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Ammonites are among the most common marine fossils from the age of the dinosaurs, but no one has found one like this before. It shows one of the swimming marine molluscs without its distinctive spiral shell 鈥� offering a rare opportunity to study ammonite internal anatomy.

On a first look, Christian Klug at the University of Zurich, Switzerland, and his colleagues struggled to make sense of the fossil. 鈥淚 wasn鈥檛 very sure what was what,鈥� he says. Although the researchers could instantly recognise the shield-shaped structure on the left as part of a Subplanites ammonite jaw, the rest was a jumbled mess. As a result, photos of the 150-million-year-old fossil from southern Germany languished on Klug鈥檚 computer for years.

Eventually, one of his colleagues tried photographing the specimen under ultraviolet light to highlight some of its subtler features. Through comparison with the soft internal structures of nautilus, a still-living relative of ammonites, it became clearer that the fossil preserved almost the entire body of an ammonite shorn of its shell.

鈥淚 recognised the oesophagus, then the stomach,鈥� says Klug. 鈥淣ext, I saw the coprolite [fossilised faeces] in its intestine, so that was clear as well. Then I identified the gills and last came the reproductive organs.鈥�

How the ammonite lost its shell is a mystery. A likely scenario is that it was a meal that got away. 鈥淎 predator might have pulled the yummy soft parts out of the shell and then dropped it [by mistake] in a place where they could preserve,鈥� says Klug.

This could explain why the ammonite is missing its tentacles. Ammonites floated in open water, and predators seem to have found it easiest to attack by nibbling a hole in the rear of the shell and then yanking the animal out. The tentacles might have been ripped off in the process.

The reproductive organs seem to include a penis, which would suggest that the 11.5-centimetre-long ammonite was male. Some Subplanites ammonite shells are 50 centimetres across, says Klug, and it is possible that the larger animals were female while males were smaller 鈥� although he says more work is needed to confirm that idea.