Hatchling tyrannosaurs would have been among the largest animals ever to hatch from an egg. They also would have looked a lot like their parents. This gives us our first window into the early life of the largest terrestrial predators of all time.
These discoveries are the two main findings of our research on the first embryonic tyrannosaur bones, which has just been published. The accepted manuscript is here (https://doi.org/10.1139/cjes-2020-0169), and our technical talk presented at the Virtual SVP 2020 is below:
Tyrannosaurs are among the best known meat-eating dinosaurs, partly because they are captivating for schoolchildren and scientists alike, but also because they are known from a very good fossil record. But while we know a lot about their anatomy and evolution, there are still major gaps in our knowledge of their growth and reproduction.
Two 70+-million-year-old bones from Alberta and Montana, a claw and a jaw, are helping to change that. I led a team from Scotland, Canada, and the USA that studied the tiny bones using a particle accelerator. Using high-powered X-rays, we were able to virtually reconstruct the bones and reveal what baby tyrannosaurs looked like.
We were immediately surprised by how much the bones looked like those of adult tyrannosaurs. This is especially true of the tiny (1 inch long) jaw bone, which still preserves the developing teeth. Even though this specimen is 20% the size of the next smallest tyrannosaur jaw, they’re remarkably similar in their proportions and features. This is a good sign for palaeontologists, because it means we’ll likely be able to easily identify any baby tyrannosaurs we find in the future.
And their likeness to adults also means we can estimate their size at birth. The result? A whopping 1 metre long upon hatching. This may seem enormous, but remember that they would have been curled up inside an egg. Even so, this ranks tyrannosaur hatchlings as among the largest babies to ever emerge from eggs on land. The now-extinct Elephant Bird, producer of the largest eggs known so far, had embryos that were 10–15% smaller than these tyrannosaur embryos!
So if they’re big and easy to identify, why haven’t we found more tyrannosaur eggs and babies by now? We don’t know the answer for sure, but we’re certain now that it’s not because we’re looking in the wrong place. Both tyrannosaur embryos were found at nesting sites of other dinosaurs, meaning that tyrannosaurs nested in these same areas. One idea we speculate upon in our paper is that tyrannosaurs could have laid soft-shelled eggs like their ancestors, but this is just a guess and it would have to go against conventional wisdom.
These are just the first clues to understanding baby tyrannosaurs, but now we know where to look, and what we’re looking for. Where does our research go from here? There are so many research avenues these bones open up. By looking in more detail at the development of the teeth and how the jaw changes over time, we’re hoping to understand more about the changes tyrannosaurs went through as they grew up. We might even be able to tell how long tyrannosaurs spent in their eggs before hatching. The similarity of the teeth and jaw to those of adults hints that baby tyrannosaurs had some of the biting adaptations that helped adults with their bone-crunching bite, and this might point to their diets early in life. And we’re definitely going to continue our fieldwork in both localities and hope that our efforts turn up more baby tyrannosaur bones!