Propping up an Allosaurus skull found in 1924: conservation at Dinosaur National Monument

The Allosaurus fragilis skull on display at Dinosaur National Monument was cracking under its own weight, and in 2019 two parks teamed up to fix it. The skull, catalogued as DINO 2560, was found in 1924 at the Carnegie Quarry Wall and stands in the Quarry Exhibit Hall in front of a cast of its skeleton. It is the only real, complete dinosaur skull material at the monument that visitors can see up close rather than in the quarry wall, and it is very popular with more than 300,000 visitors a year.

What was wrong

The way the skull was mounted made small points carry its whole weight, which caused stress fractures in the skull and lower jaws. Part of a lower jaw had also been taken off for research and could not go back on display because of the state of the mount. It had lain flat in the exhibit case for over a year, which is hard on a fossil: left unsupported, one usually touches the surface at only three or four points and cracks.

The fix

Under a technical assistance request, the monument's paleontologist took the skull to Petrified Forest National Park in Arizona, where a museum curator with 22 years of Park Service experience in building and making museum exhibits led the work. During the absence, the skull of the newest dinosaur named from the monument, Allosaurus jimmadseni, went on display instead. The two species differ in age: Allosaurus jimmadseni comes from the older Salt Wash Member of the late Jurassic Morrison Formation, and Allosaurus fragilis from the younger Brushy Basin Member, where the Carnegie Quarry lies.

• A stronger mount. The steel mount got an extra strut to cut the visible sway and bounce, which should lower the risk of damage from vibration caused by earthquakes, construction or other activity.
• A cradle. The unknown silicone-like padding was replaced with a form-fitting epoxy cradle that supports much of the underside of the palate and the sturdy quadrates (bones that join the lower jaw) while avoiding the delicate braincase. It is lined with museum-quality felt, which pads the skull and keeps it from chemical contamination at the contact points.
• Cracks. A museum technician and an intern stabilized small cracks in the skull and lower jaws with Paraloid B-72, an archival adhesive used by art conservators, and filled larger fractures with a mix of B-72 and glass microbeads. Older fillers, including plaster, epoxy putties and unknown materials, were documented and painted a neutral gray to set them apart from the fossil and keep the patchwork from distracting visitors.
• Safer jaws. Before, the skull had to be lowered slowly onto the delicate lower jaws, a risky bit of handling in which one slip could badly damage the specimen. Custom felt-lined brass mounts now hold the jaws, with small removable clips so that they can be taken out for research and still be held firmly in an earthquake.

All treatments were recorded for the monument's archives. A time-lapse video of the work was expected on the Petrified Forest YouTube channel. The skull went back on display at Dinosaur National Monument in mid-November 2019, ready for a longer, safer showing.

Sources

• National Park Service, "Preventative Conservation of an 'Allosaurus' skull from Dinosaur National Monument," by ReBecca Hunt-Foster (Dinosaur National Monument) and Matthew Smith (Petrified Forest National Park), in Park Paleontology News, spring 2020 issue (https://www.nps.gov/articles/preventative-conservation-of-an-allosaurus-skull-from-dinosaur-national-monument.htm); rewritten in hubnx's own words. The page's photographs of named staff are left out.