How Do We Know It's T. rex? (Updated)

For good reason (we scientists can get a bit long winded - we love what we do!) there isn't enough room on our Experiment.com Campaign page to provide you with all of the extra information about our project - all of the fascinating little tidbits - that we think really make this project special. So, I thought I'd take this chance to answer one of the most common questions we've gotten almost since the moment we first discovered this site in July 2014. That is, "How can we know that we have a Tyrannosaurus rex when we've only uncovered a couple of inches of a single bone?" Great question!

It's not always that easy to know what species you're uncovering. In fact, unless you have teeth, more often than not it is completely impossible to tell what species you're dealing with at such an early part of the process. But, there are a couple of good clues conveniently provided by this little bit of bone.

The first thing that jumped out to us is the general shape. The bone is clearly cylindrical. There aren't many cylindrical bones in the bodies of animals other than in the arms and legs, with the upper leg bone - the femur - being the most cylindrical (generally speaking). So, we had a good hunch right away that it was a femur, and clearly it is from a very large animal. The cross section of this bone is about 7"!!

What are more interesting and informative, though, are the features on the inside surface of this bone. The long bones (basically, the limb bones) of many animals have a hollow medullary cavity where marrow is stored. Mammals, crocodiles, birds . . . just about everything that doesn't make its living by diving in water has these cavities. The surface of that cavity is often rough and irregular, but in birds, that interior surface is just as smooth as the outside of the bone. As you can see from the picture below, the interior surface of our giant, bird-like bone is very smooth. Ah-ha! That's the big clue.

 

You see, we now know that birds evolved from theropods - the group to which all of the meat-eaters belong. And we know this because they share an extraordinary number of similar skeletal features, like very smoothly-lined, hollow limb-bones. We also know that the ornithischians - the other major group of dinosaurs - did not share this feature. So, we know we have a theropod dinosaur because we have a bone with a nice, hollow, medullary cavity.

But how can we be so sure it is T. rex, specifically? Well, that's just a very good guess based on the process of elimination. Very simply, there were no other predators this size living in this ecosystem at this time (about 67 million years ago). So it almost has to be T. rex - there just aren't any other options. It's certainly possible that there was another predator that size at that time, but it seems very unlikely that no one would have found any evidence of it by now. I'd be happy to be wrong though - the discovery of a new, giant predatory species in this part of the world would be an enormous and wonderful scientific discovery!

Of course this all depends on the success of our campaign. We're used to excavating fossils from rugged and remote places, but this situation is different. It is simply impossible for us to excavate this, and the 2 other dinosaurs just meters away, if we can't repair the roads and transport the specimens back to Philadelphia and New Jersey. And if we don't excavate them, they'll be destroyed by the elements. Please take a look at our campaign page and consider helping us with this great scientific adventure!

Take a look at the video below. Brittany does a wonderful job of explaining a bit about the bone, and the process of "field prepping" it so that it is protected from the elements for another 9 months until we can get back out to excavate it .

Expedition co-leader Jason Poole modeling beside the now-protected T. rex femur in July 2014.

 

UPDATE: As if it was written just on cue, just a day after his visit to the New Jersey State Museum, noted science writer and pale-phile, Brian Switek, published a very nice article on the Phenomenon blog about the bird-to-dinosaur link. Take a look!