Researchers have determined that a specimen of amber from Myanmar originally thought to contain a snail shell in fact contains the juvenile shell of an ammonite, a long-extinct group of cephalopods related to squid and octopuses.
In their paper in the Proceedings of the National Academy of Sciences, "An ammonite trapped in Burmese amber," the researchers use the shell's similarity to a previously known ammonite -- Puzosia (Bhimaites) Matsumoto -- to date the amber as older than the volcanoclastic matrix it was found in, to somewhere within the Albian and Cenomanian ages of the Cretaceous, or around 100 million years ago.
Amber is of course a fossilized form of tree resin which can trap objects and organisms, preserving them (more or less) for millions of years. So how did the shell of an ammonite -- supposedly a strictly aquatic organism -- get trapped in tree resin? The researchers propose three methods:
- Resin from a coastal araucarian conifer dripped down, picking up terrestrial arthropods along the way, before plopping onto an empty ammonite shell that had washed up onto the beach below.
- A tsunami flooded the forest, washing marine debris inland.
- A tropical storm blew the shell inland.
However, they've overlooked two other options: first, and least interesting, a bird might have carried it there (I live within a few miles of the Puget Sound and I've found clam shells on the roof of my house, so this is not unusual); second, and most intriguing, it might be the shell of a tree ammonite!
In my previous blog post on Dougal Dixon's 1988 book, The New Dinosaurs: An Alternative Evolution, I highlighted his hypothetical tree-climbing ammonite, the coconut grab (Nuctoceras litureperus, see illustration on the left). Dixon proposed this semi-arboreal cephalopod as something that could have evolved had the Cretaceous-Tertiary extinction not occurred.
But what if Dixon, who prides himself on applying hard science to speculative evolutionary history to produce plausible outcomes (more or less), had stumbled onto an actual evolutionary development? Could his tree ammonite have evolved before the extinction, not as a result contingent on there being no extinction?
A possible criticism of this hypothesis is the lack of other examples of ambered ammonite shells. Surely there would be more shells if ammonites roamed the forests of the Cretaceous? (Although, absence of evidence isn't evidence of absence.)
The researchers also note that the shell was damaged prior to the ambering and there's no trace of soft tissue; so presumably it was not the shell of an ammonite that was engooed by resin while climbing through the branches, but rather remains that got into or near the tree somehow. That it was deposited when the shell's previous owner died in the tree is one way that could have happened.
While these points don't positively argue in favor of the tree ammonite hypothesis, they certainly don't preclude it. In fact, the lack of evidence makes sense when we compare a hypothetical tree ammonite to similar arboreal creatures.
Unlike mindless arthropods that are commonly found in amber, we know that modern cephalopods are thinking animals, and there's no reason to believe that ammonites would be an exception.
Other intelligent animals are also notably lacking from the amber record even though they have been plentiful in trees: tree octopus beaks are completely absent, and while remains of mammals and dinosaurs (both avian and non-avian) have been found, they are exceedingly rare and are usually just bones or corpses that were trapped in resin after death. A tree ammonite, much like a bird or squirrel or octopus, would be smart enough to avoid that trap while alive.
So is it really that strange that we have thus far only found one possible example of a tree ammonite, a probably rare and unique species? Maybe we just need to look harder.
This is the only fact we have at hand: 100 million years ago, an ammonite shell was trapped in the resin of a tree. That shell had to have gotten there somehow. While we can spin stories of exceptional events requiring additional assumptions like a tsunami or a storm or a hungry proto-seagull, or a specific arrangement of tree and beach, is it not more parsimonious to allow that the ammonite simply climbed there?