March 26th, 2021

#FossilFriday inspired by tomorrow’s Virtual Speaker Series guest Dr. Greg Funston (@funstonpaleo), here is a beautiful virtual model of a jawbone from a tyrannosaur embryo. Remarkably, this jaw belonged to a tyrannosaur just over halfway through its development in the egg and it along with a claw are the only embryonic bones known for tyrannosaurs.

The fossil jawbone still encased in rock, alongside an illustration highlighting what parts of the embryonic tyrannosaur dentary are exposed at the surface. The beige colour is the bone material and teal colour reveals the teeth. Figured credit: Funston et al. 2021.
Photograph of the jawbone held by coauthor of the study Mark Powers, showing how tiny it is. Photo credit: Erin St. Jean.

This specimen was first found at Egg Gulch in Montana’s Late Cretaceous Two Medicine Formation, and collected by B. Franz and W.  Cancrow back in 1983. It was not until decades later that its true significance was revealed, when examining other baby tyrannosaur bones from Alberta.

Because of the small size and delicate nature of the specimen, the 3 cm long dentary could not be safely prepared out of the rock covering it. Instead, palaeontologists ran the specimen through the Saskatoon Synchrotron – a particle accelerator that generates powerful X-rays. After taking over 3000 images, palaeontologists were able to digitally separate the fossil from the rock encasing it. They were able to create a beautifully informative virtual model of the jaw and the developing teeth inside. After analyzing the high-resolution model, the team of palaeontologists was able to make some startling conclusions.

By comparing the shape and features to previously known juvenile tyrannosaur jaws, they found several key characters that supported its tyrannosaur identity. These include the long, straight nature of the jawbone, the pronounced chin located under the fourth tooth socket, and the presence of a deep groove on the inside of the jaw. Together, these showed that distinctive tyrannosaur features were present even in jaws only 5% the length of an adult T. rex.


Synchrotron-created virtual model of the embryonic jaw. From top to bottom: medial view, (side view from inside the mouth), dorsal view (top view looking down onto the lower jaw), lateral view (side view from outside the mouth), and ventral view (bottom view looking up to the underside of the jaw). The jaw is from a tyrannosaur embryo just over halfway through its time in the egg. Figure credit: Funston et al. 2021.

The presence of a null-generation tooth in the jaw was the ‘smoking gun’ to show that this was a tyrannosaur embryo. In combination with some of the other featured of the jaw and new framework for determining the age of the embryos, they were able to narrow down the development window of this specimen. It turns out the baby tyrannosaur was not even close to hatching, and from best estimates was just over halfway through its time in the egg. That all this information could be gathered from such a small, previously overlooked specimen truly shows that good things some in small packages.

Looking forward, the team was surprised by how much the proportions and features of this jawbone resembled those of adult tyrannosaurs. In being easily identifiable and revealing the earliest stages of tooth development, this specimen opens a vast array of new research avenues. Particularly of interest is improving our understanding of the earliest stages in tyrannosaur development, and possibly estimating how long a tyrannosaur fetus took to develop inside its egg. The team’s future work includes, returning to the sites that produced the embryonic bones in the hopes of discovering more, and re-evaluating other specimens in museums that may have been overlooked.

A baby tyrannosaur reconstruction by Canadian paleoartist Julius Csotonyi.”

For further information on the topic, please visit 

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