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Stephen Priestley wrote:

>Simulated mosasaur bites on shells (nautiloids, if memory serves) always
resulted in shattering not tooth holes - not that the mosasaur cares how it
gets at the tasty bits.  The conclusion was that the holes were made by
marine borers after the death of the ammonoid.<

There are several clues that may help determine if they are more or less
likely to be bore holes than tooth punctures, but you need to look at a
large number of shells.  A few years ago, while preparing a display of
Pleistocene fossils from the Presumpscot Formation in Maine, I had the
opportunity to collect and sift through hundreds of small fossil clams, many
of which had bore holes. We know that the holes in these shells were bore
holes, since a couple of the clam species represented still exist and are
known to be preyed upon in that manner by gastropods such as moon snails. A
few observations from that work:

1) About 30% of the clams had a single bore hole that went completely
through the shell;
2)About 5% of the clams had incomplete bore holes. Incomplete bore holes
should be easily distinguishable from the mark of a tooth that didn't
puncture a shell, since the rasps of marine borers (this holds for predatory
gastropods anyway) prescribe a circle that does not change in diameter from
first contact with the shell to completion of the hole;
3)There was never more than one completed bore hole per clam;
4)There was very little variation in the location of the bore holes. They
were always at or very close to the center of the shell; and
5) None of the above varied much based on the prey species, which consisted
mostly of Mesodesma, Hiatella and, my favorite, Yoldia glacialis.

I would be curious to know if anyone has looked more comprehensively at
ammonite fossils to see if patterns such as these exist that might help with
the bore hole v tooth hole discussion.