The X marks the problematic scute (see the text for details).
This update covers November 8-9. Much more progress has been made on the Gorgosaurus skull preparation. The eye socket area is the worst crack-wise and will likely be left until the end. Too many converging cracks make this a weak area. Late last week the thin edge of a new bone was found above the gorgosaur’s snout. This was treated at first as a disarticulated gorgosaur skull bone, but further uncovering work revealed it to be an ankylosaur (armored dinosaur) scute or piece of bony armor. This was positioned atop the skull and slightly overlapped on one side. It was an ugly piece of bone and obscured anatomical details useful for tyrannosaur researchers so I decided to remove it.
First the snout of the gorgosaur was heavily glued as was the scute and both allowed to dry. Care was taken to not glue the rock in between as we wanted to remove that. Using a sharp scalpel with a new #11 blade and an airscribe with a long stylus (tip) I was able to dig down around the scute, isolating it from the skull and pedestalling it on the rock. I then airscribed under the scute as far as I could go from nearly every direction.
Airscribing around the scute
Various preparation tools
I regularly used an old dentists mirror to inspect my progress underneath and remove any rock pieces with long-jawed forceps. In one spot the glue had soaked into the rock, thus hardening it. Because the glue is solvent-based I simply squirted as small amount of acetone on it and soon the glue gave way and I could remove the gooey glue and sandstone mass as required. When I was about to pull the scute away, it broke on the end that covered the side of the gorgosaurs snout. This might be a tragedy to some (It broke! GASP!) , but it was truly what preparators and field workers call a “lucky break”. Not only did it give me more access with the scalpel and airscribe, but importantly, it exposed a previously hidden badly cracked and weak area of the snout. This could have shattered had I pulled the scute off prior. These new cracks in the snout were now fully exposed and thus could be repaired easily. The scute was then wiggled back and forth and some water squirted underneath until it came loose without any more problems.
The cracks in the bone under the scute
On November 9th it was confirmed that some of the gorgosaur skull has in fact become disarticulated. In the antorbital region in front of the eye socket I removed what looks like an epipterygoid (part of the braincase). Research papers are very useful at this stage to identify bones and plan for their extraction. Next to it was a large bone end which has been confusing, but was eventually identified as the right postorbital (bone forming upper half of eye socket). Curiously, 75 million years ago this fell off the back of the skull, moved forward to the middle of the skull and was flipped around so the top of the bone is now the bottom (and bottom is top) and flipped again so front is back and back is front. After all that it was then pushed somehow across the middle of the skull so this right skull bone is on the left side of the head! Water currents? Scavengers?
The skull as of the 9th of November
I will be removing the errant postorbital in the next day or so. I am also now finding some of the very thin palate bones. I can only expose a few millimetres at a time, glue the exposed bone, wait an hour or two and then repeat. It is very tricky work as the bone is as thin a rice paper. One wrong move and it is ruined forever.
More to come soon. All photos here and in the series are owned by Darren Tanke and the Royal Tyrrell Museum.
Dave Hone's Archosaur Musings 
The recognizable tyrannosaur skull shape is really emerging now.
Interesting that a piece of ankylosaur armor would be mixed in with the gorgosaur skull bones. Are the armor-scutes diagnostic enough to tell what kind of ankylosaur it was?
This piece of armor is difficult to identify to generic level. It is a playing card-sized piece, lozenge-shaped and flat. I don’t work on ankylosaurs, but showed it to someone here who has more experience with them. He too said it would be hard to identify as it is flat top to bottom. Typical round to oval-shaped scutes with a keel and flat bottom are referred to nodosaurids and those with an excavated base ankylosaurids. This piece before me actually has a convex base and no keel at all! Armored dinosaurs from the formation are Euoplocepahalus, Edmontonia, and Panoplosaurus. Take your pick I guess….
OK, thanks.
Great stuff. I guess one man’s awesome ankylosaur scute is another’s pesky rock getting in the way!
The disarticulated right postorbital might have only flipped once if it rotated about the lateral axis.