How does a dinosaur bone get petrified anyway and why are they often found out west?
You may think that dinosaur all bones are like those that make up our Diplodocus – heavy stones that require a massive steel armature to hold them up. Fortunately, massive bones are not all that rare; indeed, in treeless Wyoming, someone built a small cabin exclusively of huge sauropod leg bones. If you were to examine one of these, you would find that they look very similar to a beef bone: a heavy wall around a marrow cavity and has the same composition, hydrated calcium phosphate. In many places in the American west, the void space is filled with agate, a banded form of silica that can have wonderful color patterns. Other minerals can cause much more difficulty because they are less stable. Some of the dinosaurs mounted in the Smithsonian in Washington, DC show what is called “bone disease”, where iron pyrite in the bones oxidizes and the reaction breaks the bone apart. Dipsy (the volunteers’ pet name of our Diplodocus) has not been on display very long and we do not know if there is going to be a problem. Some museums like the Sam Noble in Oklahoma, keep all their new finds in cabinets in a nitrogen-filled vault and only put replicas on display. For example, our T. rex is a replica and its lighter weight allows us to have a very life-like display. Besides, T. rex is rare and original bone specimens are very expensive.
Bone can be filled in by other minerals, too. Under certain conditions, silica may not crystallize and remain as opal – usually it fluoresces green from uranium. Calcite fillings are not uncommon and we have already mentioned pyrite and the problems it causes. Sometimes there are hematite, barite, sphalerite, and other “ore” minerals.
We have a case of striking banded agates that have been loaned to the museum by Mat Dillon of the Houston Gem and Mineral Society. The agate form by lining the inside of a cavity and sometimes is colored by iron. Iron can be yellow when it is hydrated (like rust), or red when it is a pure oxide (like rouge ), or black when concentrated, or iridescent like opal when in thin films. Other minerals can be present, but the vast majority of the colors are from iron. Most dino bone is recovered from a gravel pits dug in an ancient river beds -- their survival is greatly aided by the toughness of the agate that mineralizes the bone. The silica started off in the form of glass shards in volcanic ash or as sponge spicules, dissolved in the ground water and migrated to fill cavities such as those in bone or petrified wood.
Now look at the mineralized bone in the same case. Gary Anderson of the Houston Gem and Mineral Society loaned these pieces. He has selected pieces where the marrow cavity has been filled with a contrasting color of agate. On one piece, each cavity in the marrow has been filled by an individual fortification agate. There are millions of pounds of dino bone all over the western United States, most of it broken into small pieces. It is not uncommon for deposit of Jurassic dino bone (180 million years old) to have been eroded in the Cretaceous (70 million years old) so that all the bones were broken, scattered and redeposited. Agate has filled every cavity, but the original phosphate of the bone is still there!
The Texas Hill Country has many dino trackways (like at Glen Rose) but no dinosaur bones. Bone is easily dissolved by groundwater and the nodular limestones in the Hill Country contain very little silica to help preserve it. The only place where dino bone is more common is in Big Bend area where there is plenty of volcanic ash (remember the road in the National Park on the west side which goes to the butte named Castillon and crosses volcanic ash piled up like snow).
Dinosaur hunters in Alberta, Canada have to be very good diggers because the bone there is not mineralized. Once the bone gets within the frost zone, water that fills the marrow cavity will freeze and expansion will shatter the bone. In the Houston area, there is a very large amount of bone that comes from the period where there were elephants walking around where Herman Park is now located. Look at the big tusk at the paleo touch cart that was found near Katy. Ground water has made the once-hard ivory into something softer than your fingernail. I made a special cradle for the tusk because it is not strong enough to support itself even after I used lots of super glue. Chris Peek of the Houston Gem and Mineral Society collected the very large elephant bone from a gravel bar in the Brazos River 30 miles west of Houston. If there had been a handy volcano to provide a source of silica, the Brazos River would be an ideal locality to mineralize and preserve bone. This piece of bone, though, is much too light to have been mineralized at all. The pore structure is quite open.
On our table, we have a few of the beautiful slices of petrified wood donated to the museum by Mr. Herbert Zuhl. These were selected because they show marvelous sequences in which they were mineralized, broken, cemented by white agate, and broken and re-cemented. Check the slabs and see how many events you can see. This wood was mineralized with silica, just like the dinosaur bone was mineralized. The infilling by silica is often on a very fine scale, preserving the details of the original wood cell structure so well that the species of tree can be identified. These structures are very small, so be sure to have a look at the wood under magnification. Some trees, like conifers, have cells so small that a microscope is needed to make them out. Under magnification, one can also see how the agate fillings are built up and, sometimes, crystals or dendrites of minerals. The photographs beside the specimens show some of the beautiful structures. Often the specimens also preserve information about how the tree grew or things that happened to it – growth rates, places where there were limbs or buds, borings by insects or other animals, fungus or rotted spots,.