The Tyrannosaur Controversy – Response!

Back in March, a [paper] was published that described evidence that T. rex may actually represent 3 species and not just 1. The authors used ratios of the length and circumference of the femur and also ratios of specific teeth diameters to define the differences between the 3 species. The authors of that paper named the new species Tyrannosaurus regina and Tyrannosaurus imperator

Specimens of T. rex showing the new species. According to the study, A and B are T. rex, C and D are T. regina, and E is T. imperator. Figure 1 from the original study.

That study ignited a ton of conversation and was widely published in different news outlets, almost as if the reign of the great Tyrannosaurus rex was at an end. However, from a scientific point of view, there were several questions that needed to be tested by other research groups in order to check if T. rex was actually 3 species or not. 

That study was officially [published] in July! The authors point out a few issues with the original work. They also re-did the analyses using a more thorough dataset and slightly different methods. Buckle up, this one is a doozy.

Firstly, the characteristics that the original study used to distinguish the 3 species do not actually separate all of the existing T. rex specimens – there is some overlap, making these characters difficult to use. 

Secondly, the original study did not use all of the information available for T. rex. There are 1850 traits that have been studied on Tyrannosaurus and its relatives that are available for use. Usually, new traits are added to the ones already available so that any new analyses use all of the data to get the most thorough answer. The original study based their species divisions on only the femur and tooth measurement traits.

Thirdly, the original study indicated that the amount of variation in the skeleton among the different Tyrannosaurus specimens was unusually large. When there’s a lot of variation, then it is more likely to be showing multiple species instead of just one. The new study compared the variation in Tyrannosaurus to that of 112 other species of theropods (including birds). They found a very typical amount of variation in Tyrannosaurus, not an unusually high amount.

Figure 4 from the new study shows that the variation in Tyrannosaurus (dashed line) is about average when compared to over 100 other therapods (yellow shading shows the range).

Fourthly, the tooth measurements from the original study were not consistently made. Some of the measurements were from teeth from the same side of the mouth, some from opposite sides, and sometimes no teeth were there so they measured the tooth socket instead. 

Lastly, the analysis the new study did showed that all of the information clustered all of the Tyrannosaurus specimens into 1 group, not 3. Part of the reason for that is the original study used a method in which you tell the analysis how many groups to look for, and then it usually shows you that many groups as a result. The new study did not define how many groups to look for, leaving it to the analysis to come up with the mathematically best answer (which was 1). 

Figure 3 a and c from the new study. A shows how the specimens clustered based on their femur measurements. The different colors correspond to the new species the original study categorized the specimens in (blue – T. rex, red – T. imperator, green – T. regina, purple – uncertain). The groups do not separate out the specimens by the species the original study indicated. C is the graph showing that the most likely number of groups is 1, not 3, for these measurements.

There were more discussion points in the new study than what I’ve listed here, but I hope that the case is pretty clear – Tyrannosaurus rex is a single species, not three.

An important point that the new study made is that half of the specimens in the original study came from private collections or commercial fossil companies. When specimens are sold to individuals or to companies, they become almost impossible to study. By doing this, the original study made it difficult to replicate their results. Replication is how we confirm conclusions in science, so if we cannot see the specimens, we cannot confirm a result. Specimens belong in museums or publicly accessible collections. 

You tell ’em, Indiana Jones!

A Fantastic Fossil Find

This week a new fossil has made it to its forever home in the Burke Museum of Washington State. The fossil, the remains of a Tyrannosaurus rex, includes a very complete skull, some vertebrae, ribs, hips, and parts of the lower jaw. Even though there is not yet a scientific article that describes this fossil in detail, I wanted to talk a little bit about it here.

Tyrannosaurus rex is one of the most popular dinosaurs out there. And what’s not to love? It’s got a giant head, and tiny arms, lived at the end of the Mesozoic in the western USA, and was a carnivore.

I-Have-a-big-head-and-little-arms

© Disney.

Even though it is so popular, we do not have that many fossil specimens. Complete skulls are super rare. So this new Burke Museum find is a true gem.

Dave Demar trex

The excavation of the skull. Photo by D. DeMar.

The fossil was found in the Hell Creek Formation of Montana this summer (2016) by 2 volunteers. The fossil, nicknamed the “Tufts-Love Rex” after the 2 discoverers, took 45 volunteers over a month to excavate. The skull is over 4 feet long and weighs 2,500 pounds (1133 kilos), mostly because of the rock surrounding the fossil inside the protective plaster jacket.

Skull at museum G Hindsley seattlePI

The skull arriving at the Burke Museum. Photo by G. Hindsley (Seattle PI).

This week, the fossil made it safely to the Burke Museum, where preparators will remove the rock from the fossil and probably put it on display. This find shows how important it is to continue to search for fossils (we still have so much to find!) and that as long as you’re in the right place, *you* can find fossils, too! Look for local opportunities to join a field expedition and help paleontologists find new specimens!

Tyrannosaurs: Babies and Growth!

The middle of March brought new information about Tyrannosaurs.

We start with a [paper] by Schweitzer and others in the journal Science. These authors discovered a special type of bone on the inside of a thigh bone (a femur) of a Tyrannosaurus rex. This type of bone is only found today in birds. This makes sense because birds are a type of dinosaur, the only ones to make it through the extinction at the end of the Cretaceous Period.

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Evolutionary tree of Archosauria (crocodiles, birds, their common ancestor and all of its descendants). Birds, on the far right, are theropod dinosaurs. Drawings by Scott Hartman.

This bone is super special because in birds, it’s only found in FEMALE birds that are in the process of forming or laying eggs. This type of bone is called Medullary Bone and it forms because the calcium that is needed for the mom to make eggshells is taken from her bones. Calcium is the component in eggshells that forms the outer, hard, protective layer.

MedBone3

Figure 1 from the paper. MB is medullary bone, CB is cortical bone (the outside layer of bone), and ELB is dividing bone layer. Left is a chicken, right is Tyrannosaurus rex.

This medullary bone forms to help move the calcium from the bones to the eggshells so it is only present while the eggs are being formed and laid. Once egg laying is done, the bone is almost instantly reabsorbed. This means that if we find this bone in dinosaurs, we can tell that they are female dinosaurs and that they were able to make eggs at the time they died. This gives us a whole new window into dinosaur life!

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Pregnant versus not pregnant female Tyrannosaurus rex.

Next, a [paper] by Brusatte and others in the journal PNAS. These authors describe a newly found tyrannosaur from Uzbekistan.

uzbekistan-map

Uzbekistan is the middle pink country.

This new tyrannosaur, named Timurlengia eutica, is important because it is from the early Late Cretaceous, a time that we did not have any tyrannosaurs for before. Timurlengia fills in a large time gap and can help answer questions about how quickly tyrannosaurs went from being small animals, like Dilong, to large ones, like Tyrannosaurus rex.

dilongtrex size
Dilong on the left, Tyrannosaurus rex on the right.

What the authors found is that this new species is still small to medium sized even though it is more closely related to the largest tyrannosaurs. This means that tyrannosaurs went from being small to medium sized animals to really big, much faster than we had thought and begins to fill in information we did not have before.

TyranoTree

Figure 3 from the paper. Tyrannosaur tree of life with the time periods on the bottom. Timurlengia is in red in the middle.