Dinosaur fossils are extracted from surrounding matrix
While most visitors to Pittsburgh know that the city breaks records for its number of bridges and bars, far fewer know that Pittsburgh’s Carnegie Museum of Natural History houses the third-largest collection of mounted and displayed dinosaurs in the U.S., behind only the Smithsonian’s National Museum of Natural History in Washington, D.C. and the American Museum of Natural History in New York.
As anyone who’s seen the opening scene of Jurassic Park knows, getting dinosaur fossils from the ground to a museum hall takes a lot of work.
At the Carnegie Museum of Natural History, visitors can look through glass walls to watch the work of the fossil preparators, whose job it is to remove fossils from the rock — or matrix — surrounding them. Although fossils can be formed in several different ways, one of the more common fossilization processes is permineralization.
During permineralization, cavities in organic matter that have been buried under layers of sediment and earth are filled in with mineral-rich groundwater. Over the course of millions of years, minerals in the groundwater precipitate and harden the surrounding organic matter, which, depending on how decayed the body was when it was buried, can be just bones or teeth or even skin and feathers.
Dinosaur fossils — which are roughly between 65 and 245 million years old — are delicate, requiring a gentle attention to detail to extract from their surrounding matrix. Still, accidents happen: when a fossil breaks, preparators repair it with glue designed specifically for fossils. After a fossil is found in the field, it is taken to a museum wrapped in a plaster and burlap skin and a wooden box, a 100-year-old method that preserves the fossil and surrounding matrix while it is being transported.
There’s not just one technique for freeing a fossil from the surrounding strata. One method, according to the American Museum of Natural History, is mechanical extraction, when preparators remove the fossil from the matrix with physical force. To do this, they use tools ranging from steel needles to micro-sandblasters and mini-jackhammers, like the air scribe on display at the Carnegie Museum.
Fossil matrix materials can be relatively soft, such as chalk, siltstone, or sandstone, or much harder, such as conglomerate rock and hematitics, which are described on the American Museum of Natural History’s website as “specimens covered in a hard layer of iron concretion.” According to the Carnegie Museum’s website, it can take two days just to work away a piece of matrix the size of a deck of cards around a small fossil, and a few days to work away a piece the size of a cereal box around a larger fossil.
For some fossils, preparators prefer chemical preparation. During chemical preparation, various chemical compounds are used to dissolve the matrix around a fossil. While chemical preparation has its own set of risks — using the wrong chemicals on a fossil can damage the specimen as well as the matrix, for example — it also has great advantages for fossils too small or delicate to be extracted mechanically.
Chemical preparation requires careful analysis of the fossil and matrix to choose the right kind of chemical bath. For fossils in a limestone matrix, for example, preparators typically use an acidic solution, which would break the stone down into carbon dioxide and calcium ions. In specimens where iron oxide has formed a crust on the fossil itself, preparators may use either thioglycollic acid or the preferred Waller Method, which uses a solution of three sodium salts to break iron down into a neutral pH solution.
If the preparators want to keep a skeleton’s structure intact, they may use transfer preparation, a technique developed in the 1950s and still in use today. During this process, half of the fossil is encased in polyester resin and placed in a bath of formic acid. The acid bath eats away the matrix while leaving the bone intact and supported by the resin, preserving the relationship of articulated parts after matrix removal.
When a fossil is finally free, it’s connected into a skeleton and put on display. While most of the dinosaur skeletons visitors see at museums look like the real thing, they are often partly or wholly made up of plaster casts. It’s extremely rare to find a fully fossilized dinosaur skeleton; missing pieces of a skeleton are usually filled in with bones from another fossil of the same animal or a very similar species. Casts, which are made using latex rubber, are often exchanged between institutions for research purposes, or used to make the surface details of a fossil clearer.
For many, the most striking part of a dinosaur’s skeleton is its skull. Dinosaur skulls are, in fact, extremely rare, and are generally too heavy to safely mount. Instead, a cast of the skull is attached to the skeleton on display.
The Carnegie Museum of Natural History began its extensive fossil collection with Diplodocus carnegii, a dinosaur that lived in the late Jurassic period and whose fossil was first found in the Badlands in Wyoming in 1899, after which it was acquired by Andrew Carnegie. Carnegie sent plaster casts of the fossilized skeleton to museums around the world; casts of the Carnegie Museum’s diplodocus fossil are on display on four different continents, according to the museum’s website. Outside the museum itself, a statue of “Dippy” the diplodocus, made during the 100th anniversary year of the fossil’s discovery, recreates the dinosaur in fiberglass.