring how the diversity of extinct animals has changed through time is fraught with difficulty. One problem is that time needs to be divided up so that data can be compared from one slice of time to the next, but how we do this is controversial.

In a recent study, Chris Dean, Ale Chiarenza and the Museum's Dr Susannah Maidment proposed a new method of dividing time, by using information from the rock record. They tested their new method on 66 million-year-old dinosaurs of the Western Interior region of North America. This period of time is known as the Late Cretaceous.

During this time, the Western Interior (an area from Alberta in the north down to New Mexico in the south) was flooded by a vast, shallow sea. Some of the most iconic dinosaurs inhabited the sea's lush shores: Tyrannosaurus and Triceratops are two examples. The team investigated how dinosaur diversity changed in the run-up to the end-Cretaceous extinction.

They discovered that diversity in Western Interior dinosaurs did decrease before the meteorite hit, but only from about 68 million years ago, later than some previous studies have suggested. At that time, a large inland sea called the Western Interior Seaway was shrinking, which might indicate the two things were linked.

Species once separated from each other could have come into contact as the water disappeared and started to compete for resources. Or it could be that as the seaway withdrew, the types of rocks being deposited were less likely to preserve fossils.

While this new method allows us to pin down the timing of the dinosaurs' demise, it seems likely the decline seen was a regional pattern, and probably wasn't experienced by dinosaurs outside of North America.

A new insight into dinosaur evolution

Palaeontologists spend a lot of time attempting to work out how extinct animals are related to each other and to those alive today. They do this by looking for similarities in animals' bodies. For example, birds share feathers with non-bird dinosaurs, suggesting they are linked together.  

Compiling sets of these features, palaeontologists can build branching evolutionary trees, called cladograms, which are like our own family trees. Cladograms can also be used to understand what drove the evolution of a group as whole. They can provide insights into many aspects of a group's history, including how quickly they evolved, whether they were particularly prone to extinction and how their different specializations appeared.

Nicholas Crouch, of the University of Chicago, used a range of different dinosaur cladograms to answer some fundamental questions about how the group evolved. He was particularly interested in the factors that might control the rate of dinosaur extinction.

It is often thought that groups of animals and plants which are evolving quickly should be less likely to become extinct. This is because they can evolve new adaptations to deal with environmental crises, changes in resources or the appearance of new predators much faster than groups that are evolving more slowly. So, in theory, we might expect that high evolutionary rates should be associated with low extinction rates.

Crouch found something different: the dinosaur cladograms revealed that rates of evolution and extinction were unrelated in these animals. This suggests that the extinction rates of dinosaurs are not due to their inability to 'make' new adaptations, but that other factors must be at work. It's not currently clear what these other factors might be, but one possibility is that changes in dinosaur ecology might have been more important in driving some aspects of their evolutionary history than the ability to make new adaptations.

Discover dinosaurs

Find out what Museum scientists are revealing about how dinosaurs looked, lived and behaved.