Screenwashing, for victory!

So if you followed the link from my previous post over to the Great Plains Dinosaur Museum Facebook page, you would have seen a photo of a cute little pachycephalosaur squamosal spike that we found in the first week. Pachycephalosaurs are the famous dome-headed dinosaurs that are often shown headbutting each other (although the evidence supporting this behavior is controversial).

This pachycephalosaur spike came from "trucker's macrosite" which preserves a jumble of bones from various different species. Most of the time, bones in macrosites are water worn, or broken, but a small percentage are better preserved, so macrosites can be good places to look for skull bones, and other diagnostic elements.

One of the other interesting pieces collected from "truckers macrosite" is this fragment:

A braincase fragment from a theropod?

The specimen is constructed from quite thin-walled bone, and has numerous holes ("foramina") passing through it. I was wondering if this might be part of the braincase of a theropod dinosaur. The foramina would therefore represent the holes though which the cranial nerves passed. If anyone has a better idea for the identification of this bone, then I would be very appreciative (I am hoping it is not just parts of a large theropod vertebra).

Trucker's macrosite yielded additional pieces of theropod skull, but each of the interesting specimens from this site is incomplete, with broken ends where (presumably) pieces had broken off during recent weathering. So, in an attempt to find the missing parts of these tantalizing fragments, we returned to the site yesterday to do some intense surface collecting and screenwashing.

Peter Thomas (foreground) and Danny Anduza (shirtless) search for more treasures. Photo by Holley Flora.

Peter, Danny and Holley spent all day collecting every piece of bone from the surface and dry / wet screenwashing shovels of mud to find pieces that had been buried. Today we were trapped in camp by rain, so I started going through the surface collected material to see if we had any of our missing pieces.

Success!

Inside the third bag I looked in (collected by Holley) I found this little gem:

Pachycephalosaur squamosal spike, number 2!
The spiked tip is missing, so we will be back screenwashing again to try and find it!

Crewmember Holley Flora (one of my seasoned field crew from 2013 & 2014) had picked up another squamosal spike from a pachycephalosaur! This is almost certainly from the same specimen as our previous spike, so it is possible that there might be more at the site.

Pachycephalosaurus wyomingensis by Dr. Jordan Mallon.
The spikes we have found are from the back corners of the skull (squamosals) .
(Image from wikipedia)

You might be familiar with the recent research by my supervisor Jack Horner and Mark Goodwin from UCMP Berkeley. In 2009 they hypothesized that the dome and squamosal spikes of Pachycephalosaurus changed through growth. Young animals had no dome, but exhibited long spikes on the squamosals. The characteristic dome-head of Pachycephalosaurus developed through ontogeny, and was only fully-sized in adults. Curiously, adult Pachycephalosaurus had short rounded knobs on the squamosal, instead of the long spikes of youth.

The spikes that we found are quite small and fairly pointed (based on the angle of the sides, as the tips are missing), so we would interpret them as coming from a young Pachycephalosaurus.

As a bonus, in the next few bags of bone fragments, I found quite a few new pieces to go with the possible braincase. Overall, not a bad return for a rain day!

Reference

Horner, J. R., & Goodwin, M. B. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus.