A Visit with Two Domeheads

After Dr. Kraig Derstler's talk about comparative tyrannosaur taphonomy, I had the unexpected pleasure of viewing two nice fragments of pachycephalosaur domes. In the picture below, you'll see one from a Pachycephalosaurus - the large one on the left - and a Stygimoloch.



Pachycephalosaurs are popularly known as the "dome-head" dinosaurs who likely used their dramatically thickened skulls for sexual competition.


Image from Orin Zebest, via Flickr.

Remarkably, the Pachycephalosaurus dome was discovered on the side of the road, somewhere in Montana. How many folks passed it by without realizing its value? Luckily, it wasn't some knucklehead looking for landscaping rock who picked up the skull, but it was Dr. Derstler. He had brought them along because the Children's Museum had arranged for them to be CT scanned at a local hospital.

My experience handling dinosaur fossils is sadly lacking, so it was a thrill to be able to take a close look at these. They are not complete skulls, and therefore may not be considered special by the layperson, or the jaded veteran of the field. But it was a rare pleasure to look at the exquisitely preserved internal structure of the bone, the contours and textures and varied colors. Here's a look at the surface of the Pachycephalosaurus dome, nicely displaying the outer covering. This is a very rare occurrence, as fragile structures like this are often lost before fossilization happens, or weathered away after the fossil is revealed by erosion.



In cross section, more traces of the living tissue are revealed.





While not as spectacular as the Pachycephalosaurus dome, preserving less superficial detail, the Stygimoloch was still pretty nice.









I can't discuss these dinosaurs without mention of Dr. Jack Horner's idea that the fossils of Dracorex, Stygimoloch, and Pachycephlosaurus are all growth stages of the same species (the last of the three, that is). Derstler is, to put it mildly, unconvinced. He respects Horner, but thinks that in this case, he has not provided sufficient evidence for his conclusions. It's not a minority view in the paleontology community. Hopefully, Robert Bakker will soon publish descriptions of the fossils that will put Horner's idea to rest, as he's reported to hold.

For many more photos of these fossils, head over to my photo set at Flickr. Thanks to the folks at the Children's Museum and especially Dr. Derstler for allowing me to take these photos.

The Dragon King, Dracorex hogwartsia

One of the centerpiece exhibits at the Children's Museum is Dracorex hogwartsia, the small and somewhat controversial pachycephalosaur. Discovered in 2003 by a group of fossil hunters who then donated the skull to the Children's Museum, it was formally described by Dr. Robert Bakker in 2006. Dracorex made a big splash, famously receiving its own cover story in National Geographic.

Also, it's named for a fantasy novel series of some note.

Having been preserved in the Hell Creek Formation, Dracorex in life was a denizen of the latest Cretaceous, and contemporary with Tyrannosaurus rex and Triceratops. In his description of the fossil, Bakker's stance, as you might expect, is that Dracorex represents a unique genus. A paper published last year by Jack Horner made the case that it's more likely a young Pachycephalosaurus, as is the somewhat larger Stygimoloch. Unfortunately, all we have to go on is the skull and a few vertebrae, and finding more of Dracorex's body would help clear this up (a key point, and one that makes me lean a bit towards Bakker's view is that Horner did not examine the original fossil. More on this here).

When it comes to the Children's Museum, questions about its true identity are beside the point, Dracorex has been used to great effect to teach a larger lesson. It's the foundation of the Dragons Unearthed exhibit, which takes the striking skull of Dracorex and uses it to tell the story of paleontology. Children are asked not to stand in wide-eyed awe in front of a skeleton, but to look at this skull and other fossils and think about how paleontologists interpret them to come to a reasonable image of the animals as they lived.


One of the learning stations in the Dragons Unearthed exhibit.

Pictured below, interpreter Mookie Harris ably mans the Art Cart, one of my favorite aspects of Dragons Unearthed.


Using the skull of a tapir, Mookie asks children to help put flesh to the bone. He lets them use their own imaginations to figure out what the animal would have looked like. This part is really fun to watch, as Mookie has great chemistry with kids and really lets them call the shots, no matter how strange. Goggle-eyes on top of the head? No problem. Mookie draws them on, then suggests that such an adaptation may mean that the bizarre creature spent a lot of time hiding in the water. These features may pop out of the child's mind for fun, but Mookie does his best to give them an evolutionary reasoning.



After their attempt, Mookie reveals the true owner of the skull and shows the features that can be used to diagnose it as belonging to an herbivorous mammal. Then, he brings out a cast of Dracorex's skull, showing that the process the child just went through is just what people have been doing for thousands of years. It's a great hands-on demonstration of how mythological beasts like dragons may have been dreamed of - and that even though we know better now, they were reasonable enough for people of pre-scientific ages. It's all about critically examining the fossils to come to the best conclusions possible about its owner. Mookie told me that he's really heartened by the level of knowledge displayed by his young visitors, especially the increasing numbers of girls who have a real investment in dinosaurs. He thinks that it has a lot to do with documentaries in the style of Walking With Dinosaurs, which portray dinosaurs as animals rather than monsters.

Dracorex hogwartsia reconstruction, using the post-cranial skeleton of Pachycephalosaurus.

Dr. Bakker himself is a consultant to the museum, and Josh told me that when he pays one of his occasional visits, he really immerses himself. Anyone who has seen him on TV specials knows of his enthusiasm, and it's easy to picture him pitching in at the fossil prep lab, holding court for a group of kids and parents to teach how the work is done. No matter how the scientific consensus on Dracorex shakes out, there's no doubt that its fossilized noggin is one of the coolest dinosaur bones ever found. Its discovery and presentation to the public has been a good thing for education, and the Children's Museum has made the most of it with an attraction that promotes critical thinking over spectacle.

Jack Horner on Skeptic's Guide

Dr. John Horner was the guest on a recent episode of the podcast "The Skeptic's Guide to the Universe." It's a really well conducted interview, which you'd expect from the Skeptic's Guide crew. It mainly deals with Horner's recently published paper which proposed that Stygimoloch, and Dracorex are not distinct species, but are younger forms of Pachycephalosaurus.

They touch on Dr. Robert Bakker's contention that this is not the case, and that he is in possession of a genuine juvenile Pachycephalosaurus that does not resemble either Stygimoloch or Dracorex. You can hear a bit of Horner's irritation as he basically dares Bakker to either publish about it or shut his trap. Juicy.

Three Genera... or One?

You may remember when the pachycephalosaur Dracorex hogwartsia was revealed to the public. The reason it garnered the amount of publicity it did was that its specific name (that would be the "hogwartsia") was inspired by a certain series of novels which in turn inspire hysteria in children. It was considered striking enough to grace the cover of the December 2007 issue of Nat Geo, featuring the John Updike article Big Bad Bizarre Dinosaurs.

Not to burst any muggle-bubbles, but according to Jack Horner and Mark Goodwin in Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus, Dracorex and the slightly larger Stygimoloch aren't unique genera at all: they are juvenile specimens of the iconic "bone-head" Pachycephalosaurus. Horner and Goodwin took a close look at the available specimens and determined that they clearly share characteristics of juveniles. Their bones were still growing, and their domes weren't fully fused.

Though we may lament the loss of two such colorful dinosaur names, this does bring the pachycephalosaur family more in line with the ontological (a fancy word for growin' up) development of other dinosaurs, such as the ceratopsians and crested duck-bills, as well as birds like the modern cassowary. In all of these, juveniles begin with much different head ornamentation before developing ostentatious adult characteristics fairly quickly during puberty.


Cassowary, by rogersmithpix via flickr