New research finds that Tyrannosaurus young were leaving the nest and actively hunting not long after hatching. Credit: Raul Martin.
The dinosaur that would become Earth’s most famous land predator began life at roughly the size of a house cat. It had long legs, a body weighing perhaps 3.7 pounds (1.6 kg) at hatching and sharp teeth seemingly from birth.
This is the first somewhat informed picture of what a baby T. rex was like. It comes from the first fossils identified as hatchling Tyrannosaurus rex: a tiny foot bone, supported by a fragment of jaw and several shed teeth scattered among museum collections. The researchers also used a hatchling foot bone from the related predator Gorgosaurus.
Together, the fossils suggest that newborn tyrannosaurs moved soon after hatching, fed themselves and probably emerged in clutches numbering in the dozens.
The tiny T. rex hatchling metatarsal. Credit: Longrich et al., Biology, 2026
The researchers scanned the foot bone fossil at the Canadian Light Source at a resolution of 4.4 microns, allowing them to trace microscopic canals through the bone. The inner tissue was porous and crowded with blood-vessel channels, signs of extremely rapid growth. The denser outer layer appeared to have formed after hatching. There were no growth rings, meaning the fossil belonged to an animal less than a year old.
Most telling were tiny tunnels made as the bone repaired itself under repeated strain. Such remodeling has not been reported in dinosaur embryos and is difficult to explain by movement inside an egg. Together with the well-developed joint surface, it suggests the hatchling had already been walking and running.
Digital rendering of the Tyrannosaurus foot bone that shows an abundance of blood vessels. Credit: Alexander Claxton
“Sure enough, we found canals in the bone that indicate they were quickly out of the nest and running around,” Eric Snively, an anatomist at Oklahoma State University and a study author, said in a news release.
The interpretation is not airtight. The team could not identify a crisp “hatching line” in the bone and instead inferred the transition from gradual changes in density. But worn, shed teeth strengthen the case that these were active young animals. The teeth resemble miniature adult T. rex teeth and their worn tips show that they had already fed. The researchers believe that the hatchlings were biting relatively large vertebrate prey.
The discovery extends research on a hatchling titanosaur, whose limb bones recorded an active, mobile start to life.
×
Thank you! One more thing…
Please check your inbox and confirm your subscription.
T. rex hatchling vs house cat. Credit: Longrich et al., Biology, 2026
Out of the Nest Early
Digital rendering of an adult and hatchling Tyrannosaurus. Credit: Ji Zhongqiu.
No confirmed T. rex egg or nest has ever been found, so the researchers could not simply count the eggs in a fossilized clutch. Instead, they reconstructed one step by step.
First, they used the size of the hatchling’s foot bone to estimate its weight. The animal weighed about 5.5 pounds when it died, but changes inside the bone suggest it had already grown since leaving the egg. At hatching, it may have weighed closer to 3.7 pounds.
Young birds and crocodilians typically account for about two-thirds of the weight of the egg from which they emerge. Applying that relationship to T. rex produced an estimated egg weight of roughly 5.5 pounds. The researchers then compared adult tyrannosaurs with living birds and crocodilians to estimate how much a female could invest in an entire clutch.
Their models suggested that a 6.1-ton female might lay about 120 to 130 pounds of eggs in total, while a particularly large adult could produce roughly 160 to 175 pounds. Dividing that total by the estimated weight of each egg yielded about 20 eggs for the smaller animal and around 30 for the larger one. The precise numbers remain uncertain, and clutches of 50 or more eggs were possible. But every reasonable version of the calculation pointed in the same direction: T. rex probably laid many relatively small eggs rather than a handful of enormous ones.
But were mama T. rex-es good parents? The clutch size offers a clue.
A Special Investment
Parents have limited time and energy, and species that produce many offspring generally invest less in each one than animals that produce only a few. Turtles typically lay 80 to 125 eggs and provide little, if any, parental care. Birds, by contrast, lay between one and 20 eggs, depending on the species, and invest heavily in protecting and nurturing their young both before and after hatching.
“Because animals have limitations in what they can invest in the next generation, they can either invest in having lots of young, or invest a lot in their young, but not both,” said Longrich. “The small size of tyrannosaurs suggests they pursued the first strategy — they had lots of small young and had a light touch when it came to parenting.”
A “light touch” does not necessarily mean complete abandonment. Adults could have guarded the nest or remained near the young for a time, but the hatchlings’ sturdy feet and worn teeth suggest they soon moved about and found food for themselves.
The fossils also underline why that early chapter of T. rex life remained hidden for so long. The evidence was not a spectacular skeleton, but a handful of tiny bones and teeth stored in museum collections.
“Little fossils collected in the field and curated in museums can have the biggest long-term impact on how we study Tyrannosaurus and how it lived,” Snively said in the Oklahoma State release.
The new findings appeared in the journal Biology.