Fossil Excavation

One of the new plaster jackets.

This week, we made our first plaster jackets (also called field jackets) of the season! The fossils we collected, which are currently theorized to be from the stomach area of a mosasaur (or simply from a highly fossiliferous region) were discovered last year and were all ready to be packaged up when we returned to the site last week. The mosasaur skeleton isn’t articulated, but there are a number of vertebrae (backbones) that were deposited in a rough line and are surrounded by mosasaur rib fragments, fish vertebrae, Hesperornis legs (Hesperornis is a kind of marine bird), and even some fossil teeth (which could be from a smaller mosasaur that was eaten by the first one).

The hind limbs of a mosasaur, which are
permanently stuck in plaster.

We make plaster jackets in order to protect fossils when we transport them from the field to the museum, working in much the same way that casts do for broken arms and legs. That way, they stay safe in case the jacket is accidentally dropped or if some other calamity befalls them. Constructing a plaster jacket can take a while; first, we dig little trenches around a big fossil or a collection of smaller ones to separate them from the larger rock layer (it ends up looking like a collection of puzzle pieces carved into the rock). We then put a separating layer on top of the resulting “island”, which can be composed of paper towel, tin foil, or, in this week’s case, simply mud. This serves to keep the plaster from contacting the fossil, since it’s extremely difficult to remove plaster from rock. Thirdly, we dip strips of burlap in plaster and lay them on top of the separating layer, and finally, we cut underneath the island, flip it over, and repeat the plastering process on the bottom side.

Suzy's skull, with the tail of
Bruce, our biggest mosasaur. 

Once the jackets come to the CFDC, we saw them open (carefully, and with a hand saw, not our eyes) and clean the matrix, or surrounding rock, off the fossils. If need be, we glue fossil fragments together if they’re falling apart. Once that’s done, the fossils can accessioned or even put on display! Back in the 1970s and ‘80s, mining operations in our area resulted in a ton of fossils being found and plastered up. There were so many field jackets that we still have many of them waiting to be prepared! Our second-biggest mosasaur, Suzy, was actually lost and re-discovered because a number of years passed between the time when her fossils were jacketed and when the jackets were cut open. If you’d like to see her or any of our brand new plaster casts, come on down to the CFDC! We’re open every day. 

On the left is an old cast with a mosasaur
skull still in it. Next to it is a new
plaster jacket. 

Matt Remple
Field Tech