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Mommy Direst?

Updated on June 19, 2020
Despite numerous portrayals in dinosaur-related films and documentaries, a baby T. rex skeleton has yet to be discovered, with most portrayals of T. rex parenting being based on modern birds and crocodiles.
Despite numerous portrayals in dinosaur-related films and documentaries, a baby T. rex skeleton has yet to be discovered, with most portrayals of T. rex parenting being based on modern birds and crocodiles. | Source

Among the rarest of dinosaur fossils are the remains of eggs and babies. Almost everything we know about some of the best-known dinosaurs--including T. rex, Stegosaurus, Diplodocus, and Iguanodon--comes from the skeletons and trace fossils of adult animals.

Yet dinosaur eggs have been known to us for over a century, and since the late 1970s, we've known that at least some species were dedicated parents. With a few notable exceptions, these fossils generally tell more about the mothers than the fathers (who like most modern animals, likely played a minimal role in child-rearing) and include anything from fossilized eggs and embryos to the remains of the likely parents themselves. Clues about five particular dinosaurs show us that dinosaur parenting was more complex than one might expect.

World's oldest dinosaur embryo.
World's oldest dinosaur embryo. | Source
Massospondylus nest as depicted by Julius Csotonyi.
Massospondylus nest as depicted by Julius Csotonyi. | Source


Time: c. 200-183 million years ago

Location: South Africa, as well as Lesotho and Zimbabwe

Parenting Evidence: The earliest confirmed dinosaur nesting site was discovered in 2010 in South Africa and left by the Early Jurassic herbivore Massospondylus (though the oldest known baby dinosaur skeletons date back to the Late Triassic). The site contained clutches with as many as thirty-four eggs, as well the embryos of these baby prosauropods. Based on the fact that the nests were at different depths, the site was probably used as nesting ground by multiple generations of Massospondylus.

Despite this discovery and the fact that Massospondylus is one of the best-known prosauropods to scientists, the nature of their parenting is ambiguous: No adult skeletons were found at the site, and the nests did not appear to have been laid in the same bowl-like pits as other dinosaurs. Baby Massospondylus were born not only defenseless but toothless, and would have needed some form of nourishment to survive their first few weeks. Fossilized tracks made by feet twice the size of the newborns' (but made by feet too small to be those of adult Massospondylus) suggest that the babies received that nourishment while they were in close proximity to the nests. It's not far-fetched, then, to speculate that Massospondylus parents provided their young with food and protection until they were large enough to strike out on their own.

Adult Psittacosaurus with potential offspring.
Adult Psittacosaurus with potential offspring. | Source
Psittacosaurus family (Raúl Martín).
Psittacosaurus family (Raúl Martín). | Source


Time: c. 130-100 million years ago

Location: China and Mongolia

Parenting Evidence: Many smaller dinosaur species--especially oviraptorids--have been found brooding over their eggs. One adult specimen of Psittacosaurus unearthed in China, however, was found not with eggs but with thirty-four similarly-sized babies of the same species. The formation in which it was unearthed--the Yixian Formation in the Liaoning Province--has yielded many well-preserved dinosaurs apparently buried in volcanic sediment. It's likely this primitive, dog-sized ceratopsian was shielding its offspring from the raining ash as it died, making for one of the clearest cases of dinosaur parenting in the fossil record to date.

Maiasaura nest at the Wyoming Dinosaur Center in Thermopolis.
Maiasaura nest at the Wyoming Dinosaur Center in Thermopolis. | Source
The good mother lizard in the flesh (Luis Rey).
The good mother lizard in the flesh (Luis Rey). | Source


Time: c. 80-73 million years ago

Location: Western United States

Parenting Evidence: It was the discovery of this otherwise-typical hadrosaur in 1978 that showed the world that some dinosaurs were dedicated parents. Among the hundreds of specimens unearthed at "Egg Mountain" (in the Two Medicine Formation in Montana) were fifteen juveniles about three feet long, much too large to hatched from Maiasaura eggs. Estimated to have been four weeks old when they died (possibly buried the ash of an erupting volcano), their tiny teeth were already worn from grinding plant matter. This was likely brought to them by their parents, in part because their lower limb bones had yet to harden, meaning they would have had a great deal of difficulty trying to walk. Because of the abundance of Maiasaura specimens from Egg Mountain alone, we know that they would have left the nest about a month or two after hatching and reached adulthood around the age of seven.

Troodon nest at the Museum of the Rockies, Bozeman, MT.
Troodon nest at the Museum of the Rockies, Bozeman, MT. | Source
Troodon nests as portrayed in Dinosaur Revolution (Discovery Channel, 2011).
Troodon nests as portrayed in Dinosaur Revolution (Discovery Channel, 2011). | Source


Time: 75-65.5 million years ago

Distribution: Western United States and Canada

Parenting Evidence: Very little is known about the parenting practices of most theropods. Yet an examination of one particular Troodon skeleton among a clutch of eggs in 2008 added a new dimension to dinosaur parenting: This brooding Troodon's femur had no traces of medullary bone, a spongy lining in the bones of birds and dinosaurs. This lining produces calcium and phosphorus, which are used in the production of eggshells. Therefore, it is quite possible that the nesting Troodon was a male. Today, father emperor penguins look after their eggs while the females go out to sea to replenish themselves after a taxing pregnancy. Since birds evolved from theropods, and it would seem that male parental care was one of the behaviors passed on from dinosaurs to birds.

The world's largest dinosaur nesting site.
The world's largest dinosaur nesting site. | Source
Birth of a Saltasaurus (Luis Rey).
Birth of a Saltasaurus (Luis Rey). | Source


Time: c. 70-65.5 million years ago

Distribution: Argentina

Parenting Evidence: Saltasaurus eggs and babies have been unearthed at Auca Mahuevo, the largest known dinosaur nesting site in the world. This site was used by many generations and most likely more than one species of sauropod over a period of about twenty million years. Unlike the skeletons of the baby Maiasaura, however, the baby Saltasaurus found at the site were small enough to have recently hatched from eggs six inches in diameter, while adults of the same species are nowhere to be found. It seems, then, that mother Saltasaurus abandoned their eggs shortly after laying them and played no role in raising their offspring.

Though this would seem an impractical (and in our eyes, cruel) way to ensure the survival of the species, these sauropods constructed their nests close together and produced clutches of as many as twenty-five eggs, resulting in thousands of eggs being laid at Auca Mahuevo each year. Though most of the Saltasaurus that hatched from these eggs would perish before reaching sexual maturity (despite being born with teeth and the signature bony plates of the family), the species' survival would be guaranteed by the sheer number of eggs laid. In addition, the high infant mortality rates of Saltasaurus would have reduced the strain that these multi-ton herbivores would have placed on their ecosystem in the long run.

Hypacrosaurus nest site (Mark Hallett). These lambeosaurs laid the largest known eggs of any known non-avian dinosaur, about a gallon in volume.
Hypacrosaurus nest site (Mark Hallett). These lambeosaurs laid the largest known eggs of any known non-avian dinosaur, about a gallon in volume. | Source

Other Dinosaurs Known from Eggs, Embryos, and Babies

  • Bonapartenykus
  • Camptosaurus
  • Citipati
  • Deinonychus
  • Dryosaurus
  • Hypacrosaurus
  • Hypselosaurus
  • Mussaurus
  • Oviraptor
  • Protoceratops
  • Sinornithomimus
  • Sinosauropteryx


Barrett, Paul. National Geographic Dinosaurs; National Geographic Society, Washington, D.C., 2001

Coria, Rodolfo A. and Luis M. Chiappe. "Embryonic Skin from Late Cretaceous Sauropods (Dinosauria) of Auca Mahuevo, Patagonia, Argentina." Journal of Paleontology, November 2007

Holtz, Thomas R. Dinosaurs: The Most Complete, Up-To-Date Encyclopedia for Dinosaur Lovers of All Ages; Random House Inc., New York, NY, 2007

Switek, Brian. "Paleontologists Uncover Oldest Known Dinosaur Nest Site." Smithsonian Magazine, 25 Jan 2012

Switek, Brian. "Dino Day Care." Smithsonian Magazine, 22 Dec 2008

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