Fossilized dinosaur eggs: Nature’s way of telling dinosaur stories

Thanks to the process of fossilization, we can study dinosaur eggs, even though they were laid millions of years ago. But how exactly are fossils made, and how did the dinosaur eggs become fossils?

Fossils are formed as a result of sedimentation – small particles (sediment) of rock start covering the organic material, in our case, eggs. Over time, as more sediment layers build up on top, the bottom layers around the eggs begin to compact and become hard rock after thousands of years. Millions of years later, these ancient soils (called paleosols) are sometimes revealed because of erosion. In these paleosols, clusters of fossilized dinosaur eggs can be found. 

Dinosaurs used to lay eggs in many places, including ancient floodplains, dunes, and beaches – the places where most egg fossils have been found. It is highly unlikely that you’ll find a dinosaur egg on your nearby beach, but still, you can almost feel their ancient presence in these places.       

When were the first fossilized dinosaur eggs discovered and recorded?

It all started with the series of Central Asiatic expeditions of the American Museum of Natural History. These expeditions were led by Chapman Andrews in the 1920s. The first of these expeditions left New York in march 1921 to sail to China. On this expedition, the group found some fossilized eggshells in the Gobi in southern Mongolia, along with many other mammal and dinosaur fossils. It wasn’t until the following year (1923), when the group returned, that whole eggs were found, along with eggs that were part of nests.

When Andrews returned, everyone wanted to know about the dinosaur eggs he had found. Although scientifically recorded for the first time, this wasn’t the first time someone had found fossilized dinosaur eggs. 

Jean-Jacques Pouech had found dinosaur eggshells decades before Andrews in 1859. But, he didn’t call them dinosaur eggs, and in fact, thought they belonged to a giant bird; therefore, no one gave the discovery any importance. In 1989, after having read the accounts of Pouech’s discovery of eggshells, French paleontologists Eric Buffeataut and Jean Le Loeuff retraced Pouech’s steps and found the original site of his findings. They confirmed that the fragments truly were dinosaur eggshells. When they came back, Buffetaut and Le Loeuff published their findings in a 1994 paper and set the record straight. 

A decade after Pouech’s find, the geologist Philippe Matheron also discovered eggshells, and also in the Cretaceous stratum of southern France. Matheron hypothesized the eggs had been laid by a giant crocodile, and it wasn't until the Gobi Desert find in 1923 that they were only reinterpreted as dinosaur eggs.

A timeline of the important early discoveries

Now, let's take a look at the timeline of some dinosaur egg discoveries. Starting from Southern France and the Pouech discovery in 1859.

• In 1859, Jean-Jacques Pouech finds dinosaur eggshells in the foothills of the Pyrenees in southern France but believes them to be bird eggs.

• In 1869, Philippe Matheron, a French geologist, found eggshell fossils in the same general region (Southern France).

• In 1913, Charles W. Gilmore discovered the first dinosaur eggshells in the United States.

• In 1923, the expedition group of the American Museum of Natural History discovered fossilized dinosaur eggs in Mongolia and W. Granger, a paleontologist in the group, recognized them as dinosaur eggs.  

• In 1957, M. Sahni discovered the first dinosaur eggs in India.

•  In 1969, A.H. Muller reported fossilized eggs from Muschelkalk, Germany.

• In 1975, Chinese paleontologist Zhao Zhi-Kui developed a system for the classification of fossilized eggs.

• In 1979, the first whole dinosaur egg in North America (an area now known as Egg Mountain, Montana) was discovered by student Fran Tannenbaum on an expedition led by paleontologist Jack Horner.

• In 1987, teenage fossil enthusiast Wendy Sloboda discovered dinosaur eggshell fragments along the Milk River Ridge in southern Alberta, Canada. A team sent from the University of Calgary found Canada’s first dinosaur nesting site, including fossilized nests, eggs, and embryonic remains of Hypacrosaurus stebingeri, a new species of hadrosaur.

• In the 1990s’, Konstantin Mikhailov, a Russian paleontologist, published research about Mongolian fossilized eggs using Zhao’s system. This work was translated into English, bringing a wider awareness of Zhao’s work.

At this point, fossilized dinosaur eggs were a hot topic worldwide. You can almost guess what must have happened next. In 1993, Hollywood released the film "Jurrasic Park" (based on a book written by Michael Chrichton), and, to no one's surprise, it was a hit. To quote a joke from the movie, “What do you call a blind dinosaur? A do-you-think-he-saurus?” The thrill of the dinosaur movie and its many sequels, humor, and one-liners, like “life finds a way,” are still popular today.

How old are fossilized dinosaur eggs?

Scientists use sophisticated methods, like radiometric dating, to determine the age of a fossil. Based on fossil discoveries and their ages, scientists have classified the age of dinosaurs into three main categories: Triassic (252-201 million years old), Jurassic (201-145 million years old), and Cretaceous (145-66 million years old).

Where did dinosaurs live?

No, the dinosaurs were everywhere, and their fossils have been discovered all over the world. Just before the start of the millennia (2000), the number of dinosaur fossil discoveries, of both fossilized eggs and bones, had already reached a staggering number. You can see that from the maps shown below. They show the locations where the dinosaur fossils were found, all before the year 2000. 

Dinosaur fossils have been found on every continent except Antarctica, although most of the dinosaur fossils and the greatest variety of species have been found high in the deserts and badlands of North America, China, and Argentina. 

During the Cretaceous period, Antarctica was ice-free and contained forests. Scientists believe that dinosaurs roamed there, but most of their remains, including any fossilized eggs, are buried too deep under the ice to reach.

Why study fossilized dinosaur eggs?

Scientists study these fossilized eggs to understand more about how dinosaurs reproduced. More importantly, the structure of the eggs and the very rare finding of a fossilized embryo inside an egg can help scientists answer questions about the ways that dinosaurs lived and could also provide information about how some birds evolved. 

This brings us now to the question of which one, reptiles or birds, is more closely related to dinosaurs.

Birds are not just related to dinosaurs; they ARE dinosaurs!

Today, it is a well-established and widely accepted theory that birds evolved from dinosaurs. Some dinosaurs had already developed feathers a hundred and fifty million years ago. Birds and dinosaurs – the debate is over; an article published in Nature in 1998 discusses the impact of two dinosaur fossils discovered in China. Both discoveries were new species of dinosaurs that had feathers but lacked wings. The article's authors argue that their discovery effectively closed the debate on whether or not birds and dinosaurs share a close evolutionary heritage.

Today, most of the research and debate on the relationship between birds and dinosaurs centers on the evolutionary details. The evolution of birds can be understood from three kinds of studies:

1- Structure and composition of the eggshells,

2- Nesting behavior in dinosaurs,

3- The study of the preserved embryo inside the egg. These studies tell us a lot about the evolution of modern birds.

Evaluation of the fossil evidence, including eggs, has shown features common to birds, such as feathers, actually began to emerge in dinosaurs long before the evolution of birds. This indicates that birds evolved by adapting these features to new use, such as flight. This would have occurred over a long period of time and included a shrinking in size, the evolution of the wishbone, more complex feathers, wings, and flight. 

Paleontologists believe that one key to this is rapidly shrinking – from large dinosaurs to smaller birds – was a necessary precursor to flight. In fact, it has been found that birds closely resemble dinosaur embryos – which could not have been determined before the discovery of fossilized eggs.

So did chickens evolve from dinosaurs? The answer is a resounding YES!