Baby Dinosaurs: How Dinosaurs Were Born and Raised Their Young

For most of the twentieth century, dinosaurs were assumed to reproduce like large reptiles: lay eggs in a nest, walk away, let the hatchlings fend for themselves. It was a convenient narrative that fit with the broader picture of dinosaurs as cold-blooded, sluggish, reptile-like animals. Almost all of it turned out to be wrong.

The fossil evidence for dinosaur reproduction and parenting that has accumulated since the 1970s tells a substantially different story. Some dinosaur parents were attentive, remained with their young, and actively fed them in the nest. The eggs themselves came in a remarkable variety of shapes and sizes, and the development of embryos from egg to adult involved growth rates that exceed anything alive today.

The Eggs

All dinosaurs laid eggs, and those eggs have been found on every continent. What’s less appreciated is how diverse dinosaur eggs were. They ranged from roughly tennis-ball-sized (small theropods) to about 45 centimetres long — the eggs of Gigantoraptor, which are the largest dinosaur eggs known. The shapes varied considerably: round, oval, elongated, some with almost cylindrical proportions.

The largest sauropods — Argentinosaurus, which weighed around 70,000 kilograms as an adult — laid eggs no larger than a football. This isn’t carelessness; it’s physics. An egg’s shell has to be thick enough to structurally support itself and resist pressure, but thin enough that the embryo can actually breathe through the shell’s pores and eventually break through. Beyond a certain size, you can’t meet both requirements simultaneously. So even the largest animals in Earth’s history started life at perhaps 5 kilograms.

That means a baby Argentinosaurus faced a 14,000-fold weight gain over its lifetime. For comparison, a human infant increases its birth weight by roughly 20 times over a lifetime of growth.

In 2021, researchers studying an oviraptorosaur embryo called “Baby Yingliang” found it preserved in the exact pre-hatching posture that modern birds use — head tucked under the right wing, body curled — a position called “tucking.” The implication is that this behaviour evolved in dinosaurs tens of millions of years before the first bird.

Parental Care: Maiasaura and What It Changed

The discovery that changed palaeontological thinking on dinosaur parenting happened in Montana in the 1970s. Palaeontologist Jack Horner and his colleague Bob Makela found a nest of Maiasaura — a hadrosaur — that contained not just eggs but hatchlings with worn teeth.

Worn teeth in hatchlings mean the babies were eating solid food while still in the nest. The only way to explain that is if an adult was bringing them food. The hatchlings’ legs were also not fully developed for independent locomotion — they were, essentially, helpless babies that required care. The site contained multiple nests with similar evidence, and the nests were spaced roughly one adult body-length apart, suggesting a colonial nesting ground. Horner named it “Egg Mountain.”

Maiasaura, which means “good mother lizard,” became the first strong evidence that at least some dinosaurs were attentive parents who fed their young in the nest — a behaviour we associate with birds and mammals, not reptiles. Whether Maiasaura was typical or exceptional among dinosaurs is still debated, but it permanently changed the default assumption.

Oviraptor: A Case of Mistaken Identity

Oviraptor means “egg thief.” It’s one of the more unfortunate names in palaeontology, because it was given based on a misinterpretation that took decades to correct. When the first Oviraptor specimen was found in Mongolia in 1923, it was lying near a nest of eggs that were assumed to belong to a nearby Protoceratops. The natural conclusion was that Oviraptor had been caught stealing eggs.

In the 1990s, better specimens were found in Mongolia — including several Oviraptor individuals in brooding posture, sitting directly on nests with their arms spread over the eggs, exactly as a modern bird sits on a clutch. The eggs in these nests were eventually identified as Oviraptor’s own. The animal hadn’t been stealing eggs in 1923; it had been brooding them.

Multiple brooding Oviraptor specimens have since been found, some apparently killed by sudden sandstorms while protecting their nests. The picture that emerges is of an animal with genuinely bird-like parental behaviour — covering eggs to regulate temperature, protecting them from predators, dying rather than abandoning them.

Growth: The Numbers Are Strange

Dinosaur growth rates are determined through bone histology — cutting thin sections of fossil bone and examining the growth rings under a microscope, analogously to counting rings in a tree. The rings record annual growth, and their width records how fast the animal was growing each year.

What this reveals about the largest dinosaurs is difficult to fully process. Large theropods like T-Rex gained roughly 2 kilograms per day during their teenage growth spurt — around ages 14 to 18 in most estimates. Sauropods grew faster. An Argentinosaurus hatchling of around 5 kilograms had to reach 70,000 kilograms over a lifespan estimated at perhaps 40 years. The mathematics of that growth rate, sustained over decades, requires an enormous daily food intake and a metabolism that looks nothing like a cold-blooded reptile.

This growth rate evidence is one of the primary lines of support for the now-dominant view that non-avian dinosaurs, or at least many of them, were warm-blooded (or at minimum, had metabolic rates far higher than modern reptiles). You cannot grow at the rates indicated by the bone rings while running on a reptilian metabolism.

The flip side: baby dinosaurs of the largest species were absurdly vulnerable. A hatchling T-Rex was about the size of a turkey. A hatchling Argentinosaurus was the size of a large dog. Both existed in ecosystems full of predators that would have had little difficulty catching them.

What Baby Dinosaurs Looked Like

Hatchling dinosaurs didn’t look like small adults, and we know this from actual embryo fossils. Baby dinosaurs had the proportional features that characterise infants across many animal groups: larger head relative to body, larger eyes relative to skull, shorter snout. The “baby face” that triggers parental responses in mammals appears to have been present in at least some dinosaur hatchlings.

Baby theropods were almost certainly fluffy. Embryo and hatchling specimens from China, preserved with feather impressions, show downy coverings that look very similar to a newborn chick — regardless of whether the adult of the species retained extensive feathering. This suggests that insulation was particularly important for hatchlings, which had a high surface-area-to-volume ratio and would have lost body heat rapidly.

Survival

Palaeontologists estimate that the vast majority of baby dinosaurs didn’t reach adulthood. For large sauropods, survival from egg to adult may have been as low as 1-5%. The strategy for managing this was the same one many animals use: produce a lot of eggs. A Maiasaura nest could hold 30-40 eggs. Producing many offspring and letting most of them die before they’re old enough to matter reproductively is evolutionarily viable, even if it looks brutal.

The animals that did survive infancy faced a different set of problems. The growth phase in large dinosaurs — particularly the rapid teenage growth spurt — left them in a vulnerable middle size, too big to use the hiding strategies available to small animals but not yet large enough to be safe from the largest predators. The bone record of T-Rex, for example, shows dramatic depletion of juvenile age classes in the known fossil assemblages, consistent with high mortality during the growth phase.

Those that survived to adulthood had, in at least some cases, remarkably long lifespans. Large sauropods probably lived for 40-70 years. T-Rex specimens have been aged at around 28-32 years. The journey from a 5-kilogram hatchling to a 9-tonne apex predator or a 70-tonne sauropod took decades — and most of them didn’t complete it.