Tyrannosaurus rex was a giant of the prehistoric world, standing up to 13 feet (4 meters) tall and weighing around 9 tons (8 tonnes). So you might expect that its hatchlings were also a considerable size.
But according to new research published in the journal Biology, they were surprisingly small, about the size of a house cat and weighing less than 5 pounds (2.3 kilograms). Unlike many newborn animals, these tiny predators were born ready to run and feed themselves with little help from their parents.
Researchers came to these conclusions after surveying museum collections for tiny bones and shed teeth from Late Cretaceous deposits in North America.
Forgotten fossils
Baby dinosaur bones are small and fragile and consequently are often overlooked in museum collections, sitting unidentified in drawers and on dusty shelves. To make sure they were studying tyrannosaurs, the team looked for telltale anatomical features such as the characteristic middle foot bone. In the tyrannosaur family, these bones have a deep pit on the front and a ridge on the back.
They also examined tiny teeth, selecting those that matched known features of T. rex teeth. Then, to study the internal structure of the bones without cutting them, they performed high-resolution synchrotron X-ray scans.
Tiny terrors
The scans revealed that the bone tissue showed signs of remodeling associated with mechanical stress, meaning the baby dinosaurs were most likely already walking and running. Many of the teeth were worn, suggesting they were feeding on relatively large vertebrate prey rather than insects or very small animals. The researchers argue that this could mean they were already hunting and feeding on other animals soon after hatching.
The paper also suggests that a newly hatched T. rex likely weighed about 1.7 kilograms (3.7 pounds) and measured roughly 75 centimeters (about 2.5 feet) long. This is based on mathematical models and comparisons with living birds and crocodiles.
"They show that tyrannosaur eggs and hatchlings were small, <2.5 kg, and suggest that young were highly precocial, able to move and feed themselves soon after hatching," the authors commented in their study.
In addition to these remains, the scientists studied fossils of Gorgosaurus, a slightly smaller cousin of T. rex that lived millions of years earlier. These hatchlings were estimated to be even smaller, weighing only about 1.3 kilograms (2.9 pounds).
They also estimated the number of eggs a T. rex would have laid per clutch. "What we can therefore say with some confidence is that tyrannosaurs probably laid at least 15–30 eggs. Larger clutches of 50 or even 100 eggs are not impossible."
In terms of the wider evolutionary picture, the research team argues that these fossils suggest dinosaur parenting was a stepping stone between reptiles and birds. Reptiles like crocodiles lay large batches of eggs and offer relatively limited care to their babies. Birds, on the other hand, lay fewer eggs and invest a lot of energy in caring for their young. Dinosaurs like T. rex appear to have fallen somewhere in between, producing many offspring but showing signs of less parental investment than modern birds.
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More information
Nicholas R. Longrich et al, Hatchlings of Tyrannosaurus rex and the Evolution of Dinosaur Reproductive Strategies, Biology (2026). DOI: 10.3390/biology15131090
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Citation: T. rex babies were born ready to run and feed themselves (2026, July 15) retrieved 16 July 2026 from https://phys.org/news/2026-07-rex-babies-born-ready.html
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