Online exhibits : Field notes : Randy Irmis Investigating the rise of dinosaurs: Field notes from Randy Irmis By UCMP grad student Randy Irmis, late June, 2007

We recently finished up our excavations at Ghost Ranch. It was a very successful season; we collected over 1,000 specimens ranging from fragments to skulls and skeletons, and several hundred pounds of screenwash concentrate to be picked through at the lab. This year seemed to be the year of the phytosaur — we found many more specimens of this group of animals compared to last year.

The last week of excavations we were joined by the Ghost Ranch paleontology field seminar. This group is a bunch of folks who are interested in paleontology and excited to have a chance to join excavations. They are also invaluable in the field — we had a dozen people this year so it tripled our workforce. The seminar participants found many significant specimens and were a great help!

Making a plaster jacket
Last time I talked about how we recover the smallest specimens using a technique called screenwashing. This is inappropriate for larger specimens (1 cm or larger). For these, we want to use a method that will allow us to remove the bone whole and undamaged — preferably with a protective layer of rock that can be removed slowly in the controlled environment of the lab (where we have specialized tools, glues, microscopes, etc).

The key is to immobilize the rock and fossil so it comes back to the lab in one piece. The first step is to dig a trench around the entire bone, so that it is elevated on a pedestal, sort of like a mushroom. Later we will "pop" the "mushroom cap" away from the ground in one piece. The second step is to cover the pedestaled bone and rock with damp toilet paper. We apply the water with a brush so that the toilet paper conforms tightly to the surface. The reason for applying this layer is that it separates the next layer, plaster, from the bone, and prevents them from sticking to each other.

Left: Sterling Nesbitt applies damp toilet paper to the pedestaled fossil. This acts as a separator between the fossil and plaster. Right: Drew Eddy from North Carolina State University (left) and Randy Irmis (right) cut burlap strips.

The next step is to make some kind of jacket to protect the fossil. The type of jacket depends on how large your pedestaled fossil is. If it is smaller than a foot across, you can cover it with plaster medical bandages, the exact same thing that a doctor will use to put a protective cast around a broken bone. Because our pedestal is much larger, we'll be using plaster and burlap strips. Both of these methods achieve the same result; they form a hard plaster shell that keeps the inside contents from moving about or breaking apart.

Left: Randy Irmis wrings excess water out of burlap strips that have been soaking for several minutes. Center: Randy mixes water and plaster just before making the plaster jacket. Right: Randy saturates the burlap strips in plaster. The damp burlap soaks up plaster better than dry burlap.

Sterling Nesbitt applies the plaster-soaked burlap strips to the pedestal. He will cover the entire fossil with several layers. The plaster has cured so Sterling starts to loosen the jacket with a pick. To make the second type of jacket, first we cut the burlap into strips that are 5–6 inches wide and 2–3 feet long. These strips are then soaked in plain water for several minutes. After soaking, excess water is wrung out. Soaking the strips helps them absorb plaster. The plaster is mixed with water just prior to using it — if you wait too long, the plaster will cure into a hard solid. We soak the burlap strips in the wet plaster and make sure they are completely saturated. Each strip is applied to the pedestal in a criss-crossing fashion that will give it added strength. We make sure to cover the entire pedestal with several layers of burlap and plaster. Then the waiting game begins. We have to make sure that the plaster is completely dry. If the plaster hasn't cured, there is always the possibility that it will fail because it cannot support the weight of the rock and fossil inside. After making sure the plaster is dry, we drive chisels and picks underneath the pedestal (now known as a plaster jacket) to loosen it. The plaster jacket is then flipped, with the hope that nothing will fall out. After a successful flip, the plaster jacket is transported back to the lab where rock will be slowly removed to expose the fossil underneath.

Left: The entire group helps to flip the very heavy plaster jacket. Right: The jacket is flipped successfully. Randy Irmis inspects the bottom side to make sure no fossil bone is exposed.

The method explained here is decidedly low-tech — it hasn't changed in over 100 years! No one has found a better way to remove fossils, so we continue to follow the method of vertebrate paleontology pioneers.

All photos by Sarah Werning, UCMP.