Dinosaurs with ouchies on their noggins

A pair of Pachycephalosaurus put their heads together at Drayton Manor Theme Park. Photo by Matthew Wells, via Flickr.

Since I was a kid, one image has been stuck in my head when I think of the pachycephalosaurs, those rugged ornithischians with the outrageously thick frontoparietal bone: two males ramming into each other headlong like the two dudes in the photo above. I'm pretty sure I'm not alone in this. Even when I read a piece expressing skepticism about the behavior, I think to myself, "okay, but we're going to stick with the rule of coolness here." It's the same attitude that annoys me when people choose to not believe that many theropods wore feathery integument of some fashion. Again, I remind myself that science isn't about accepting things because they seem to make sense or they're pleasant to believe. So it's good that he head-butting hypothesis has recently had a spate of research devoted to it. The latest is a new paper in PLoS ONE, Cranial Pathologies in a Specimen of Pachycephalosaurus.

The rock has offered us scant remains to work from, which is probably a good reason why the pachycephalosaurs - who Tom Holtz refers to lovingly as "buttheads" in his Dinosaurs encyclopedia- have long been the subject of debate. Why were those heads so thickened? How many taxa might simply represent different growth stages of one species? Could they sustain direct head-to-head impacts or was it more likely that they smacked each other sidelong? Because of the patchiness of their fossil record, it's been difficult to put many of these ideas to the test.

In this new study, University of Wisconsin paleontologist Joseph Peterson, with the help of Christopher Vittore from the Department of Radiology at Rockford Memorial Hospital in Rockford, IL employ CT scanning of a Pachycephalosaurus frontoparietal dome at the Burpee Museum of Natural History and compare its apparent signs of injury to lesions in modern animals. Are these spots best interpreted as sites of bone resorption, as the rigors of decay, fossilization, and erosion, or are they evidence of traumatic impact?

Peterson's study concludes that it ain't the first two. Bone resorption, studied extensively in chasmosaurine fossils, leaves a smooth surface, as opposed to the roughness of the lesions in the studied Pachycephalosaurus dome. Signs of likely taphonomic suspects - such as scavenging, insect traces, and water wear - are also absent. But the lesions do match the marks left on the the crania of modern birds which have sustained impact and lived to tell the tale. Moreover, Peterson writes that the marks "are clustered over the thickest region of the dome; this distribution corresponds well to the location of expected traumatic pathologies resulting from agonistic behavior proposed for pachycephalosaurids." That would be head-butting, though they could have been smacking into boulders instead, I suppose. It's less absurd than some recent dinosaur stories in the popular press.

This follows another recent study by Eric Snively and Jessica Theodor, who compared the skull of Stegoceras to modern "buttheads" and found that "some pachycephalosaurs were as competent at head-to-head impacts as extant analogs displaying such combat," including musk oxen and duikers. (check out Eric's guest post at Archosaur Musings for more on this). On Facebook, Andy Farke joked that the "P" in PLoS ONE may as well stand for "pachycephalosaur." That wouldn't be so bad, would it?

Also be sure to follow Dr. Peterson on Twitter and at his blog (which has been dutifully added to the LITC blogroll).

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.

Vintage Dinosaur Art: Tim and Greg Hildebrandt

The Brothers Hildebrandt, Tim and Greg, are illustrating legends, and I was heavily stoked when I came across this title at a resale shop, stuffed unceremoniously between two lesser books. Pinup girls, Darth Vader, hobbits, and Hollywood monsters have been brought to life in their unmistakable style, so it's only fitting that the greatest creatures to ever walk the Earth should as well. Titled simply Dinosaurs, it was published by Merrigold Press in 1976.



Dig these hairy Pteranodons, buzzing a Pachycephalosaurus. A bonus ChasmoPointTM for not calling them dinosaurs!



Even the Hildebrandts weren't immune to copping a bit of Charles M. Knight's portfolio, though. Witness the latest installment in the Ornitholestes library I'm slowly amassing.



It may look like a man in a suit, but this version of the tyrant king is total evil.



I would like to see a post-renaissance take on dinosaurs from the Hildebrandts - maybe a pulpy, lurid series on Jurassic Park. Something like they did with the original Star Wars poster, you know the one with Luke raising his lightsaber above his head and Leia striking a pose at his feet. Only substitute in Alan Grant and Ellie Sattler, with T. rex instead of Darth Vader. I bet their take on the ice cream eating scene would be mind-blowing.

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

Anatotattin'

I am not a tattoo dude. There are plenty of reasons that people like marking their skin. While I understand the desire, none of the reasons are worth the procedure or its permanence of its results. But I suppose that if I was of the tattooing persuasion, I would do something like this:

A pachycephalosaur and (Flickr Image since removed by user) a Protoceratops couple. Admirable in that they're not the classic five or six dinos. The only problem with this approach is that as paleontologists make further discoveries, I might be stuck with an obsolete piece of paleoart on my skin. Not attractive. It might be best to play it safe, like these folks:

I think my ideal dinosaur tattoo would be something like a Troodon on my tummy, and my chest hair would be a cute little hairdo. I would also need to solve the problem of growing hair on my chest.

Also: Face to face with dinosaurs, via virtual reality - Video from Scientific American. It looks like a big ViewMaster toy. Speaking of which...