This ammonite was fossilised without its shell

If anxious humans have nightmares of being naked in public, an anxious ammonite may have dreamed about swimming around without its shell, its soft body exposed to the elements and the leering eyes of predators.

For one unfortunate ammonite in the Late Jurassic, this was no dream but a harsh reality. The animal died utterly unclad, outside its whorled shell, and was buried this way. According to a study published recently in the Swiss Journal of Palaeontology, the ammonite’s death made it an extraordinary fossil — one of very few records of soft tissue in a creature that is most often immortalised as a shell.

“We know millions and millions of ammonites that have been preserved from their shell, so something exceptional had to happen here,” said Thomas Clements, a paleobiologist at the University of Birmingham in England who was not involved with the research. “It’s like finding ——” Clements said, trailing off. “Well, I don’t even know what it’s like finding, it’s that bizarre.”

René Hoffmann, an ammonitologist at the Ruhr-University Bochum in Germany who reviewed the study, called the fossil a “paleontological jackpot you have only once in a lifetime.”

To the untrained eye, the fossil looks more like an Impressionist painting than an ammonite: a pink, bean-shaped smear surrounded by bulges, veins and ovals. It was discovered in the Solnhofen-Eichstätt region of southern Germany which was, in the ammonite’s day, around 150 million years ago, an archipelago studded with serene, oxygen-deprived lagoons. These conditions allowed soft, dead creatures to sink into the mud unscathed by predators or bacteria, according to Christian Klug, a palaeontologist at the University of Zurich in Switzerland and the first author of the paper.

When Klug first saw the fossil, he knew it represented the soft parts of an ammonite, but exactly which soft parts, he did not know. He left it alone for months until Helmut Tischlinger, a fossil collector and an author on the paper, sent him photos of the fossil taken with ultraviolet light, which revealed the minute elevations and mineral stainings in the fossil.

Klug reconstructed the creature’s anatomy sequentially, from the most visible organs to the most obscure. First, he identified the aptychus, a shelly lower jaw that indicated the fossil was an ammonite. Behind the jaws, he found the chitinous layer of the oesophagus, and then a lump that suggested a digestive tract with a cololite — faecal matter (he used a different word) “that is still within the intestine,” Klug clarified.

“For the most part, the soft body reconstruction makes perfect sense,” said Margaret Yacobucci, a paleobiologist at Bowling Green State University in Ohio who was not involved with the research.

Solving the fossil’s other mystery — how the ammonite came to be separated from its shell — was far more difficult. The soft parts were so intact that they appeared to still be coiled. The authors propose several alternate endings to the ammonite’s life, each possible but uncertain. One suggests that the soft parts of a dead ammonite slipped out when the tissue connecting its body to its conch began to decay.

Another, more elaborate explanation imagines a predator breaking the ammonite’s shell from behind and sucking out its body only to drop the naked ammonite. “The best explanation is that some squid-like organism pulled out the soft parts and could not retrieve it,” Klug said.

Clements finds the clumsy predator theory “awesome” if unlikely; presumably a chomped-on ammonite body would show more visible damage. But he has no good alternative. Interpreting a fossil always invites some degree of doubt, and Clements predicts that the unarmed ammonite will be analysed again in the future with robust chemical analyses.

Curiously, the fossilised ammonite is missing its arms, leaving unresolved one of the outstanding mysteries of ammonite anatomy. “Did they have many thin, delicate arms, like modern nautiluses, or a few strong arms, like modern coleoids?” Yacobucci asked. “If I gained access to a time machine, the very first thing I would do is zip back to the Jurassic to see what kind of arms ammonoids had.”

If a squidlike predator did in fact free the ammonite from its shell, it may have munched on the creature’s unknown quantity of arms as a consolation prize, nourishing both ancient cephalopods and the scientists who study them.