Displaying objects as heavy, fragile and complicated as large articulated fossil skeletons requires a great deal of engineering and metalworking expertise. We’ve been dismantling historic skeletal mounts as part of the Deep Time exhibit renovation project, and in the process fossil preparators in the Department of Paleobiology and NMNH conservators have learned a lot about the array of fossil mounting techniques used by our predecessors during nearly 150 years of exhibit building. This series of posts, based on a recent presentation by preparator Michelle Pinsdorf, describes different styles of mounting and considers their advantages and disadvantages in terms of aesthetics, specimen conservation, and, for lack of a better word, “convenience” for those of us tasked with taking the mounts apart.
Mounts with internal armatures look very natural because there is little visible metal holding the bones in position, but this historic style of mounting came with a trade-off -- the fossils were permanently damaged as metal armatures were built to form supports within the bones. Metal rods and pipes might pass internally through natural gaps and holes, such as the neural arch in vertebrae, but much of the armature was inserted into holes drilled into, or completely through, the fossils. Mounting in this style resulted in the permanent loss of fossil material during coring and drilling, and risked bones shattering or sustaining other types of damage during the drilling process. Several skeletal mounts formerly exhibited in the Ice Ages Hall were built with internal armatures. The mount of Paramylodon harlani (USNM 15164), an extinct ground sloth, is shown below.
Part of the collection of pipes, rods and fasteners used to construct the metal armature for the Paramylodon. Connections between pieces of the armature were easy to undo, which allowed us to dismantle the mount rapidly.
The damage done to the ground sloth fossils during construction of this mount prompts the question of why an internal armature was used to mount it. All specimen mount armatures are designed to balance the need for a lifelike display and the need to preserve the research value of easily accessed, undamaged fossil specimens. When this mount was built, around 1965, priority must have been given to its aesthetic appearance rather than to maintaining the research value of the bones used to build it. This skeleton is made up of fossils from an untold number of ground sloths, having been originally excavated from a locality where fossils were abundant but skeletons were not preserved intact. Perhaps the specimens were considered less important for research use than those that could be connected to a single animal, or maybe their relative abundance made the sacrifice seem acceptable. Whatever the reason, this is not a decision we would make today. We do use this mounting technique to display cast replicas, however, both because of the more natural appearance it affords and because it reinforces cast stability. Cast bones may even be constructed around their armatures, as was done a few years ago when we constructed a cast of one of our dinosaur specimens, Camptosaurus browni (USNM 4282). A detail of this mount is shown to the right.
Photos by Michelle Pinsdorf and Abby Telfer.