If frightened people have nightmares about being naked in public, a frightened ammonite may have dreamed of swimming around without its shell, its soft body exposed to the elements, and the lurking eyes of predators.
For an unfortunate ammonite in the late Jurassic, this was not a dream but a harsh reality. The animal died completely naked, beyond its whirling shell, and was buried in this way. According to a study recently published in the Swiss Journal of Palaeontology, the death of the ammonite turned it into an extraordinary fossil – one of the few soft tissue records in a creature usually immortalized as a shell..
“We know of millions upon millions of ammonites that have been preserved from their shells, so something extraordinary had to be done here,” said Thomas Clements, a paleobiologist at the University of Birmingham in England who was not involved in the study. “It’s like you find …” said Dr. Clements, as he walked away. “Well I don’t even know what it’s like to find, it’s so bizarre.”
René Hoffmann, an ammonitologist at Ruhr University Bochum in Germany who reviewed the study, called the fossil a “once in a lifetime paleontological jackpot”.
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 in the time of the ammonites was an archipelago dotted with serene, oxygen-poor lagoons about 150 million years ago. These conditions allowed soft, dead creatures to sink into the mud unharmed by predators or bacteria, said Christian Klug, a paleontologist at the University of Zurich in Switzerland and the paper’s lead author.
When Dr. Klug saw the fossil for the first time, 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 author of the paper, sent him photos of the fossil made with ultraviolet light, which revealed the fossil’s minute elevations and mineral flecks.
Dr. Klug sequentially reconstructed the creature’s anatomy, from the most visible organs to the most obscure. First he identified the aptychus, a shell jaw that indicated the fossil was an ammonite. Behind the jaws he found the chitinous layer of the esophagus, and then a lump that suggested a digestive tract with a cololite – stool (he used a different word) “that’s still in the gut,” clarified Dr. Klug.
“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 in the study.
Solving the fossil’s other mystery – how the ammonite got separated from its shell – was much more difficult. The soft parts were so intact that they still appeared rolled up. The authors propose several alternate endings for the ammonite’s life, each possible but uncertain. It is suggested that the soft parts of a dead ammonite slipped out when the tissue connecting its body to its shell began to rot.
Another more elaborate explanation envisions a predator breaking the shell of the ammonite from behind and sucking its body out to drop the naked ammonite. “The best explanation is that some squid-like organism pulled the soft parts out and couldn’t get it back,” said Dr. Klug.
Dr. Clements thinks the clumsy predator theory is “great” when unlikely; a chewed ammonite body would probably show more visible damage. But he has no good alternative. Interpreting a fossil always raises some doubt, and Dr. Clements predicts that the unreinforced ammonite will be re-analyzed in the future with robust chemical analyzes.
Oddly, the fossilized ammonite is missing its arms, leaving one of the outstanding mysteries of ammonite anatomy unsolved. “Did they have a lot of thin, delicate arms, like modern nautiluses, or some strong arms, like modern coleoids?” Dr. Yacobucci. “If I got access to a time machine, the very first thing I would do was drive back to the Jurassic to see what kind of arms had ammonoids.”
If a squid-like predator has indeed released the ammonite from its shell, it may have chewed the creature’s unknown amount of arms as a consolation prize, feeding old cephalopods as well as the scientists who study them.
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