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Tuesday, April 26, 2011

A fish out of water: why did the shark cross the berm?

A fascinating fossil find

Shark teeth are some of the most spectacular remnants of life one may come across in the field. Thus, I can only imagine the surprise on Kentucky coal miner Jay Wright's face when he pulled half a jaw (teeth and all) from a coal seam back in February. The marine shark, Edestus, is estimated to have been more than 20 ft. long, making it one of the larger specimens of that genus.

The shark-bearing coal is Pennsylvanian in age (~299–318 Ma), and surrounded stratigraphically by marine shale and carbonate rocks (shallow subtidal/supratidal grainstone and boundstone). Fossils of small, marine invertebrates are apparently not uncommon in the coal, but the shark specimen represents the first, large vertebrate find.

A fish out of water?

To any keen observer, one obvious question may arise: how did the remains of a giant marine shark end up in the swampy backwoods of ancient Kentucky? Traditionally, coal is thought to have formed in densely vegetated swamps, landward of a sandy berm. Presumably, the shark did not "make a jump for it" at high tide, Free Willy style, only to find himself flopping around a freshwater mire. Thus the fossil find seems to strain at the conventional wisdom behind coal geology.

Brian Thomas at the Institute for Creation Research (ICR) took this approach in a recent article entitled "Shark Jaw Opens Questions about Coal Formation". He cites an introductory geology textbook to show that coal is thought to have formed "when millions of years of plant debris accumulated into peat bogs at the bottom of ancient swamps," and then asks, "...how did a huge shark find its way into a swamp?"

Before answering this question, we should consider Mr. Thomas's alternative explanation. Following a model by Steve Austin of ICR (whose Ph.D. dissertation focused on these very coal beds), he suggests:

"A catastrophic flood event ripped up whole ancient forests, and then transported plant and animal debris into low-lying areas. A subsequent series of tsunami-like waves then carried sediments over the top of the plant debris."

According to Mr. Thomas, this scenario could explain some geological features of the coal (absence of root casts, sharp transition to the bounding rock types, broad lateral continuity), as well as the "out of place" marine fossils.

Coal formation: the rest of the story

While potentially convincing at the surface level, Mr. Thomas's argument hardly does justice to the complex nature of sedimentary deposition, as well as the process of coal formation. Here are a few facts to consider:

1. Mr. Thomas begins by telling us the "standard textbook story is that coal seams were formed when millions of years of plant debris accumulated into peat bogs..." (emphasis mine). Unfortunately, it seems many YEC's believe that geologists fall back on "millions of years" to explain every process, and the phrase now possesses a decidedly pejorative connotation among the YEC community. With regard to peat accumulation in swamps, however, the timescale is significantly shorter (on the order of hundreds to thousands of years).

2. Coastal mires, though located on land, are not free from marine influence. If you've lived in the southeastern U.S., or simply kept up hurricane news over the past few years, then you've already seen this in action. Storms, and even tsunamis, are capable of bringing saltwater, sediment, shells, and yes, the occasional, disarticulated jaw of a large shark, onto the land. If the latter items end up in a calm, oxygen-deprived swamp, then the preservation potential is quite high. Such events are relatively rare, to be sure, but coal seams in western Kentucky contain abundant evidence of such marine influence (Eble et al., 2001). In fact, Hower and Williams (2001, p. 147) cite Dr. Austin's Ph.D. thesis (Austin, 1979), which "described marine shale partings bearing marine fossils within the coal."

3. The close association of marine rocks with coal is also due to the fact that many coals form in interdistributary bays, in addition to terrestrial swamps. In both cases, the oxygen-poor, heavily vegetated ecosystems are immediately adjacent to marine depositional environments (river delta, shoreline sands). In the rock record, adjacent depositional environments are recorded as a succession of distinct layers—a process described by Walther's Law. This is precisely what we find in the coals of western Kentucky (e.g. Dewet et al., 1991), which only comprise about 5% of the rock layers, because...

4. Coal seams form when plant matter accumulates in swamps, but in a prograding, marginal marine setting. Mr. Thomas cites an article by Stuart Nevins (found here) to suggest that the shear size (lateral extent, not thickness) of Pennsylvanian coals in this region is more consistent with catastrophic deposition. There, Mr. Nevins states that "no modern swamp has an area remotely approaching the great Pennsylvanian coals." Both authors fail to take into account, however, that coastal swamps migrate as sedimentation moves the coastline seaward during periods of low sea level. The result is a horizontally continuous layer of rock that is much larger than the depositional environment in which it was formed.

4. Coal seems thin and thicken as one traces them out laterally. They also tend to interfinger with marine lithologies (Eble et al., 2001). This phenomenon is very well explained by the process described above, but makes little sense in terms of catastrophic deposition.

5. Western Kentucky coal seams are relatively rich in sulfur (2–13 wt. %) due to fluid interaction during mire development. Seawater, for example, is very rich in sulfate, and may contribute to the high sulfur content through repeated incursion (from storms, etc.) over thousands of years. Mr. Thomas must account for the high sulfur content of these coals and, simultaneously, the low sulfur content of others.

6. Mr. Thomas states that "modern peat bogs are thoroughly penetrated by roots. Coal seams show no trace of these root masses." Eble et al. (2001) point out, however, that most coal seams are underlain by paleosols with abundant roots. Root structures are typically lost within the coal seam due to degradation of organic matter before and after burial (thermal maturation). In fact, the low oxygen content of swamps is due to the constant breakdown of organic matter, roots included.

Concluding thoughts: the culinary art of coalification

Perhaps the most important aspect of western Kentucky coal beds is the long journey from decaying plant matter to an economically useful resource. In the article cited by Mr. Thomas, Stuart Nevin erroneously rules out time as a factor in coalification. He bases his reasoning on the fact that some geologically old coals are less mature than geologically young coals. But when it comes to any petroleum product (including coal), maturation is a function of both time and temperature.

Imagine thermal maturity as a fancy term for the 'doneness' of coal. As with cooking a roast, the 'doneness' depends on the oven temperature and the cooking time. If the temperature is very low, the roast can cook for many hours, while an extremely hot oven will blacken the meat within minutes.

Mr. Nevin notes that coal can be converted from plant matter in a number of hours, given enough heat. That is true, but at what temperature have the Pennsylvanian coals of this region been since burial? Substantially lower, at less than 100–150°C. In general, reaction rates (i.e. cooking times) are cut in half for each additional 10°C. So a peat layer at 100°C will take 64 times longer to reach the same thermal maturity as a peat layer at 150°C, and more than 2,000 times longer than a peat layer at 200°C.

Thermal maturity in coal (just like in oil) is a function of both time and temperature, and thus depends on the specific burial history and tectonic setting for each region. It is not enigmatic that some geologically old coal deposits are less mature than more recent ones, since not all sedimentary rocks are buried to the same depth or exposed to the same heat source. Moreover, the current thermal maturity of coals in western Kentucky poses a serious problem for Mr. Thomas's interpretation. What takes only hours at very high (300°C+) temperatures can take thousands to millions of years at the current temperature (~100°C) of coals in the subsurface. Unless Mr. Thomas wants to propose that these coal beds were exposed to extreme heat since the flood (a testable and falsifiable hypothesis), he must admit his model does not allow for nearly enough time to explain all the facts.

References Cited:

Austin, S.A., 1979, Depositional Environment of the Kentucky No. 12 Coal Bed (Middle Pennsylvanian) of Western Kentucky, with Special Reference to the Origin of Coal Lithotypes: Ph.D. Thesis, Pennsylvania State University, 411 p.

Dewet, C.B., Moshier, S.O., Hower, J.C., Rimmer, S.M., 1991, Deposition and diagenesis of a marine-swamp margin; the providence limestone and adjacent coals, western Kentucky: Society of Economic Paleontologists and Mineralogists Core Workshop, p. 169–204.

Eble, C.F., Greb, S.F., Williams, D.A., 2001, The geology and palynology of Lower and Middle Pennsylvanian strata in the Western Kentucky Coal Field: International Journal of Coal Geology, v. 47, p. 189–206.

Hower, J.C., and Williams, D.A., 2001, Further examination of the ragged edge of the Herrin Coal Bed, Webster County, Western Kentucky Coal Field: International Journal of Coal Geology, v. 46, p. 145–155.


  1. Another great article, Chemostrat. I was reminded of a story I heard that a bull shark was found in the Mississippi River well north of St. Louis. Apparently they are able to tolerate fresh water.

  2. Thanks Tim, and thank you for the insight. Freshwater sharks are indeed common, and more so in the fossil record (they can be distinguished by the shape of their teeth, primarily).

    I actually didn't consider this when writing the article, but you are absolutely right. This is another reason the fossil discovery is less enigmatic—though still fascinating!—than ICR has proposed.

  3. Interesting - I didn't know that the shape of the teeth was different for freshwater sharks. In what way are they different?

    I have personally seen sting rays in fresh water. I shot some underwater video of one in Silver Glen Springs in central Florida.

  4. In general, freshwater shark teeth are shorter and broader than marine sharks (which are typically tall and pointy). I'm not sure how well this rule applies across the board, and I know there are some exceptions. But I've sorted out a large number of Late Cretaceous shark teeth from freshwater riverine sediments, and they were fairly distinct (in size and shape) from their saltwater counterparts.

    Ray teeth are also very common in freshwater sediments from the late Mesozoic (and very oddly shaped!). It's amazing to me how well some aquatic fauna can adapt to salt and freshwater habitats. I wish I had the opportunity to actually swim with them, though. :)