I volunteer in the collection at the Fair Park Campus of The Perot Museum of Nature and Science where I work primarily with specimens stored in alcohol, otherwise known as fluid storage. This week another volunteer was labeling and boxing fossil specimens. From a distance these specimens look pretty boring, just a bunch of odd-shaped, grey rocks. But when I sat down to eat my lunch at the table where she was working and saw the fossils up close, my interest was piqued.
I noticed this ‘distal portion of a tibia’ (the farthest end from the knee of the larger and stronger of the two bones in the leg below the knee), which has a somewhat lovely core of whitish crystal. Why is this so? It looks to me as if the marrow was replaced with something different from the rest of the fossil, but I freely admit that I know very little about fossils, so I went to the museum’s expert, vertebrate paleontologist and fossil preparator Dr. Ron Tykoski. The first surprise he revealed to me is that not all fossils are rock! According to Dr. Ron, anything that is physical evidence of life greater than 10,000 years old is considered a fossil. [Some definitions use ‘from a period of time prior to recorded human history’ (Shepherd)]. So those mummified mammoths that occasionally pop up are considered fossils right along with the rock-solid trilobites that you can find in a museum store. In any case, the reason we discussed this fact is that this particular fossil is in an incomplete state of permineralization, a type of fossilization in which water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal cast with deposited minerals (Babcock). Because water will take the path of least resistance and go where the most space is, the mineral deposits will generally start where there is more space in the bone. And wouldn’t you know, according to Wisegeek.com:
Spongy bone is lighter, softer, and weaker than compact or cortical bone, the other type of calcium tissue, but it has a greater surface area and is much more vascular, or supplied with blood vessels. Spongy bone is found on the inside of some bones, and it is surrounded by the stronger, more protective compact bone. (Foster, 2012)
So, the center of the bone, being spongy and with more surface area (more spaces in it) would likely be the first to permineralize and this specimen demonstrates it perfectly! Dr. Ron was able to find three or four other examples of this in the various trays around the fossil prep lab, so it seems to be a common occurrence. But here is what I found to be really cool. The parts of this fossil that have not undergone permineralization are in effect just old bones! I am talking really old bones from the Cretaceous, which spans the time from about 145 to 66 million years ago (the specimen in the photo is in the range of 70 to 69 million years in age). You can feel the difference between one of these specimens and a completely permineralized fossil by their weight; the first is lighter because it still has all of the open spaces found in living bone while the second has been filled with minerals and is much heavier.
What does all this mean? It means that not all fossils are solid rock and that sometimes what you think is just an old bone, might actually be a fossil. Fascinating!
Babcock, L. E. (n.d.). Permineralization. Retrieved Dec 5, 2012, from Access Science by McGraw Hill: http://www.accessscience.com/abstract.aspx?id=803250&referURL=http%3a%2f%2fwww.accessscience.com%2fcontent.aspx%3fsearchStr%3dpermineralization%26id%3d803250
Foster, N. (2012, Sep 20). What is spongy bone? Retrieved Dec 5, 2012, from Wisegeek: http://www.wisegeek.com/what-is-spongy-bone.htm
Shepherd, R. (n.d.). What is a fossil? Retrieved Dec 5, 2012, from Discovering Fossils: Introducing the Paleontology of Great Britain: http://www.discoveringfossils.co.uk/whatisafossil.htm