In today's installment, I shall address two questions with an answer that should come to me as easily as breathing:
1) What are fossils?
2) How do fossils form?
While these are remarkably simple questions, I feel that anyone who cares enough to ask deserves a comprehensive answer derived from actual research rather than hearsay. (This belief has been the greatest factor in my procrastination regarding the questions.)
What are fossils?If one hopes to understand paleontology, they must first acquire the fundamental knowledge of what fossils actually are. Failure to do so would be like owning the entire run of Game of Thrones on DVD but not having any idea how to turn on your TV.
Thanks to my insatiable need to acquire college textbooks about paleontology and biology, I have an easily citable reference providing a formal definition for "fossil". Per this source, a fossil is "any physical or chemical remains or traces of past life" (Foote, 2007, p. 324). This may seem quite vague, but it allows for the seemingly infinite factors paleontologists must account for when delving into Earth's history.
How do fossils form?As you hopefully surmised, explaining the formation of fossils is much more complicated than defining what they are. Thankfully, my previously cited source goes into great detail about this, so my APA-formatted reference list will only have one entry. Foote and Miller (2007, p. 5-8) identify eight modes by which fossilization occurs, listed in order by increasing likelihood:
2) Preservation in amber
7) Molds and Casts
8) Trace fossils
FreezingThis one is self explanatory. Freezing is very rare and only specimens dating back a few thousand years have been found this way. While this mode of preservation is remarkable, sometimes even providing scientists with actual DNA, the specimens themselves died too recently to offer much of a glimpse into evolutionary history.
Preservation in amberAnyone who loves dinosaurs is probably familiar with this on some level thanks to Jurassic Park (the book or the movie). Sadly, we're not finding any dinosaur DNA in the amber, just small organisms like insects and spiders. Thankfully, the movie does a great job of describing this mechanism for fossilization, which is one of the few times it did science right.
For the uninitiated, the bug, a mosquito perhaps, lands on a tree in some moment in time. As it turns out, it plopped squarely in some fresh sap and slowly becomes enveloped in the goo. Over time, this goo, or resin, hardens into amber and under the right circumstances can be preserved to be found today. If we're really lucky, this mosquito was fossilized in a chunk of amber that included air bubbles, because geochemists use those to study the composition of Earth's ancient atmosphere.
CarbonizationThis one is a bit more complicated. Soft parts such as leaves can be fossilized when carbon films are formed under heat and pressure through distillation, removing hydrogen and oxygen. Unfortunately, this process also substantially alters the chemical form of any recovered organic material, but carbonization is still a great way for the soft anatomy of organisms to be preserved.
PermineralizationIn this process, pore water percolates through rock and into the trapped skeletal material. Materials dissolved in the water precipitate out into existing spaces in the bone. These inorganic substances, including silica, phosphate, and pyrite, permeate the bone, hardening it while preserving finer details like growth bands, skeletal pores, and shell layers.
A similar process, known as petrifaction, preserves organic non-skeletal material by converting it to mineral. Petrified wood is the most commonly known example.
ReplacementThis mode is very similar to permineralization, albeit more extreme. Instead of the skeletal material being hardened by precipitated materials, it is replaced, sometimes molecule for molecule. As with permineralization, the fine details of the bone are preserved. The nature of the replacement depends on the chemistry of the pore water. Three examples are pyritization (pyrite replacement), silicification (silica replacement), and phosphatization (phosphate replacement).
RecrystallizationThis is a chemical process quite similar to the transformation that creates metamorphic rocks. In this case, bone undergoes an increase in temperature and pressure to convert to a more thermodynamically stable form. At first glance, this new form is nearly indistinguishable from the original, but the transformation logically diminishes the fine features often preserved by previously discussed modes.
Molds and CastsMolds are negative impressions of hard parts, and the process is actually quite simple. An organism's skeletal material is encased in sediment and over time pore water dissolves it completely, leaving an impression in the rock. This can be simulated by using Silly Putty, with your thumb representing the bone. Pushing your thumb into the putty simulates the action of sedimentation, and removing your thumb represents dissolution in the pore water. What remains is the mold of your thumb. Obviously, this process is just a tiny bit (re: whole freaking lot) faster than the natural mode of fossilization.
Another type of mold is called a steinkern, or internal mold. Instead of rubbing complicated science all over this paragraph, I'll just explain it using Jell-o. (I apologize to anyone who has not had the opportunity to eat a Jell-o jiggler.)
Let's say we buy one of the special molds available on the Kraft online store. (College students may prefer to use plastic shot glasses.) The mold will act as the shell of a clam, since those are easy to visualize. We then fill the pan with delicious watermelon-flavored Jell-o, simulating the act of intrusive sedimentation. By placing our mold in the fridge, we have begun the process of lithification. An eternity later (Anyone who has made Jell-o will agree with me.), the sediment has turned to rock. The final step in the process is the erosion of the original skeletal material, which we represent by flipping the mold over, dumping the contents on a table. After this arduous process, what remains are sweet, tasty steinkerns.
Since I spent entirely too much time entertaining myself with a discussion on molds, I'm only going to gloss over the definition of a cast. Basically, it's when a mold is filled with a new material that in turn becomes lithified. (Fill your thumb imprint from earlier with plaster to achieve this effect. If the drying time seems unappealing, Play-Doh will suffice, albeit with your cast having a weaker constitution.) While this process does occur naturally, paleontologists have been known to use an epoxy resin to create casts from fossil molds.
Since this mode by definition contains no original organic material, only the physical appearance of the fossilized organism remains.
Trace fossilsEach of the above listed modes of fossilization are referred to as body fossils, which are remains of actual parts of organisms. Trace fossils basically encompass every other piece of evidence of past life mentioned in the definition of "fossil". The most well-known specimens in this category are footprints and burrows.
Trace fossils are actually quite useful in establishing behavior in extinct species, such as using trackways to estimate movement speed or identify migratory herd behavior. Another interesting note is that trace fossils, including boreholes and bite marks, are commonly found on body fossils. The study of trace fossils is called ichnology.
ConclusionFossils are far more complicated than traditionally represented. Many people associate the word with dinosaur bones, picturing a digger lying next to a giant sauropod femur. This is characteristic of the nearly universal misconception that all paleontologists study dinosaurs. Hopefully, the rise of online science communication can help inform the public regarding these incorrect assumptions and shine a light on the importance of all branches of paleontology. At the very least, I'll be doing my part to educate the public, especially my nieces and nephews, through every medium accessible to me.
ReferencesFoote, M., & Miller, A. I. (2007). Principles of Paleontology: Third Edition. New York, NY: W. H. Freeman and Company.
Notes: Since my writing this post was also meant as a learning experience for me, I welcome any corrections in the comments and I will see to it that my mistakes are amended accordingly. Any other feedback is also encouraged.
Work at CLDQ has been going well, and getting to hang out with Dr. Joseph Peterson and his crew from University of Wisconsin-Oshkosh was a valuable experience. Sadly, work obligations prevented me from visiting the Burpee Museum crew down in Hanksville. Hopefully I get another opportunity to participate in some excavations, but while I wait for that, I'll continue to act as a geology teacher at the visitor center.
Since I've been recommending these books to visitors that express interest in evolutionary study, I'll drop a plug for them here as well. Buy Written in Stone by Brian Switek! (Just as well get his other book too. I still need to read it. . .) Buy Your Inner Fish by Dr. Neil Shubin! (Other book. . . Still haven't read. . .)
For some educational television, check out Cosmos: A Spacetime Odyssey and Your Inner Fish. I still have two parts of Your Inner Fish to view, but I'll vouch for it now. Dr. Shubin seems like a cool guy, and he knows how to put a fun spin on science without diminishing the value.
Cosmos is just full of win. Don't blindly accept it as truth though, because that opposes the intent of the show. Question the presented information and search for answers yourself through proper channels. (Yahoo Answers is not a proper channel.)
Before I close this thing out, I would like to mention my appreciation for Eric Prokopi putting in some good work and stifling the black market fossil trade in Mongolia. It's unfortunate that he did it because his back was against the wall, but an individual's reasons for fighting crime have little relevance.
|Jillian Marie Whalin takes the mound, ready to put out the fire and clinch the team victory!|
I'd be remiss if I failed to mention that my niece Jillian pitched in a baseball game for the first time last Thursday. She came in late with two on, one out, and an 8–5 lead. The first batter she faced reached on an error, and she struck out the next two! I don't think Little League records saves, but she earned one from me. (I'm honored to say that I taught her a little about this and more so that she learned so well.) My younger niece Paige is also on the team and has been doing well. (If you couldn't tell, I'm one proud uncle.) As of this posting, I don't know how their Monday game went, but if they won, they're tournament champions! (If not, they play again Tuesday in a true championship game.)
I'll hopefully be moving on to my next post about dinosaur eggs next week during my time off. I won't promise anything though. Thanks for reading!