The Thagomizer of Stegosaurus (Stegosaurus Week)

This Saturday at the Morrison Natural History Museum in Morrison, Colorado, we will be celebrating Stegosaurus Day, in honor of Colorado's state fossil!  (To learn more, click HERE to be redirected to the Facebook page of the Morrison Natural History Museum!)  So, in honor of Stegosaurus Day, The Natural World is going to have ourselves a little Stegosaurus Week!  Each day, we are going to be looking at a different aspect of Stegosaurus, and today, we are going to be looking at the tail spikes on the tail of Stegosaurus, nicknamed the thagomizer!  What were they used for?  Did they actually use their tail spikes for defense?  And how about that funky name: where did the term "thagomizer" come from?  Well, learn those answers and more in today's post for Stegosaurus Week!

Remember IN THE LAST POST OF STEGOSAURUS WEEK when we mentioned paleontologist Ken Carpenter and his very complete and articulated Stegosaurus skeleton?  Well, in 1993, when Carpenter was presenting his findings, he first used the term "thagomizer" to describe the tail and spikes of Stegosaurus.  Without even knowing its backstory, it seems like a fitting name: but its true origin is even more interesting!  For those of you who have enjoyed Gary Larson's fantastic "The Far Side" comic strip, then you may already know where we are heading with this!  One of my favorite "Far Side" strips is the one above, and, interestingly, it is from this strip that Ken Carpenter got the name "thagomizer!"

Now, one question that has stumped paleontologists for years is, how were the spikes arranged on the thagomizer?  Nowadays, we know that there were two sets of spikes, and they are thought to have been about 180 degrees from each other, forming a horizontal line.  (For a more complete discussion, see the last post in our Stegosaurus Week series, entitled THE GENUS STEGOSAURUS THROUGH TIME.)  But other questions stumped paleontologists, too.  For example: what was the thagomizer used for?  It definitely looks like a very apt defensive weapon, but for a long time, paleontologists had no clues to help them figure out whether defense was actually the answer.

One source of evidence that Stegosaurus and other stegosaurs were using their thagomizers to defend themselves is that many of the spikes have broken tips.  Now, just because a fossil is broken, doesn't necessarily mean that it was broken during the animals life.  Paleontologists can tell whether or not a bone was broken during the life of the animal by looking to see whether the bone shows any signs of healing.  If the fossilized bone shows signs of "remodeling," then the bone broke during the life of the animal, and then started to heal while the animal was still alive.  Following the death of an animal, if somehow a bone becomes broken, it's not going to heal: the animal is already dead!  In a study that examined 51 tail spikes of Stegosaurus, researchers found that about 10% of these spikes had broken tips whose bone had started to grow back.  So clearly, these spikes weren't just for show, and were actually being used for something.

The best evidence that paleontologists have right now that indicates that Stegosaurus was using its thagomizer to defend itself against predators is an Allosaurus tail vertebrae with a hole in it: a hole exactly matching the kind of hole that a thagomizer would have made!  What's very interesting about this fossil is that, while damaged bone in the vicinity of the hole shows signs of healing (indicating that the Allosaurus survived, at least for a little while, following its encounter with Stegosaurus, and that the damage to the vertebrae was not post-mortem), the hole itself doesn't seem to have healed at all.  This has caused some paleontologists to hypothesize that part of the tough outer sheath that would have surrounded the tail spikes in life, probably making them sharper and pointier, of a Stegosaurus became stuck in the tail vertebrae, remaining lodged within the tail vertebrae of that particular Allosaurus, until the animal died!

Another picture that I took at the Denver Museum of Nature and Science of an Allosaurus attacking a Stegosaurus adult and juvenile

Want to learn more about Stegosaurus and it's relatives? Well, check out the Homebase for Stegosaurus Week HERE to partake in more of the festivities!

The Genus Stegosaurus Through Time (Stegosaurus Week)

This Saturday at the Morrison Natural History Museum in Morrison, Colorado, we will be celebrating Stegosaurus Day, in honor of Colorado's state fossil!  (To learn more, click HERE to be redirected to the Facebook page of the Morrison Natural History Museum!)  So, in honor of Stegosaurus Day, The Natural World is going to have ourselves a little Stegosaurus Week!  Each day, we are going to be looking at a different aspect of Stegosaurus, and tonight, we are going to be looking at the genus Stegosaurus as a whole, and how our concept of Stegosaurus has changed over time!  Let's dive on in!

Stegosaurus was first discovered by famous paleontologist Othniel Charles Marsh during the Bone Wars (a paleontological competition from Marsh and rival paleontologist Edward Drinker Cope) in the late 1800s, and was first described by him in 1877.  When Stegosaurus was first described by Marsh, he wasn't really sure what it was: he actually thought that it might be a turtle-like creature, as you can see in his illustration from above!  This reconstruction explains why Stegosaurus has its name: covered, shingled, or roofed reptile.  It wasn't until Marsh and his crew found other, more complete specimens of Stegosaurus that he was able to figure out what the animal looked like with a greater degree of accuracy, which you can see in the reconstruction below.

The reconstruction above, while much closer to what we think Stegosaurus looks like today than the first reconstruction, nevertheless has several key differences from today's reconstructions.  One of the main issues that paleontologists face when reconstructing Stegosaurus and its relatives from fossils is that the plates aren't attached to any bones.  The plates are modified osteoderms, used in many different animals to protect themselves from attack (a more extreme example of which can be seen in the close relatives of stegosaurs, the ankylosaurs).  Like the osteoderms in other animals, such as the ankylosaurs, the osteoderms would sort of "float" in the skin, only attaching to the rest of the skeleton by means of softer tissues, softer tissues that don't typically fossilize.  What paleontologists really needed was an articulated specimen of Stegosaurus.

Well, that's exactly what paleontologist Ken Carpenter got in the 1990s!  Using this very complete and articulated specimen, Carpenter and his colleagues were able to solve a number of Stegosaurus mysteries.  For example: the exact placement of the plates.  In Marsh's 1890s reconstruction above, you can see that he had positioned the plates in a single row running down the back.  Later reconstructions by other scientists had been created with a double row of plates, which was proven to be correct by Carpenter's specimen.  It was also shown that the plates alternated down the back, as opposed to the side-by-side reconstructions sometimes seen.

Another mystery that Carpenter's specimen was able to solve is the number and placement of the tail spikes.  As you can see in Marsh's 1890s reconstruction above, he hypothesized that Stegosaurus had four pairs of spikes, and that they pointed upwards at around 10-15 degrees from the vertical.  Carpenter's specimen, coupled with further research, has shown that, to the best of our knowledge, no species of Stegosaurus had that many tail spikes: in fact, from what we know, all species of Stegosaurus had two sets of tail spikes, for a grand total of four.  We also now know that, instead of the spikes being about 10-15 degrees from the vertical, they were almost certainly horizontal to the ground!  This hypothesis is backed up by investigations into the flexibility of Stegosaurus's tail: to successfully bring its tail spikes into play in the 10-15 degree arrangement, Stegosaurus would have had to have a tail much like a scorpion, and all research done up to this point indicates that Stegosaurus had nowhere near that much vertical flexibility in the tail.  However, the horizontal reconstruction makes much more sense, as the tail seems like it would have had a great deal of side-to-side flexibility.

The final main difference between Marsh's 1890s reconstruction and our reconstructions of Stegosaurus today lies in the way it held its tail and its neck.  Due to the Dinosaur Renaissance in the 1960s and 1970s led by John Ostrom and his pupil Robert Bakker, the perception of dinosaurs as lumbering failures changed dramatically to what it is today: not failures of evolution, but instead, a remarkable success that shaped the evolutionary course of the Earth for millions of years.  This change in perception is reflected in how we think dinosaurs moved: we no longer think that they dragged their tails on the ground, barely able to keep their heads from dragging in the mud.  Instead, we view them as much more nimble than we once thought.  And while Stegosaurus may not have been the nimblest of them all, you can clearly see how our ideas of how we think this animal moved around have changed over the years.

Want to learn more about Stegosaurus and it's relatives? Well, check out the Homebase for Stegosaurus Week HERE to partake in more of the festivities!

The Thagomizer of Stegosaurus

Stegosaurus is a very famous dinosaurs, one of the most famous, along with Triceratops and Tyrannosaurus.  All three of these dinosaurs have been found in Colorado; in fact, the first Triceratops bones were actually found in Denver, and Stegosaurus is the state dinosaur of Colorado!  Stegosaurus has also been found in Wyoming and Utah, in the Morrison Formation, as well as in Portugal.  The row of plates along its back make it very interesting looking, as do the spikes on the end of its tail, nicknamed a "thagomizer."  Holes in the vertebrae of a potential predator of Stegosaurus, Allosaurus, fit exactly with the size of one of the spikes on the tail of the Stegosaurus, showing that there was almost certainly a predator-prey relationship between the two, as was previously suspected.  Studies have shown that the tail end of Kentrosaurus, a close relative of Stegosaurus, also with a thagomizer on its tail, could have been brought around to the side of the dinosaur, potentially swatting at enemies trying to attack the Stegosaur from the side.

Finally, discoveries of articulated Stegosaurus skeletons show that the spikes were actually horizontal from the ground, as opposed to held at an angle, as you can see in the outdated and incorrect picture below.

It's an interesting word, though, isn't it: "thagomizer."  It certainly doesn't sound like a very sciency name; in fact, when I first heard it, I immediately thought it sounded like something out of one of my favorite comic strips, Calvin and Hobbes, by Bill Watterson.  Well, if you thought that as well, then you were actually surprisingly close to the mark, as the term did in fact come from a comic strip, another one of my favorites: The Far Side.

The term wasn't used scientifically until the year 1993, at the annual meeting of the Society of Vertebrate Paleontology, by the one-time Curator of Paleontology at the Denver Museum of Nature and Science Ken Carpenter.  The name has been used multiple times since then, in different mediums; books and television shows (including another personal favorite, BBC's Planet Dinosaur), as well as places such as the Smithsonian Institution and Dinosaur National Monument.  Although an informal name, it is popular, and has amusing origins.

Gary Larson created a great number of hilarious The Far Side cartoons covering all sorts of topics.  However, it seems like a large number of them featured animals, or were in some way nature related.  You will undoubtedly see a great many of them in this blog as time goes on, but here are a few other amusing dinosaur ones.