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Smithsonian National Museum of Natural History Logo The Department of Paleobiology - The Life of a Vertebrate Fossil
Level 3 - Preparing the New Find - Supporting Cast
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Here's a photo of the molding process of the skull and some neck vertebrae of the plesiosaur holotype Brachauchenius lucasi (short necked plesiosaur) collected in 1903 from the 'Benton Formation' in Ottawa County, Kansas. This marine reptile lived during the late Cretaceous Period, about 90 million years ago and was a fish eater. The skull is about 45 inches (115 cm) long, and the light blue substance is the silicone rubber molding compound. The skull is upside down, so what you see are the lower jaws and the skull palate.

Often preparators make exact replicas of the specimen, known as casts. Before doing this, they must first make a mold of the fossil. Molds are made by painting on thin layers of a special liquid that solidifies into rubber. Once the rubber is completely dried or cured, the specimen is removed from the mold. If the specimen in the mold is large, the mold is encased in a removable plaster jacket before the specimen is removed. Then, the mold is ready for making casts. The casts are made by pouring or painting into the mold a high quality plaster, which when dry can be painted the color of the specimen, or of colored epoxy, a kind of plastic. The casts resemble the original so closely that they are used for study when:

  • Several scientists want to study the specimen at the same time.
  • Several museums want to display a particular fossil, but there aren't enough fossil bones to go around.
  • The original fossil must be returned to the country or state where it was found.
  • The original fossil is too fragile to be displayed or studied.
  • Museum visitors want to be able to handle the fossil.

After making a mold of the fossil, vertebrate fossil preparator Steve Jabo paints several layers of a special epoxy into the mold. The epoxy then dries and hardens into a solid piece of plastic. The solid piece of plastic is called the cast.   In our FossiLab Exhibit, prepartor Jennifer Young, makes molds of fossils. She uses clay to help plug up any gaps around the specimen so that the mold takes the form of just the specimen.


In this video Steve Jabo, a vertebrate paleo preparator, takes off the protective jacket and the mold. Inside is the hardened cast of a jaw.

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