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Norki

I have heard that it's not uncommon to find examples of the ammonite placenticeras meeki with evidence of supposed mosasaur predation marks. A certain example of mine has since stood out as a possible contender. This example comes from deposits of the late Campanian Bearpaw Formation, a unit that is already well known for its good preservation of late Cretaceous molluscs, including placenticeras meeki with the supposed predation marks.

 

I know that there are competing theories about the origin of these marks, including abrasion by limpets or other gastropods, so I'm curious about whether any of you are in agreement that this conspicuous pattern is evidence that this particular placenticeras was chomped by a mosasaur.

 

A note about the specimen - I somewhat foolishly decided that a fine grain sandpaper was the solution for getting rid of the stubborn bits of sandstone matrix and pyrite that clung to the nacre, so most of the surface, including the rims of the matrix filling the puncture holes, is slightly polished. Also unfortunate is the fact that this ammonite, on account of most of the internal chambers being completely hollow, smashed into hundreds of little pieces once the concretion containing it was split. Fitting these fragments back together is essentially impossible, and I'm regretful that the specimen was ruined slightly by not being extracted carefully enough, but thankfully there's still a significant amount of it that's still intact. If anything, it seems to be telling that the only part of the fossil that isn't hollow (and therefore more durable) is where the puncture holes are, given that these holes would have allowed water and sediment to enter the chambers they had breached. The chambers which did not fill with matrix, on the other hand, could not handle the stress of the concretion being split, and shattered.

 

Anyway, the first photo here shows the first two holes. These are on the left side of the ammonite. Note that the nacre around the punctures is cracked, where otherwise it is smooth and unblemished.

 

baae2d8364.JPG

 

The right side, showing the third puncture hole. It is difficult to tell in the photo, but this hole is depressed slightly into the ammonite. The bit near the end of the tape measure could also be a hole, but it's difficult to tell with so much pyrite encrusting it.

 

5e8dc9f342.JPG

 

Finally, a front-facing view. I've added arrows to approximate the location of the holes on either side. Note the preservation of the nacre of the septum, and how much of it is still covered by pyrite. Note that the other end of the fossil has no obvious septa, leading me to believe that this fragment is from near the body chamber.

 

737d09f423.JPG

 

So, thoughts? I know that the origin of this type of trace fossil is still somewhat contested in paleontology, and I'm really curious about what the forum's consensus will be.

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Tidgy's Dad

Very interesting and a very pretty specimen.:) 

I don't know if it's mosasaur predation or not, but I can say I've seen specimens with very similar marks that are claimed to be mosasaur bites.  

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FossilDAWG

Pretty specimen! :wub:  If these are punctures from mosasaur teeth, I wonder what process could have resulted in two substantial holes on one side and one on the other without evidence of the other teeth, given the spacing of teeth in the jaw.  To me that suggests some other cause.

 

Don

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Jeffrey P

I've heard it suggested that a mosasaur biting an ammonite's shell would have shattered it. A mosasaur puncturing a shell like that would have been impossible. 

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DevonianDigger

Big ammonite, tiny mosasaur, lol. Just kidding obviously. I am inclined to agree with Jeffrey.

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DevonianDigger

I could see it being plausible that it might not totally destroy it if it didn't fully bite down, but even still I would think there would be crush damage around the punctures, not a clean circle as presented in your piece.

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Mark Kmiecik

The holes may have caused by some other predator that we haven't found in the fossil record yet. What percentage of all extinct life has been fossilized?

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jpc

There is great discussion about these things among academic paleos.  Some say mosasur bite marks,some say limpet scars on a dead shell.  

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olorotitan

I would lean towards saying no to the mosasaur theory..  It is peculiar that the 3 holes line up in a straight-ish line on both sides of the shell, but there should be more holes from other teeth in the jaw as well.. unless the teeth had fallen out prior.  Even if they did, the absence of additional punctures makes it seem mosasaur bite did not make these holes.  

 

My mosasaur (prognathodon) tooth from morocco has a strong carine on the posterior side- if such a tooth made the puncture, it should display a similar imprint.  Now I don't know the dentition of mosasaurs in the area but these holes look too perfectly circular to be made by teeth designed for cutting / slicing flesh.

 

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