In 2003, Nick Fraser scouted a new locality near Shell, Wyoming, where he had been told that dinosaur bone fragments were present at the surface. Looking around on the surface, he found several bones, including these two sauropod sacral vertebrae:
These bones had rolled down a hill. Higher up on the hill, Nick found some bone fragments weathering out, and decided to excavate the next season.
In 2004, we began excavations at what became known as the Two Sisters site. Digging into the area where the bone was weathering out, we quickly found more extensive remains:
These were clearly the paired pubes and ischia, the lower four of the six bones that make up the pelvis. We immediately began to speculate that the first jacket possibly contained the ilia, the other two pelvic bones (and the ones that are attached to the vertebral column).
When the new museum opened, we opened both of these jackets. The one containing the lower bones was completed first:
Largely through the efforts of Decker Chaney and Amy Garrett, we also made steady progress on the other jacket, the suspected ilium. The image below, which is several months old, has been turned so that dorsal is at the top. The lateral side of the bone is visible. The hip socket is the curved brown area just above the spray can, confirming that this is the ilium. Most of the top half had weathered away.
The three knobs of bone sticking up at the top are attachments for the sacral vertebrae. In adult sauropods the sacrals are often fused to the pelvis, but this was a young animal, so the sacral vertebrae were still detached.
Which brings us back to the photo at the top of the page. I decided to pull out the vertebrae from Nick’s original discovery of the site and compare them to the pelvis. As is clear in the closeup, they are a perfect fit:
Now it wasn’t totally shocking that these bones all came from the same animal. They were, after all, found in the same place, and when you find sacrals and a pelvis in such close proximity you have to suspect that they may go together. But, considering that the vertebrae had weathered out of the ground and rolled down a hill, and were found a year before the pelvis, I had no real expectation that we would be able to prove that they were from the same animal; I certainly didn’t expect a perfect fit!
There is a third sacral attachment, and we have additional vertebrae from the site, so it’s possible we may be able to attach another vertebra, but so far we haven’t identified that one.