Jacketed Fossil Specimens
Janet Spiller/© AMNH
The jacketed fossil array in the Louis V. Gerstner, Jr. Collections Core on the first floor of the Richard Gilder Center for Science, Education, and Innovation presents a group of specimens that are quite distinct from the articulated fossils and fossil slabs visitors can find in the Museum’s fourth-floor fossil halls.
Curated by Curator Jin Meng in the Division of Paleontology, this exhibit showcases specimens that remain partially or completely enclosed in their field jackets, illustrating some of the steps in the delicate process of recovering and protecting specimens as they move out of the ground and into scientific collections or onto display.
Jacketing in the Field
When extracting large specimens, paleontologists sometimes completely encase the fossilized bone and surrounding matrix in a “jacket”, a rigid plaster shell often made from strips of fabric soaked in plaster that provides stability and support.
A barrier layer is usually placed between the plaster and the matrix to allow for easy separation and removal of the jacket later. The barrier layer may be made from paper towels, plastic wrap, aluminum foil, or even toilet paper. Jacketed specimens are later transported, unopened, to a lab, where the fossil can be prepared under controlled conditions.
Closed Jackets
Phytosaur closed jacket
This specimen is a phytosaur fossil embedded in reddish-brown rock matrix and encased in a jacket of textile and plaster. It was collected in 1969 from Pekin, North Carolina. There are two portions of tree branch incorporated into one side of the jacket. It is likely that in its original field orientation, the surface with the branches was on the bottom. The branches were used like the handles of a stretcher to carry the block away from the quarry site. Securely protected in its jacket, this fossil specimen can be safely stored until preparatory are ready to open and prepare it for study.
The team approached this specimen by assessing condition and the need to clean and stabilize it for display. The jacket arrived with a thick layer of dust, obscuring the plaster. In this image of the partially cleaned jacket, the level of dirt that had accumulated on the surface is clearly visible. Cleaning with a brush and vacuum was followed by cleaning with cosmetic sponges.
Phytosaur, closed jacket fossil specimen, FARB 33701, before treatment. Janet Spiller/© AMNH
Phytosaur, closed jacket fossil specimen, FARB 33701, after treatment. Janet Spiller/© AMNH
There was also a tear in the jacket along the bottom edge, revealing the reddish-brown matrix inside. The plaster surrounding the tear was loose and crumbling away. The unstable plaster was carefully stabilized and covered with very thin Japanese tissue paper and a stable adhesive. This method allows for reversibility, while also providing support to the tear and the crumbling plaster.
In different contexts, excavators, preparators, and conservators might approach the same damage in contrasting ways. For example, the excavator might add a quick patch of plaster-impregnated gauze to shore up the torn jacket. The preparator might use the opening to use as a window to better understand the fossil and plan its opening. While each approach is reasonable, in the display context, it was important to use materials that would effectively stabilize the crumbling, but are easily distinguished from the original, and reversible if needed.
Hadrosaur femur closed jacket
This Hadrosaur femur is encased in a thin, unopened jacket made from several layers of plaster and burlap textile. There is a large mobile crack through the center of the specimen that penetrates both the jacket and the enclosed femur. The two halves of the fossil femur move relative to one another at this crack if force is applied without sufficient support. Additionally, the jacket was covered in a thick layer of dirt with large muddy accretions.
Unfortunately, the intact plaster jacket made it impossible to internally stabilize the large crack. A contoured mount was created from a rigid panel and foam, and used to prevent movement during handling. This mount could then be used to move the specimen more safely.
An additional question concerned the red painted label on the specimen. The display orientation was selected to present the femur’s form as clearly as possible to the viewer. But when positioned this way, the prominent red label on the jacket was upside down. Would the inverted label be distracting? Perhaps, but given the unstable central crack, a change in the object’s orientation presented unacceptable risks to its physical integrity.
Janet Spiller/© AMNH
After careful consideration, the simplest solution was to capture hi-resolution photographs and then obscure the number in a reversible manner. As the label is primary data intrinsic to the history of the specimen, it was important that the number remain clearly associated with the fossil through these records and that the mark not be permanently removed. Two layers of Japanese tissue paper were attached over the numbers, taking care to make sure that the adhesive did not touch the red paint. Once dry, pigments were applied to the tissue paper to mimic the plaster surface.
Opened Jackets
Coelphysis bauri
This fossil specimen, Coelophysis bauri, is embedded in red and green matrix, and was collected from Ghost Ranch in Abiquiu, New Mexico, USA in the mid-20th century by a team led by Curator of Vertebrate Paleontology Edwin H. Colbert, who uncovered an extensive dinosaur graveyard there.
The jacket is partially open to reveal the fossil skull, vertebrae, pelvis, and limbs. Toilet paper and aluminum foil, cheap materials that are easily available in the field, were used as a separation layer between the plaster jacket and matrix.
When examined for conservation, the specimen initially appeared to be in poor condition, with little delineation between the fossil and a loose crumbly matrix. However, closer examination revealed that the specimen had been heavily consolidated, likely in the field, adhering fragments that had broken free from the surface. Fossil elements that at first appeared precariously attached were, in fact, quite secure.
Conservation treatment needs for the partially open jacket were minimal. It was cleaned with a vacuum and brush, followed by cosmetic sponges. Loose bits of plaster on the jacket were adhered in place with a reversible adhesive system and Japanese tissue paper.
A bronze cast of a well-preserved and complete Coelophysis bauri specimen from Ghost Rach can be seen (and touched!) in the Museum's 81st street subway station.
Platecarpus ictericus
A gray-brown fossilized skull and partial skeleton of a Platecarpus ictericus illustrates another stage in the preparation of fossil material. Contained in a wooden tray and its original gray limestone-carbonate matrix, the specimen was collected in Kansas by H. T. Martin in the late 19th century.
The stone matrix has been chiseled back to reveal the fossil skeleton. Jaw bones, with both the dentary and maxilla bones in five separate parts, are visible. The slab specimen is supported by a wooden tray, framed with boards on three sides. The fourth side of the frame was missing when it arrived at the lab.
There was some spalling in the stone matrix and a few breaks in the bone layer with separation from the matrix. The fossilized bone was unevenly coated with a shiny consolidant. Imaging under ultraviolet radiation revealed a deep blueish-gray fluorescence over the fossilized bones, suggesting an acrylic consolidant was used. Hover over the image below to see the slab specimen under UV radiation.
The specimen was first cleaned using a small brush and vacuum, as well as cosmetic sponges. Then the loose matrix was stabilized, and cracks in the stone matrix were filled with a stable, reversible fill material. View a time-lapse video of cleaning this specimen below:
The object on display includes not just the fossil slab, but also an original wooden frame highlighted by the story told in this exhibit, so the wooden board that had been lost from the bottom edge, as well as a diagonal brace in the bottom right corner, were replaced, stabilizing the frame and presenting a realistic representation of its role in preparation. Wooden planks, provided by the Division of Paleontology, were distressed to match the surrounding wooden frame with watercolors, and held in place with the original nails.