Scientists have discovered how Tyrannosaurus rex hatchlings were born
The most fearsome predator ever to have roamed the Earth did not start out as a giant. An international team of palaeontologists has discovered that newborn Tyrannosaurus rex were about the size of a domestic cat and weighed around 1.7 kilograms.
That said, they probably did not require long-term parental care. Analysis of their bones and teeth suggests that the young were able to walk, run and feed themselves shortly after birth.
The study has been published in the journal *Biology*.
Tyrannosaurus hatchlings
An adult T. rex stood almost four metres tall, was over 12 metres long and weighed up to nine tonnes. Until now, scientists knew virtually nothing about what its newborn hatchlings were like, as their tiny bones are extremely rarely preserved.
The researchers re-examined museum collections in North America and discovered several miniature bones and teeth that had remained unidentified for decades. To confirm that the finds did indeed belong to Tyrannosaurus, the scientists compared their structure with the bones of other members of the family and scanned the specimens using synchrotron X-rays.
They were running almost immediately after birth
The internal structure of the bones showed signs of tissue remodelling under mechanical stress. This suggests that the animals were already actively moving at an early age.
Another surprise was the teeth. They showed signs of wear characteristic of feeding on relatively large prey. This suggests that young tyrannosaurs may have been able to hunt independently, or at least feed on the meat of vertebrates, almost immediately after hatching.
The authors emphasise that this is the most likely interpretation of the available data, although it is, of course, impossible to observe the behaviour of the hatchlings directly.
According to calculations, a newborn T. rex weighed around 1.7 kilograms, and its body length was approximately 75 centimetres.
By way of comparison, the hatchling of the tyrannosaur’s close relative, the gorgosaurus, was even smaller and weighed around 1.3 kilograms.
The researchers also estimated the size of the clutch. In their view, a female T. rex could lay at least 15–30 eggs, and possibly significantly more.
What does this change?
The authors believe that the new data helps to better understand the evolution of parental care in dinosaurs.
Modern crocodiles lay many eggs and show relatively little care for their young, whereas birds focus on producing a small number of offspring, to which they devote a great deal of attention. Tyrannosaurs probably occupied an intermediate position: they produced numerous offspring, but the young were sufficiently developed to become independent quickly.
Source
Article: Hatchlings of Tyrannosaurus rex and the Evolution of Dinosaur Reproductive Strategies.
Journal: Biology (2026).