If you’ve ever been caught short in your birthday suit, mid-danger dash from the bathroom to the bedroom, you may sympathize with a fossil which was recently identified as an ammonite who was committed to the immortal fossil record without its shell on.
You might be thinking the find - published in the Swiss Journal of Palaeontology - is an unusual one, and you’d be right. Usually soft tissues don’t survive the preservation process. Rare specimens with soft tissues intact have fallen out of the permafrost, and sometimes fossils are found with dribs and drabs of soft-tissue-signs still attached (sometimes even a butthole). So how does a (presumably once wet and squishy) shell-free ammonite survive for 150 million years?
The soft tissue remains are believed to have once been a male perisphinctid ammonite from the early Tithonian. The researchers suspect as much because spermatophores were among what evidence remained of the unusual specimen. The squishy ammonite innards have survived for so long as a result of the special depositional conditions in the marine basins of the Solnhofen Archipelago, where our nudey mollusc hails from.
It was a rocky road for Christian Klug and colleagues from discovering the specimen to putting a label on its constituent parts. Understandable considering all other ammonites are known for their shells (called a conch), not a lack-there-of. The giveaway was the presence of a jaw morphology which was a characteristic of Subplanites ammonites.
“I wasn’t very sure what was what,” said Klug in an interview with New Scientist. “I recognised the oesophagus, then the stomach. Next, 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.”
The other big question was: Why was this streaker swimming about with no shell on? The researchers suggest two theories. One is that the ammonite suffered from a condition or illness that resulted in the breakdown of tissue which kept the conch on the animal’s body, meaning the shell spontaneously sloughed off one day. The other is that it was the result of a failed predation attempt by a marine predator who dropped the mushy ball whilst trying to relieve it of its shell.
An unfortunate demise for our naked ammonite, but a fortuitous one for science. The unique specimen has lifted the lid on one of the most prolific fossils known to science, revealing for the first time structures that are usually hidden inside the conch.
[H/T: New Scientist]