Elementary Paleontology Education - Did Dinos Drag Their Tails

Coming home from school a couple of years ago (3rd grade), my daughter had some work to do in her Reading textbook. Looking over the questions, I was rather appalled by the implication that they had in this book that still persist to this day.

The question in question is number 15: "Write the letter of the part that shows the mark left by the animal's tail."

Analyzing the traces in the image above, the footprints indicate that we are likely dealing with a large theropod dinosaur, based on the shape of the foot. These three-toed impressions looks similar to a Tyrannosaurus rex or a Dilophosaurus footprint. Within the image above we can not entirely differentiate between the two because we don't have a scale to identify the size of the footprints. There are also other dinosaurs that make three-toed footprints. These include the ornithopods (like Parasaurolophus and Hadrosaurus), however their footprints are generally more rounded in nature, with less toe definition. 

Since these are likely theropod footprints, lets look at the anatomy of a theropod dinosaur. Science has basically determined that these large theropod dinosaurs walked as sort of a lever, with the hip joint working as a fulcrum. You have the front half of the dinosaur, with the head and arms on one side of the hips, laid out perpendicular to the legs, and the tail sticking straight out behind the dinosaur, also perpendicular to the legs. And if you look at the illustration given in the reading textbook, this is actually almost what is shown.

Although this is a more upright version of what this type of dinosaur looked like, it was likely closer to reality than what they were implying with the tail drag question. And even looking at the dinosaur pictured, there is no way that that dinosaur is even dragging its tail. There is a complete disconnect between the pictured animal and what the traces that it left behind indicate. Based on the size, shape, and number of fingers of the animal pictured, this is likely a Tyrannosaurus rex. And we can look at historical and modern interpretations of what the animal may have looked like.

Original mount for the Tyrannosaurus rex by Barnum Brown at the American Museum of Natural History. Image courtesy of Benjamin Burger. 

Looking at the original mount for the T. rex above, we can clearly see that this animal looked like it dragged its tail. However, for Barnum Brown to even get the skeleton to bend like this he had to break some of the vertebrate. Which, any modern day scientist will tell you, if you need to break the skeleton to do what you want it to do, you are doing something wrong.

As mentioned above, the modern day interpretation of the T. rex stance is much more of a lever and fulcrum situation as can be seen on SUE below from the Chicago Field Museum.

SUE the T. rex from the Chicago Field Museum.

The modern interpretation of the T. rex stance clearly did not drag its tail. I can't even imagine how this animal would have gotten its tail on the ground.

Other potential tail dragging dinosaurs?

The question then comes that if theropod dinosaurs didn't drag their tails, did any dinosaurs? And the answer to that is a "not usually, but maybe at times?" There have been some indications of tail dragging by dinosaurs in the fossil record, however these are by non-theropod dinosaurs, with the most obvious one being tied to a sauropod (Foster et al., 2000). Foster et al. describe a sauropod trackway that shows evidence of a midline tail drag mark from Twentymile Wash within the Grand Staircase-Escalante National Monument, preserved within the Middle Jurassic aged Entrada Sandstone. 

However, besides this one instance, there are numerous dinosaur trackways preserved around the world with the number of tail drag traces almost nonexistent. There has been a few others tail drag traces mentioned in the literature, which have been attributed to sauropods and ornithopods, however the identification of tail drag in some of these cases were ambiguous and uncertain. So, although it is possible that some dinosaurs may have been dragging their tails, it is far from common place, and more of an abnormality than anything consistent.

What if this trace was real?

But thinking like a scientist, and knowing what we know about modern T. rex anatomy, if we did come across the trace fossils as seen above (the footprints and tail drag), how would it be classified. We can narrow this down to several possibilities:

1. This is a true tail dragging theropod. 
• If this were the case, it would likely be due to an injury that the animal sustained, causing it to drag its tail. An anomaly that we haven't currently found in the fossil record, but one that cannot be ruled out.
2.  The animal itself was carrying something that was dragging along the ground.
• This is the theory that I like to think makes the most sense. Suppose the animal had its dinner and was carry part of it back to its home or somewhere else. With little use of its arms, its mouth and legs would provide most of the functions that it would need to carry and tear apart any meat. Therefore, it is possible that a large enough body of meat would drag along the ground while being carried by the theropod.
3. Something else made the drag mark.
• There are numerous animals that do drag their tails, specifically reptiles, lizards, and other small animals. Although it is a possibility that a theropod came along and walked along the other animals footprints, essentially erasing them, while preserving the tail drag, this is an extremely unlikely scenario. It is also possible that is was a natural drag mark, perhaps a stick being floated down a river that drags along the riverbed. However, again it is unlikely that any animal would walk with this drag mark perfectly aligned between the footprints. 

So with the evidence displayed above, which is a set of theropod footprints and a groove aligned medially between them, I think the most likely scenario to produce such a trace would be the dragging of something, likely food, by the large theropod dinosaur. That would make this most likely a drag trace and not a tail drag trace.