A New Tyrannosaur Just Dropped

We all know Tyrannosaurus rex, with its face full of knives and adorable tiny arms. Paleontologists have a lot of information for T. rex because of the numerous fossils specimens we’ve found – over 40 of them. That amount of data gives us the ability to judge if a feature is a within the range of typical variation or something different. 

The T. rex at the American Museum of Natural History.

Most of the time, paleontologists only have a handful of specimens of any given species and we have to make inferences from those. Sometimes we only have 1 specimen and the inferences get really tough (Google the Nanotyrannus debate for an example). 

But not with T. rex. We have a lot of them. We know what they looked like and the range of many traits they had.

Figure 1A from the paper, showing the locality where the specimen was found.

And that’s the context for the new Tyrannosaurus species found in New Mexico. The original specimen was found a while back and analyses showed that it was a T. rex. However, the discovery of a few more skull bones and a re-analysis done in a [study] published in January cast doubt on its original diagnosis. 

Figure 1C from the paper showing the found bones of the skull and neck.

The skull bones show features that are outside the known range for T. rex. And not just one feature on one bone… every bone shows a feature different from what’s seen in T. rex

The authors note that these differences are not due to size or developmental differences because this new tyrannosaur- Tyrannosaurus mcraeensis (named for the McRae Formation where it was found) is the size of any adult T. rex. They also note that the differences are subtle, but so are differences between other species within the tyrannosaur family. 

Figure 5A showing the size of the new specimen (left) compared to “Sue” (middle), the largest T. rex specimen to date, and the holotype (right) of T. rex.

The McRae Formation has rocks that date to 7 million years older than the next T. rex-containing rocks, lending more evidence to this specimen being a new species.

A reconstruction of Tyrannosaurus mcraeensis. Credit to Sergei Krasinksi (with edits).

My one wish for this is to find a long bone, so that we can get an age for the specimen – so far it’s just part of a skull and some neck. And I will say, I approached this with a lot of doubts. Many researchers want to overthrow T. rex, or split it into species, or discover something that is novel about it, so I started reading this study with a big grain of salt. However, the authors do a good job of addressing those doubts with the evidence the fossil shows. Only time, and more fossils, will tell if this hypothesis holds out. 

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