Tuesday, November 30, 2010

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.

Cranial domes

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

Pachycephalosaurus
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.

Pachycephalosaurus dome

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

Pachycephalosaurus dome

Pachycephalosaurus dome

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

Stygimoloch dome

Stygimoloch dome

IMG_4818

Stygimoloch dome

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.

2 comments:

  1. "Hopefully, Robert Bakker will soon publish descriptions of the fossils that will put Horner's idea to rest, as he's reported to hold."

    Hasn't Bakker already done so (See the following quotes)? I figured Horner was just ignoring relevant/contradictory evidence (as usual).

    Quoting Bakker et al. (See "DISCUSSION": http://uuu.childrensmuseum.org/themuseum/dinosphere/draco_rex/dracorex_hogwartsia.pdf ): "Dracorex hogwartsia is most similar to Stygimoloch spinifer and to
    a lesser extent Pachycephalosaurus wyomingensis. It is considered to be a
    member of the Pachycephalosaurini as defined by Sullivan (2003). However,
    it is readily distinguishable from these two taxa based on a number of
    key features. Stygimoloch spinifer is characterized by a huge spike cluster,
    consisting of 3 enlarged (hypertrophied) spikes. This differs from the shorter
    4 spike arrangement in Dracorex hogwartsia. Galton and Sues (1983)
    characterized Stygimoloch spinifer as having three-to-four spikes on the
    squamosal. This characterization allowed them to include a smaller, isolated
    squamosal with four spikes (YPM 335), a specimen we here consider
    to be referable to Dracorex hogwartsia. We have been able to determine,
    based on comparison with other documented specimens of
    Stygimoloch spinifer (MPM 7111 and MPM 8111), and two undescribed
    specimens in private collections, that S. spinifer consistently has these enlarged
    spikes coupled with an incipient, laterally compressed dome, made
    up of only the frontals and parietal. S. spinifer lacks open supratemporal
    fenestrae. Moreover, these skulls are of the same size as the holotype of
    Dracorex hogwartsia, so we conclude that these differences are not the
    result of ontogenetic development."

    Quoting Bakker et al. (See "DISCUSSION": http://uuu.childrensmuseum.org/themuseum/dinosphere/draco_rex/dracorex_hogwartsia.pdf ): "Flat-headed pachycephalosaurids include the Asian taxa
    Homalocephale calathocercos, Goyocephale lattimorei, and
    Wannanosaurus yansiensis (Sereno, 2000). Prior to this discovery of
    Dracorex hogwartsia, the only semi-flat headed pachycephalosaurid from
    North America was Stegoceras validum (which includes the flat-headed
    Ornatotholus browni), and the flat-headed nature of this taxon is only demonstrable
    in juveniles of the species (Sullivan 2003, 2005, 2006). Some
    specimens of S. validum retain a well-developed parietosquamosal shelf
    and moderate-sized supratemporal fenestrae (Sullivan, 2003, fig. 2). D.
    hogwartsia, however, has a skull that is totally flat, with no hint of doming.
    Although the specimen is considered to represent a young adult, we believe,
    based on the beginning of coosification of mid-cervical arch with
    centrum, that the animal was probably near maturity."

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