Do All Dinosaurs Hatch From Eggs? What We Know About Dino Eggs


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Movies like Jurassic Park have shown us how big dinosaurs can really get, and it can be a bit tricky imagining them as young creatures freshly hatched from eggs. Science tells us that dinosaurs were closely related to modern reptiles and birds. Both of these groups of animals reproduce by eggs, and they are called oviparous. They are also distant relatives to snakes, turtles, and sharks. However, some of these animals also reproduce by live birth, for instance, like sharks. Additionally, there are different kinds of eggs – some are hard-shelled, while others are soft or leathery. 

There are millions of species of animals. In the earth’s natural history, the Mesozoic era lasted some 180 million years, and it was during this time when the majority of the dinosaurs existed. With all these different reproduction methods of animals, it makes me wonder about dinosaurs.

So, do all dinosaurs hatch from eggs? Paleontologists have discovered fossilized eggs and even eggs with dinosaur embryos intact, which is evidence that dinosaurs hatched from eggs. Whether every genera and species of dinosaur hatched from eggs is something that no one can say for sure. Live birth evidence in some marine reptiles from the Mesozoic era have been found, but also evidence that indicates that the majority of therapod dinosaurs laid eggs, including the giant sauropods and apex predators like T. Rex and Allosaurus.

In this article, you’ll know about why all dinosaurs are believed to have laid eggs. I’ll also cover the known egg-laying dinosaurs such as the sauropods, theropods, and hadrosaurs. You’ll also read on discoveries related to fossilized dinosaur eggs, such as their characteristics and the smallest and the biggest fossilized dinosaur eggs that have been discovered.

Because modern birds, reptiles, and other animal species laid eggs, we will briefly examine their behaviors, such as how bird eggs are hatched. Along with this, you’ll also get to know the nesting habits of egg-laying dinosaurs.

How Dinosaurs Give Birth

Although I’ve already mentioned in the previous section that dinosaurs are believed to lay eggs, let’s delve into this topic more. Dinosaur reproduction is one topic that has accelerated because the number of dinosaur eggs being discovered has increased. Scientific technology to examine fossils (such as CT scans) enables us to study fossilized eggs better.

Why It Is Believed That All Dinosaurs Laid Eggs

Up to this day, there is no evidence yet that there were dinosaurs that are viviparous. Viviparous creatures are capable of developing embryos inside their bodies, eventually leading to live birth. On the other hand, numerous documentations show that most dinosaurs actually laid eggs ages ago. 

The Evidence Of Egg Fossils In Nesting Sites

The earliest documented discovery of a dinosaur eggshell dates back to 1859. However, it was still unusual for paleontologists (scientists who specifically study fossils) to unearth fossilized eggs and young dinosaurs’ bones. 

During the 1980s, numerous fossilized egg discoveries were made in different areas such as the USA, France, Spain, Mongolia, China, Argentina, and India. In these discoveries, signs of nesting and brooding similar to bird-like parenting behavior were documented. Of specific interest were the nesting grounds of the Maiasaurus found in Montana. 

Additionally, paleontologists discovered and used new technology to study dinosaur eggs. New technologies proved useful when examining, for instance, Protoceratops eggs in Mongolia that have a dinosaur embryos intact or fossilized baby Protoceratops dinosaurs. The fossils are from the Late Cretaceous and were found in the Gobi desert. 

Modern Descendants Of Dinosaurs All Lay Eggs

It might not be physically obvious, but birds and reptiles are closely related to dinosaurs, if not modern descendants. Birds descended from feathered dinosaurs that survived the massive dinosaur extinction 65 million years ago. 

Reptiles, specifically crocodiles, came from the same ancestors as dinosaurs. These modern-day animals all lay eggs, and it’s a safe assumption that their closest ancient relatives, which are dinosaurs, have laid eggs also. Fossil evidence supports this.

YouTube Video About Fossilized Dinosaur Eggs and Egg Embryos

YouTube video by “Amazing Facts” that details examines new research on dinosaur eggs found in the Gobi Desert as well as some interesting results from CT Scans of dinosaur egg fossils

Interestingly, scientists suggest that herbivorous dinosaurs laid more roundish or oval eggs, and carnivorous dinosaurs had long oblong or ellipse-shaped eggs. It was also seen in the nests of sauropods, hadrosaurs, and carnivorous theropods.

Egg-laying Dinosaurs

The discovery of various fossilized eggs from dinosaurs is confirmation that all dinosaurs must have reproduced by laying eggs. Some dinosaurs that were verified to be the sources of these fossil eggs are the sauropods, theropods, and the hadrosaurs. 

Many of the dinosaur eggs fossilized in nests were just broken shells; however, there were also some unhatched dinosaur eggs. Sometimes the eggs were completely unbroken, and other times they were cracked but still held the shape of an egg.

Gigantic Sauropods

Sauropods are the largest dinosaurs that ever existed, known for both long necks and tails. Their fossilized eggs, which are oblong or football-shaped, are discovered in many locations like India and South America. Due to their enormous height, scientists believe that they had an extended tube from their bodies to lay their eggs to the ground safely. The famous sauropods, due to their enormous height and length, are, for instance, the Diplodocus and Brachiosaurus.

Some of the earliest dinosaur eggs discovered was in France in 1846. The fossil eggs were linked to the Hypselosaurus, a sauropod with partial fossil skeletal remains found in the same area as the eggs. What’s unusual about these eggs, which were not even recognized as dinosaur eggs for decades after discover, is that they were really large, about 1 foot (0.30 meters) in diameter. (Source)

Interestingly, the Brachiosaurus and Diplodocus’s fossilized eggs are found in a straight line indicating that these dinosaurs might have laid eggs while walking.

Paleontologists have also found Apatosaurus and Brontosaurus skeleton fossils and their eggs in North America. 

Carnivorous Theropods

Theropods are the usual predators during their time due to their flesh-eating nature. They are also the most diverse looking group of dinosaurs. The positioning of egg fossils on their nesting sites suggests that they laid eggs two at a time. Specific theropods, like the oviraptors, have been discovered brooding over their eggs in their fossilized forms. Probably the most famous dinosaur, the Tyrannosaurus rex has no record yet of any fossilized egg. Other well-known theropods are the Oviraptorids, which are initially thought of as egg thieves but then further testing on discovered egg nests with their remains showed that they are just protecting their eggs.

Duck-billed Hadrosaurs

Hadrosaurs are herbivores that traveled in herds during migration. These duck-billed dinosaurs are usually preyed upon by carnivores. They lay eggs near rivers once a year and usually return to the same nesting sites grouped. A distinct hadrosaur is named Maiasaura, which means “good mother lizard.” Fossilized remains of Maiasaurs and their eggs are the first manifestations that some dinosaurs can care for and raise their offspring. Parental care and nesting habits of the Maiasaurus have been studied thanks to an amazing fossil find in Montana. Fifteen nests were discovered in one area. 

The Possibility That Dinosaurs Had Given Live Birth

No actual evidence has been found proving that there are dinosaurs that had given live birth. But there is still a possibility that such an occurrence existed millions of years ago. Given these reasons, you can suspect that some dinosaurs might have given live birth before.

Evidence Of Live Birth In Ancestors Of Dinosaurs

Dinosaurs, along with other animals, descended from a group called archosauromorpha. The Dinocephalosaurus belongs to this same group. In China, a pregnant Dinocephalosaurus fossil was found with an embryo fossil inside its body. It was initially thought of as a result of cannibalism. Later, this was negated after a closer inspection of the embryo position and species.

Adaptation To Aquatic Life

There is a theory that marine dinosaurs would have likely given live birth to produce offspring. Dinosaurs are considered as reptiles, and reptilian eggs must be incubated. The incubation of eggs cannot happen underwater. With the inability to leave the water, marine dinosaurs must have given live birth instead. Plesiosaurs, the long-necked marine reptiles that existed with dinosaurs before, were proven to have given live birth. A fossilized plesiosaur was found in 1987 to have remains of an unborn in its abdominal cavity. It’s not impossible to think that the same ability was present to dinosaurs living underwater.

Dinosaur Egg Sizes And Types

Now that we’ve discussed how dinosaurs produced their offspring, it’s time to know more about various dinosaur eggs’ characteristics. After all, dinosaur eggs are very different from the animal eggs that we encounter nowadays.

Characteristics Of Different Dinosaur Eggs

Dinosaur eggs are different due to various factors such as shell type, size, and color. The classifications of dinosaur eggs are limited because fossilized eggs’ characteristics may have been altered by the environment where they are excavated.

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Types Of Dinosaur Eggs Based On Shells

Dinosaur eggs can be either hard-shelled or soft-shelled. Most of the eggs of the first dinosaurs were soft. It is supported by fossil discoveries of baby dinosaurs surrounded by thin films that matched the soft shell fingerprint similar to modern turtles. (Source)

Although this type of egg is easier to hatch, it offers minimal protection from the outside. The known soft-shelled eggs are Mussaurus eggs, Protoceratops eggs, and the Antarcticoolithus egg (highly suspected to be laid by a dinosaur). The ability to lay hard-shelled eggs is an evolutionary feature for survival after millions of years. The commonly discovered hard-shelled eggs are from the Sauropods. (Source)

Hatching from these shells might take up long periods, usually 3 to 6 months, which probably contributed to the extinction of dinosaurs. It is believed that bird-like dinosaurs have used their beaks for hatching. Unlike their hatching period, baby dinosaurs grow up fast. They double in size after just six weeks.

It’s interesting to note that some paleontologists think that mother dinosaurs would help the hatchling, similar to modern-day crocodiles and alligators. In this case, the dinosaur would pick up the egg in its mouth and crack it open so that the hatchling could easily get out of the egg. (Source)

Egg Size And Shape

Theropods laid elongated eggs while non-theropods, such as the hadrosaurs and sauropods, laid spherical eggs. Eggs of hadrosaurs are believed to grow as big as the large African bull elephant. Although sauropods are the largest dinosaurs, a specific sauropod, the Mussaurus, produced the smallest known fossilized dinosaur egg. Sauropods, in general, laid relatively small eggs compared to their adult size.

Colors Of Dinosaur Eggs

Most fossil dinosaur eggs are colored black and brown due to fossilization, but there is evidence of dinosaur eggs having blue-green eggs. These discovered eggs are from an omnivorous dinosaur, the Heyuannia. Paleontologists claim that birds have colored eggs due to having dinosaurs as ancestors.

Amazing Discoveries Of Fossilized Eggs

Discovering fossilized dinosaur eggs is still rare, and unearthing dinosaur egg fossils of extreme sizes even make it more remarkable.

Fossilized Dinosaur Eggs Are Rarely Discovered

It is claimed that all dinosaurs initially had soft and feathery eggs, which are hard to fossilize. These soft-shelled eggs were easy to deform physically. Aside from this, most dinosaur eggs are eaten by predators. It also takes a hard time determining fossilized eggs because little to no traces of embryos are usually found due to bacterial decomposition.

Remarkable Fossilized Dinosaur Egg Discoveries

In Montana, duckbill dinosaurs’ fossils, including eggs, nests, hatchlings, and adults, were discovered in what appeared like a mass grave in a nesting colony. In 1869, the first and biggest fossilized dinosaur eggs were the Hypselosaurus eggs found in France. The smallest dinosaur eggs found are the Mussaurus eggs, which are about 1 inch across.

About Dinosaur Egg Nesting Habits

It was initially thought that all dinosaurs never attempted to care for their young, but this was debunked through findings based on their fossil remains and fossilized eggs. In this section, you’ll get to know how dinosaurs cared for their offspring, which was probably brought by parental instincts or merely just for survival.

The Survival Rate Of Dinosaur Eggs

The nesting of dinosaur eggs is very crucial to the survival of dinosaur eggs. Long nesting periods, coupled with low egg survival rates during a catastrophic event, might have played an important role in the extinction of dinosaurs.

Causes Of Low Dinosaur Egg Survival

Remains of dinosaur eggs are hard to find, probably because of the low egg survival rates millions of years ago. The long hatching times of these eggs make them highly susceptible to predators’ attacks and being destroyed by floods and drought. Parent dinosaurs who are very protective of their eggs might have died of hunger or predator attacks, leading to decreased egg survival.

Practices By Dinosaurs To Protect Their Eggs

Parental instincts of dinosaurs have led them to do some practices to ensure the safety of their eggs. In southeast Mongolia, nesting sites of theropods seemed to be in colonies, which shows that mother dinosaurs could have joined together to protect their eggs. Some dinosaurs, liked the oviraptors, stay with their eggs to provide protection. Some oviraptor fossils found show that they died while protecting their eggs using their wings. Since most dinosaurs are too heavy to sit on their eggs, spiral rings with central openings are seen in dinosaur fossils that depict eggs’ intricate arrangement to prevent parents from crushing them. Another tactic done by parent dinosaurs is placing many eggs in a single nesting grounds site to ensure that at least one egg survives an attack. (Source)

Nesting Habits Of Dinosaurs

Dinosaurs might have been terrifying creatures, but their nesting habits show that they would have cared for their young.

Theropods

Most of the identified fossilized eggs of theropods are from the Oviraptorids. The mother dinosaur of these theropods would lay her pair of eggs and arrange them carefully to circles. They created nests that are bowl-shaped and usually had a maximum of 24 eggs. These parent dinosaurs would sit on these nests every day for two weeks to a month.

Hadrosaurs

Eggs of hadrosaurs are found to contain 7 to at most 32 eggs in their nests. Maiasaura, a hadrosaur, created 2 meters in diameter holes dug out of the ground as nests for their eggs. They usually created egg nests for their offspring and constructed multiple nests that are 9 meters apart, which indicated their colonial nesting behavior. These dinosaurs are also known as good mothers because they feed their babies with chewed plants while they were very young. If you are interested in hadrosaurs and their nesting habits, read my article on this website: Types of Duck Billed Dinosaurs -Names, Habitats, Nests.

Sauropods

For proper incubation, sauropods would lay their eggs in warm areas caused by geothermal activity. However, parent sauropods are not clingy to their eggs. They would leave as soon as they already found a place for their eggs. They usually lay eggs in coarse sandy sediments and fine soil with plant material. Mussaurus, a sauropod, had egg nests, but fossilized sauropod eggs are rarely found in nests in general.

Final Thoughts

Dinosaurs have existed long ago, but they can always be a topic of interest because there is so much to learn and discover from their existence. During the Mesozoic era, habitats changed and were different during, for instance, the Jurassic compared to the Cretaceous period. It’s interesting to think about how egg-laying and nesting might have changed over the millions of years.

As big as their sizes are, the unraveled questions behind them rivals this. By specifically looking into fossilized dinosaurs hatching from eggs, many discoveries can be made, such as things related to their egg hatching and nesting. Dinosaur parenting styles can also be determined by studying fossilized eggs. Indeed, dinosaurs are terrifying but interesting creatures too!

Michael Haralson

I'm the owner of Adventure Dinosaurs website. Although I have an extensive business background, I am fascinated with dinosaurs and have been since childhood. I'm fortunate enough to have visited fossil museums in Europe (UK, Germany, and Spain), the US (California, Texas) and in Asia (China). Currently, I'm location independent with a home base in Kirkkonummi, Finland.

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