Baby Tyrannosaurus rex hatchlings weighed just 5.5 pounds and hunted like miniature apex predators right out of the shell, according to a study published in the journal Biology. Paleontologists analyzing newly examined fossils from T. rex and its close cousin Gorgosaurus libratus found dental and vascular evidence showing that infant tyrannosaurids were active and lethal shortly after hatching, rather than lingering defensively in the nest.
Fossil Evidence Reveals Active Infant Predators
Baby tyrannosaurids were roughly the size of a house cat at 2.5 kilograms, but they possessed the physiological traits needed to hunt immediately. Oklahoma State University anatomy professor and study co-author Eric Snively stated that tyrannosaurs were unsafe at any size, noting that babies quickly left the nest and actively killed prey. According to Snively, whose work applies biomedical engineering and 3D imaging to dinosaur anatomy, these infants smashed into the bones of their food just like giant adults.
Researchers worked with fossilized metatarsals—long bones found in the midfoot and toes—originally recovered by paleontologist Nicholas Longrich within the collection of the Royal Saskatchewan Museum’s T. rex Discovery Centre in Canada. Snively explained that baby metatarsals help determine whether the juveniles hunted right away or stayed in the nest. Synchrotron computed microtomography, a sophisticated form of X-ray CT scanning, confirmed both the relative ages and the active lifestyles of the specimens.
Bone Scans and Dental Wear Confirm Rapid Growth
The T. rex metatarsal showed no growth rings, confirming the fossil came from a nearly newborn animal. Scans revealed a dense, intricate web of blood vessels coursing through the bone, which is a strong indicator of rapid growth in young dinosaurs. Additionally, researchers examined fossilized teeth from the tyrannosaurids and found wear and tear, such as enamel worn away at the apex, matching routine dental damage seen in adult fossils of the same species.
Did you know?
At just 5.5 pounds (2.5 kilograms), a newly hatched baby T. rex was comparable in weight to an average house cat.
Reproductive Strategy and Parental Investment
Prior fossil evidence combined with the new findings suggests that T. rex laid around 20 to 30 eggs at a time. Lead study author Nicholas Longrich, a senior research fellow at the University of Bath in England, stated that animals face limitations in how much energy they can invest in offspring. According to Longrich, parents either invest in having many young or invest heavily in their young, but not both.
The small size of tyrannosaur hatchlings indicates they pursued the first strategy by producing numerous small young with a light touch regarding parenting. Longrich added that while there was likely some parental investment—such as nest guarding and guiding the young for a while, similar to modern crocodilians and larger ground birds like ostriches—the babies could ultimately fend for themselves shortly after hatching.
Frequently Asked Questions
How big was a baby T. rex?
A newborn baby T. rex weighed about 5.5 pounds (2.5 kilograms), making it roughly the size of a house cat.
Did baby T. rex hunt their own food?
Yes. According to researchers analyzing fossilized foot bones and teeth, infant tyrannosaurids were active hunters that left the nest quickly and showed dental wear consistent with adult feeding habits.
How many eggs did a T. rex lay at one time?
Evidence indicates that T. rex typically laid between 20 and 30 eggs per clutch.
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