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.

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