Science and Technology Palaeontology Remember the tooth
科技 古生物学 牙齿的痕迹
A link is made in an ancient ecosystem
WHO ate whom in the food chains of the past is rarely clear.
Though it is obvious which species were predators and which prey, the subtle specialisations of feeding habit that allow many types of carnivore to co-exist are rarely preserved in the fossil record.
Rarely, but not never.
That is why the recent discovery of a Jurassic ammonite with a shark's tooth embedded in its shell has excited palaeontologists.
The ammonite in question, Orthaspidoceras, a species that thrived 155m years ago, was part of the collection of an amateur who did not realise the significance of what he had found.
Romain Vullo of the University of Rennes, however, did-and he has brought the discovery to the attention of the scientific world in the pages of Naturwissenschaften.
Ammonites were both predators and prey.
They occupied a position in the Mesozoic oceans similar to that of modern squid.
Like squid, they were swimming tentacled molluscs-a group called the cephalopods.
Unlike squid, though, they had protective shells.
These shells were divided internally into gas-filled chambers.
The result had neutral buoyancy, allowing the animal to move (like squid) by jet propulsion.
Modern pearly nautiluses, whose relationship to ammonites is much debated, have a similar arrangement.
What ammonites-or, at least, some of them-ate became clear earlier this year when an X-ray showed a small crustacean in the jaws of a species called Baculites.
But what ate ammonites has never been shown in such an unambiguous manner.
Some ammonite fossils have tooth marks that look as though they were made by huge reptilian predators called mosasaurs.
Some appear to have been attacked by the beaks of other cephalopods.
And some seem to have been bitten by sharks.
On top of that, coprolites, as palaeontologists politely describe fossil faeces, have turned up with ammonite shells in them.
Based on comparisons with modern evacua, these are probably from sharks.
But which sharks? Dr Vullo's ammonite nails one culprit.
The tooth belongs to a species called Planohybodus.
And that is a surprise.
Those modern sharks that eat shelled animals have robust teeth for crunching through hard exteriors.
The teeth of Planohybodus, in contrast, were slender and pointed-the sort usually associated with grasping and tearing at flesh.
In a modern predator, that would indicate the habit of eating fish.
That a shark with teeth like this would try to make a meal of an ammonite is, at first sight, odd.
But second thoughts provide a possible explanation.
Ammonites' manoeuvrability would have depended crucially on their buoyancy control.
Even a small puncture to the shell, which a pointed tooth would be well able to deliver, would let the water in and cause that control to vanish.
Since ammonites could not withdraw entirely into their shells for self-defence, it would then just be a question of dragging the creature out of its chamber in order to eat it.
And for that, sharp, pointed teeth are ideal.