This update covers November 17-18. There have been a major development on the Gorgosaurus project. Some people are having trouble figuring out how a femur could end up near the head so in this posting I include a rough schematic sketch I made on Nov. 17th showing the rough placement of known bones and presumed ones. When the drawing was done, the presence of a tibia and hind foot were not known but presumed- more on that later.
Work on the 17th confirmed the presence of a femur and fibula. The bone in both (femur especially) was well preserved in many places, but badly shattered, splintered and collapsed. Other places the bone is like a completely dried out chocolate cake and just as crumbly. It truly is awful bone to work on, but fortunately all the pieces are still in place or nearly so and the rock comes off the bone very well. In the last posting I briefly discussed 2 ton epoxy glue and in this posting I want to detail some common sense approaches and tricks I have used with epoxy glue in the past and have/will use on this Gorgosaurus.
Epoxy glue is typically mixed in a 50:50 ratio and has the consistency and viscosity close to that of honey. Techniques:
1. When using glue to fill cracks in bone or in the rock (or both), slowly pour the glue in at one end of the crack (usually the widest end) so it flows along driving the air in the crack ahead and out of the way. Simply pouring the glue into the crack can form large trapped air pockets of bubbles, weakening the join. Always pour at one end and let it flow along. If it is an angled but nearly horizontal crack, pour at the higher end, taking care that it does not flow out the lower end onto bone surfaces.
2. For a shallow pour, cram the bottom of the crack to the depth desired with packed tissue paper, then pour the glue in. Alternatively, you can mix the glue, wait until it begins to cure then pour the thickening glue into the crack. If the crack is two-sided with a vertical crack and a horizontal one, the poured glue will escape out of the vertical crack. Plug that off with tissue paper crammed in or use putty/plastecine to hold the glue in. I prefer the tissue paper technique.
3. If you find the epoxy too runny, you can thicken it up. Much like adding flour to gravy to thicken it, you can make the epoxy glue thicker by adding various inert materials. Materials such as clean sand, powdered glass (which may give the epoxy an unwanted white color), chopped strand (short lengths of fibreglass) can be mixed in. Another way to thicken the glue without adding other materials is to chill it in a fridge first, mix it up and then pour. It will still set, but will take its time doing it.
4. If you want the epoxy to be colored, the best way is to simply stir in powdered tempera paint until the desired color is achieved. The wet color of the paint/epoxy mix during the mixing phase may be different from the dry and hardened epoxy, so advance testing should be done.
5a. Epoxy, like any fluid, will follow the path of least resistance. If the glue is not flowing where you want it to, then raise the fossil or specimen jacket on one end and block it up with lengths of wood, or, being very careful, tilt the table slightly. 5b. Another trick is to use your airscribe. Air blows out the tip. On a low setting, this air can be used to push around and steer the glue to places where you want it to go.
6. Epoxy curing is a chemical reaction, so if you want, a desk lamp can be aimed close to the setting glue and the warmth from the bulb help speed up the curing time. Don’t put the lamp too close- just close enough to keep the work area warm. This often works for other solvent-based glues too, especially those using acetone.
As with any new technique, test on a safe area first. Techniques 1,2, 5b, and 6 were used on the Gorgosaurus‘s knee region on the 17th. The glue cured overnight and on the 18th the rock airscribed away. This eventually resulted in epoxy glue “dikes” appearing atop the cracked (but no glue-filled) bones and this was simply airscribed away until near the bone. At this point a sharp knife (#15 scalpel blade) was used to shave and trim the excess glue away.
On the 18th, additional work around the fibula and cracked bone/rock there neccesitated my clearing a new work area. While brushing away loose matrix the extreme edges of three toe bones were found! These needed exposing and stabilizing so by the end of the day three complete toe bones and part of another were exposed. In one picture the plaster I poured in at the beginning of the project can be seen. It flowed deep into the smaller rock cracks and held the rock pieces together. These toe bones were exactly where I predicted they would be. No I don’t have X-ray vision, but this is where an extensive lab and fieldwork background comes in handy. Tyrannosaurids often pull their limbs up towards their body as part of their “death pose”. The legs fold up into a compressed “Z”, much like a scissors jack folded up. It is quite rare to find an articulated tyrannosaur femur and fibula together and not have a foot, so I was pretty sure the foot was there. The tibia is still unseen, but it has to be there- most of the foot articulates to it. Without the tibia being there, the foot would fall off. In the field, once we had the pelvis, I was pretty sure there was a skull, as the head is often pulled backwards over the pelvis (more of the “death pose” again). That prediction came true as well. Another thing I predicted this specimen might have was evidence of healing bite marks and it did; that prediction based on the fact about 50% of small tyrannosaurids from Alberta show evidence of this. So experience is invaluable in the field and laboratory when working on these remarkable specimens.
Work on the Gorgosaurus will likely slow down substantially now because the bone in the pelvic region appears to be very soft and crumbly, requiring much glue and glue drying time. Updates here may come now every 3-4 days, though I may fire off some brief fossil preparation tips from time to time. Don’t forget that the Royal Tyrrell Museum Facebook page has the daily preparation of this specimen posted there too, though postings there may slow down as well for the reasons above.
Note: Darren and I are still looking for questions on the specimen and preparation techniques , their effect on research etc. We’ll answer them later this week in a special post. Just leave your question as a comment below.
All photos here and in the series are owned by Darren Tanke and the Royal Tyrrell Museum.