The Mystery of the Magnetic Dinosaur Bones
Originally published in October 1987 "News Nuggets", Albuquerque Gem and Mineral Club
Many of you know that the New Mexico Museum of Natural History is engaged in the excavation of a huge, new species of sauropod (Brontosaurus-like) dinosaur in New Mexico. In fact, some of you may be volunteers actually digging "Seismosaurus" bones under the direction of the New Mexico Museum of Natural History curator of vertebrate paleontology, Dr. David Gilette. But perhaps you are not familiar with the fact that a large number of sophisticated scientific techniques have been tried out, in the vicinity of the exposed bones, in an attempt to locate the more deeply buried material. Of all the techniques, one has proven to be a consistent winner with a batting average of a mere 100%. This is the magnetometer survey.
Why should dinosaur bone be magnetic? Well, this bone contains about 2% iron, while the rock is essentially iron-free. But this doesn’t answer all the questions we have. I know and you know that bones don’t usually have any iron in them to speak of. So how did this iron get there and what form is it in? It was in order to answer these questions that I procured some of the material for detailed study. But I found the answer as soon as I thoroughly cleaned the bones and perused them with an ordinary binocular microscope. Some of the bone is dark gray and some is very light gray and all of it is rich in holes that used to contain blood vessels, nerves, etc. In one of the best pieces of bone, of a "bone" white color, I saw tiny (1/4 mm) crystals in some of the holes. At high power these were resolvable as perfect cubes of a chocolate brown color. What do you think they are? Most of you should be able to answer that because you know they are brown cubes and they are probably rich in iron. O.K? Well, I hope a lot of you realized that these crystals are "limonite" pseudomorphs after pyrite. And all that iron was easily detected by the magnetometer.
What’s that, you say? You don’t remember finding pyrite in bones either! That’s true, but have you ever noticed that some fossils are pyritized? Well, they are! You can find brachiopods, snails, clams, sand dollars, sea urchins, beautiful ammonites, and even coprolites (fossil excrement) that have been replaced by pyrite. This is how it happens. When the animal material is first buried, it still contains a lot of organic material such as fleshy parts in with the hard parts. This stuff wants to rot away, but it needs oxygen to do so. Ground water percolates through the sediments around the future fossil and, if it carries some oxygen, the organic remains might use the oxygen to decay further. But then the ground water changes chemically from oxidizing to reducing. IF there are ferric and sulfate ions in the water, these may be reduced to ferrous and sulfide ions, and these would immediately precipitate to form pyrite. These might form crystals in open cavities or they may fill the area formerly occupied by the organic material.
If you dig up these fossils before they are attacked by weathering, the pyrite will be bright and shiny. Otherwise, the sulfide will be reconverted back to sulfate and wash out, while the iron reverts back to a ferric hydroxide ("limonite"), which is insolvable and stays put. If the weathering is done right, the original shape of the pyrite will be retained - the limonite will form a pseudomorph after the pyrite. That’s what happened to the "Seismosaurus" pyrite, except that some of the crystals still have some pyrite left in the core of the crystals. And the pyrite and limonite have enough iron to make the bones magnetic - mystery solved!
P.S. The "best"! example of this type of fossil that I’ve ever seen is a specimen found by one of my students when I was teaching at Wisconsin State University-River Falls. It was a coprolite from the Eocene Fort Union Formation in Wyoming. It must have been produced by a mammal about the size of a 30 - 40 pound dog. It looks just like the modern equivalent because the pyrite was replaced by brown "limonite" (a crack near the middle of the 6" specimen shows tiny cubic crystals) and the size and shape are just right - even down to the slightly curved, tapered end! The owner delighted in placing it in the middle of the corridor outside the classroom where everyone studiously avoided stepping on it!
l. Kues, B. S., l982, Fossils of New Mexico, University of New Mexico Press, Albuquerque, NM, 226pp.
2. Moore, R. C., Lalicker, C.G. and Fischer, A. G., l952, Invertebrate Fossils, McGraw-Hill Book Co., Inc., New York, NY, 766 pp.
3. Romer, A.S., l966, Vertebrate Paleontology, University of Chicago Press, Chicago, Illinois, 468 pp.
4. Jackson, Kern C., l970, Textbook of Lithology, McGraw-Hill, Inc., 552 pp.
5. Well, J. M., l960, Statigraphic Principals and Practice, Harper & Row, New York, NY, 725 pp.
Republished by permission of Albuquerque Gem and Mineral Club
Paul Hlava, author