I found a fossil on eBay some weeks ago. It was a 74-million-year-old ammonite from Montana, USA. Ammonites are an extinct group of shelled cephalopods, common as fossils in Mesozoic marine sediments. But this fossil looked different from hundreds of other ammonites offered on eBay. I arranged for the Friends of the Palaeontological Museum in Oslo, Norway, to buy the fossil and donate it to the museum.
The ammonite is named Placenticeras meeki and is rather large, about 35 cm across, flattened and boringly white, no nice mother of pearl preserved. Why did I bother to get this for the museum? It was because of small round holes in the shell. A scientific discussion of the origin of such holes in ammonite shells has raged since the 1960s. There are two main contestants: the boring gastropod theory and the more exciting bite theory.
Boring gastropods, in this instance limpets, are seen by some palaeontologists as the most likely perpetrators of the round holes. Limpets do not usually penetrate the shell they are grazing on, but they can thin it considerably. The thin spot could then break during compaction or preparation leaving nice round holes in the shell. In many instances the holes in ammonite shells looks rather haphazardly placed and could support the idea of limpets moving around on a shell.
The more exciting bite theory is based on the presence of one of the fiercest predators in the Cretaceous sea, the mosasaur. A mosasaur was a swimming relative of the Komodo dragon, only bigger. They grew to be up to 17.5 m long and had large pointy teeth with a round cross section. They ate everything in the Cretaceous ocean — stomachs of mosasaurs have yielded birds, bony fish, sharks, and smaller individuals of their own species. The holes in the ammonite shells from Montana are the same size as the teeth of medium-sized mosasaurs found in the same rock layers. But many of the holes are singular or not in a very convincing pattern.
Enter my eBay catch. The main holes form two slightly converging lines. Looks like the jaws of a mosasaur clamped down on the poor ammonite some 74 million years ago. But the holes in one line are smaller than in the other line. Is this evidence against mosasaur attack? Are the boring limpets the real perpetrators?
I think not, this is actually strong evidence for the mosasaur theory. The ammonite was not flattened when the bite came. The shell was a three-dimensional structure with the living chamber being more expanded than the inner whorls of the shell. And this is just what is shown on this specimen. The teeth are going deeper into the higher, expanded part and only slightly piercing the lower part.
But are all holes bite marks? Probably not. Perhaps it is a typical example of a Winnie-the-Pooh moment:
Rabbit said, ‘Honey or condensed milk with your bread?’
[Pooh] was so excited that he said, ‘Both’…
A A Milne (1926) Winnie-the-Pooh