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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.
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.
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.
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.
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.
Time: c. 70-65.5 million years ago
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.
Other Dinosaurs Known from Eggs, Embryos, and Babies
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