Tyrannosaurus rex is without a doubt the most famous dinosaur in the world, and one of the lasting questions people have about this amazing dinosaur is what it was like as a teenager before it was full grown. A paper I co-authored with Bruce Rothschild, a former research affiliate with the University of Kansas Biodiversity Institute, now published online in the journal Cretaceous Research addresses this interesting question.
A nearly complete dinosaur skeleton labeled as BMR P2002.4.1, but more affectionately referred to as 'Jane' in honor of the woman who discovered it, has been the center of a decades long dispute over the validity of a dinosaur called Nanotyrannus lancensis. Nanotyrannus was named by a team led by the famous paleontologist Bob Bakker as a 'pygmy tyrannosaur' from the Late Cretaceous of Montana1. Not all dinosaur paleontologists are convinced of this assessment, and many prominent studies have asserted that Nanotyrannus—specifically 'Jane', the original holotype fossil skull at the Cleveland Museum of Natural History, and a handful of other isolated remains—are instead remanants of immature T. rex. In fact, if you visit wikipedia's page for Tyrannosaurus rex, you will find a proudly displayed image of 'Jane' from the Burpee Museum of Natural History. While paleontologists in this debate have focused on the number of teeth in the jaws2, the overall shape and proportion of the skull3, and whether the texture of the bone is more similar to that of adults of immature individuals4, we observed an isolated character on the skeleton of 'Jane' that shed some additional insight on this debate.
A portion of 'Jane's' lower jaw (called the dentary bone) is marked by a deep groove containing numerous small openings. Bruce Rothschild, who is an expert on ancient diseases and has looked at many jaws from theropod dinosaurs, was unaccustomed to seeing such a feature in a tyrannosaur, and thought this groove was possibly a sign of some disease. It turns out that the other specimens of the embattled genus Nanotyrannus also shared this feature, so it likely wasn't evidence of a disease. After examining additional dinosaur fossils, we found out that, in fact, this groove is found on nearly all theropod dinosaurs outside of the tyrannosauroid group (the group more closely related to T. rex than other meat-eaters like Allosaurus, Spinosaurus, and Coelophysis). Among tyrannosaurs, however, we found an opposite trend: only 7 of 18 tyrannosaurs had this feature, and half of those occurences were found in the group of the earliest tyrannosaurs. We further investigated this question by examining known T. rex material, ranging in age from "baby" all the way to full grown adult, and found that none of these fossils showed the groove we found on 'Jane'!
So what does this mean? It could be that 'Jane' and all the other fossils we call Nanotyrannus really are juvenile T. rex, and they are undergoing a really dramatic bodily transformation during their growth into adults (puberty sure is rough!), but this is unlikely given that none of the undisputed T. rex fossils we investigated have this feature. This groove is a passageway for nerves and blood vessels to move through the bones of the skull, and short of saying that the nerves and veins of the head dramatically changed their placement as the animal grew, if a baby has no groove, a sub-adult has no groove, and a full-grown adult has no groove, one would logically not expect a juvenile to have a groove either. To us (and some of the other scientists arguing in favor of Nanotyrannus), this is evidence that Nanotyrannus is a different dinosaur from T. rex, and they likely preferred different environments and prey even though they lived at the same time.
What does this mean about how Nanotyrannus fits in to the dinosaur family tree? Even though Nanotyrannus has been variously proposed to be a young T. rex or a closely related species, our phylogenetic analysis actually places Nanotyrannus as a close relative of the albertosaurine tyrannosaurs (moderate-sized theropods that lived in what is now Canada). We obtained this result because they are the only group of advanced tyrannosaurs to possesses the groove we studied. This result was interesting, however, because Charles Gilmore, the paleontologist that described the original Nanotyrannus on display at the Cleveland Museum5, thought it was an example of a new species of Gorgosaurus, one of the types of albertosaurines. History seems to have come full circle.
So now what? Well to the fan club of Nanotyrannus, we have some additional evidence that this was in fact a separate dinosaur species. And for now, the hunt is back on for a complete fossil that shows us what the mighty T. rex was like as a teenage terror.
1. Bakker et al., 1988. Nanotyrannus, a new genus of pygmy tyrannosaur, from the Latest Cretaceous of Montana. Hunteria 1:1-28.
2. Larson, P. 2013. The case for Nanotyrannus. Pp. 14-53 in Parish et al. (eds.), Tyrannosaur Paleobiology. Indiana University Press.
3. Carr, T. 1999. Craniofacial anatomy in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology 19:497-520.
4. Currie, P. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48:191-226.
5. Gilmore, C. 1946. A new carnivorous dinosaur from the Lance Formation of Montana. Smithsonian Miscellaneous Collections 106:1-19.
This Thanksgiving, don’t think of the yearly tradition as just carving up a turkey. In reality, you’re dissecting your very own dinosaur.
KU Paleontologist David Burnham studies ancient raptors of all sizes. Studying these ancient relatives fills the gaps between raptors of the past and the turkeys we eat today. Upon studying this lineage, one can see that turkeys and raptors have much more in common than you may think, despite differences in how we traditionally picture a “bird.”
“The public’s perception of what a bird may be might not be the definition a scientist would use,” said Burnham.
The public largely defines birds by their feathers and flight capabilities. By comparison to their ancestors, not only do both prehistoric raptors and modern birds share feathers, but many living birds also either rarely or never use flight including ostriches, emus, cassowaries and turkeys.
“The loss of flight has evolved several times throughout that lineage,” said Burnham. “If we want to draw a line when theropod dinosaurs became strictly avian, well, we’re still refining that even today due to the enormous amount of new discoveries.”
What’s important to remember is that dinosaurs never fully became extinct. The ones that survived mass extinction merely changed. Birds such as turkeys and chickens share their lineage with theropods, or two-legged meat-eating dinosaurs. The skeletal structures of turkeys and extinct theropods such as Velociraptor, Bambiraptor and Microraptor retain several similarities in particular.
Here are some points to look for while dissecting your “dinosaur” this Thanksgiving:
- Wishbone - The furcula, or wishbone, is a major connection between the turkey and its ancient theropod ancestors. The furcula is made up of two formerly separate collarbones, fused together. This evolutionary change aided in flight capabilities of ancient raptors such as Microraptor, and continues to help modern birds, such as turkeys, reach liftoff.
- Wings - The turkey wing deserves careful inspection. The fleshed-over tip is where claws protruded from theropod arms such as those of Velociraptor, Bambiraptor, and even the massive Dakotaraptor. Imagine those on your dinner plate! As theropod dinosaurs evolved, their arms became longer and those claws were covered by flesh forming wings suitable for extended flight – an easily recognizable feature of avian species we see today.
- Thighs and drumsticks - These are often the most sought after pieces of the feast, and still quite similar to the legs of theropods. This leg structure allowed raptors to reach impressive ground speeds; Velociraptor is thought to have been able to run as fast as 40 miles per hour! The turkey on your table is no slowpoke either thanks to this ancient design, with a top running speed of 25 miles per hour.
While the turkey still possesses many remarkable features harkening back to its raptor relatives, there are some things we can be thankful were lost during evolution.
“Of course, turkeys don’t have teeth,” said Burnham, “and that’s probably a good thing.”
vertebrate-paleontology, David Burnham, turkey, museum