Reworked fossils, Part 2

I had planned for this piece on reworked fossils to be two parts, but when I saw how big Part 2 was getting I decided to go with three parts instead. So today we’ll look at some examples of reworked teeth from the Carmel Church bonebed.

The teeth I’m going to talk about are all from the shark genera Otodus and Carcharocles. There are many paleontologists (including me) that believe that Otodus and Carcharocles are a more-or-less continuous evolutionary series that shows gradual changes from the Paleocene to the Pliocene. This is not universally accepted, and there are many paleontologists who believe that the similarities between Otodus and Carcharocles are convergences or sympleisiomorphies (in fact, they place Carcharocles into synonymy with Carcharodon). I’m not going to get into that debate here, but if you’re interested there is discussion about it at elasmo.com. At any rate, the exact phylogeny has little effect on the biostratigraphic and sedimentologic principles we’re looking at here.

On a related point, there are nominally a large number of Carcharocles species currently floating around in the literature. The species problem in paleontology generally and shark tooth nomenclature specifically are also worthy topics of discussion, that I’m not going to talk about right now. But since these Carcharocles species are based on small differences that are difficult to identify in reworked specimens, they grade into one another, and some of them are of questionable validity anyway, we’re going to use a greatly simplified system that suits our purposes.

Now, with all that out of the way, what do we have at Carmel Church?

Here are two examples of Otodus obliquus; the one on the left is reasonably fresh, and the one on the right is heavily reworked:

Otodus has prominent lateral cusps and a non-serrated crown (the fresh example has some small breaks along the cutting edge, but no serrations). The genus is found in place from the Paleocene to the early Eocene.

Here’s a reworked example of an early Carcharocles sp.:

Again, there are several names applied to these teeth (C. auriculatusC. angustidens, and C. chubutensis are the most common ones), but they all have serrated crowns and lateral cusps. They range from the middle Eocene to the middle Miocene.

Finally, here are fresh (left) and reworked (right) examples of Carcharocles megalodon:

This is the final species of the Carcharocles lineage, and ranges from the middle Miocene to the early Pliocene. They have serrate crowns and no lateral cusps (although there is sometimes a wrinkle in the enamel where the lateral cusp would be located).

(As I look at these photos, I notice that my reworked teeth are smaller than their fresh counterparts. This is just coincidence; the reworked teeth aren’t always smaller.)

So we have three different taxa here, with all three represented by reworked specimens and two represented by fresh specimens.  There is no point in time at which all three of these taxa were alive simultaneously, as you can see in this range chart:

The thing is, all these teeth were found in the same bed at Carmel Church! How do we reconcile finding all these teeth, apparently of different ages and with differing amount of wear, in the same stratigraphic bed? That’s what we’ll look at in Part 3.

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This entry was posted in Carmel Church Chondrichthyans, Carmel Church Geology, Carmel Church Quarry, Chesapeake Group, General Geology. Bookmark the permalink.

5 Responses to Reworked fossils, Part 2

  1. Tony Edger says:

    Excellent sequence of postings. Looking forward to #3. I’m struck not just that the reworked specimens are smaller than the fresh specimens as you note, but by how dramatic the differences are. Just luck of the draw?

  2. Alton Dooley says:

    I think it was just luck. When I went to the cases and pulled out specimens for the post, I picked ones that had the visible (easily photographed) features I wanted, like serrations. I really didn’t realize the sizes were like that until I was organizing the photos for the post. I have plenty of reworked Otodus and Carcharocles that are just as large as the fresh ones shown here (although our very largest Carcharocles are all fresh).

  3. Seth says:

    Very interesting. I was also wondering if some of the larger speicmens remained in-situ because they are harder to move through bioturbation and weathering. I would think that would have some effect, but maybe just on the largest specimens.

  4. Alton Dooley says:

    Size probably matters to a point, but no shark tooth is so big that it can’t be reworked. Reworked whale vertebrae are fairly common, for example.

    A large tooth can avoid some bioturbation, but the burrowing I depicted in the diagram isn’t the only possible type of bioturbation. For example, Brian Beatty and I published a paper suggesting that Diorocetus was a bottom feeder, like modern gray whales. That’s a whole lot of sediment being ripped up at one time, but it’s still bioturbation.

  5. boesse says:

    In my field experience in bonebed and transgressive lag-riddled Neogene rocks in California, size doesn’t guarantee anything with regards to being ripped up and eroded form the seafloor so much as it affects transport capability of an object. There are entire reworked skulls in phosphate nodules in my thesis field area in the Purisima Formation; in some cases these nodules include multiple bones, or even phosphate cemented “disks” of shell beds up two feet in diameter. Now, they’ve travelled vertically (i.e. down with the erosional surface) but probably not laterally very far. In fact, I’ve got a partial articulated fur seal in a phosphate nodule that was eroded and then redeposited.

    Additionally, there are lots of places along the california coast where you can go into tidepools in the wave-cut platform in fossiliferous strata, and find nearly complete baleen whale crania eroded out and just sitting in the tidepools; similar finds have been made in Pleistocene terrace deposits (which are basically pleistocene, buried ‘tidepools’/wave cut platforms).

    I thought I’d also mention that while transport, winnowing, and erosion can cause abrasion, it doesn’t always (and it obviously varies with the type of skeletal element) -so lack of abrasion in certain cases cannot be used to eliminate certain depositional processes from the mode of bonebed/shellbed genesis.

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