Fossil Evidence Indicates Baby T. rex Were Tiny, but Deadly

An adult Tyrannosaurus rex could grow to be 12 feet tall and 40 feet long, putting it on par witha whale shark, according to NOAA. Their babies, however, were surprisingly tiny.

Analyses of the first-known fossils of T. rex hatchlings suggest they weighed just five pounds and were roughly the size of a small house cat. And yet, they hit the ground running, scarpering around and devouring “relatively large” prey shortly after birth.

“Tyrannosaurus were unsafe at any size,” Eric Snively, an Oklahoma State University Center for Health Sciences professor of anatomy and cell biology, said in a statement. “Babies were quickly out of the nest and killing the heck out of things.”

The results have been published in the journalBiology.

What Was T. rex Like as Hatchlings?

When it comes to tyrannosaurus young, the fossil record is pretty scant, meaning relatively little is known about the dinosaur’s reproductive strategy and parenting style. The fossils investigated by Snively and team were found amongst an assemblage at the T. rex Discovery Center of the Royal Saskatchewan Museum in Canada.

Using high-tech computed tomography (CT) equipment, the team examined the structure of a foot bone from a young T. rex and found an abundance of blood vessels, suggesting a growth spurt. The absence of growth rings led paleontologists to believe the specimen in question was less than a year old at the time of death.

Meanwhile, dental fossils attributed to tyrannosaurus hatchlings show wear most likely caused by gnawing on vertebrates.

“The babies were smashing into the bones of their food, just like giant adults,” said Snively.

What Were T. rex Parents Like?

Based on the fossil evidence studied, the researchers conclude the infamous land predator likely laid a relatively large clutch of eggs.

According to the study, a small adult T. rex likely laid about 20 eggs, while a larger adult T. rex could lay about 30 eggs.

This number is notably higher than that of modern-day dinosaurs (birds) but, crucially, lower than that of primitive reptiles, and could offer a glimpse into the species’ approach to parenting.

“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,” the paper’s lead author Nicholas Longrich, a paleontologist at the University of Bath, U.K., said in a statement.

Turtles adopt the first strategy, laying as many as 125 eggs but letting their young fend for themselves, whereas birds lay fewer eggs but provide a far higher degree of parental care, often involving both parents. The T. rexprobably fell somewhere between the two. The dinosaur likely parented with “a light touch,” added Longrich.

The comparatively large clutch size also suggests childhood mortality rates were reasonably high. The researchers say the hatchlings may have been preyed upon by larger theropods (including those of their own species).

Parenting Strategies Evolving Over Time

The T. rex’s approach to parenting may highlight a shift from an offhand style to a more involved one.

The researchers point out that tyrannosaur hatchlings may have been small, relatively speaking, compared with today's birds and mammals, but they were larger than those of earlier dinosaurs, whilst the size of their clutch may have been smaller than that of more ancient species.

These shifts did not simply occur in dinosaurs, the researchers wrote. The trend towards higher levels of parental investment during the Mesozoic (252 million to 66 million years ago) is evident in other animal groups.

The authors note that these trends continued after the dinosaurs went extinct. It’s seen in mammals and birds.

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