150 Million-Year-Old Fossil Of An Ammonite That Lost Its Shell Offers A Rare Glimpse Into Its Internal Anatomy

Ammonites are a group of fossil marine mollusk animals closely related to living cephalopods (i.e., octopuses, squid, and cuttlefish) and shelled nautiloids such as the living Nautilus species. The earliest ammonites appeared during the Devonian some 419 million years ago, and the last species vanished in the Cretaceous–Paleogene extinction event 65 million years ago. Over this vast period of time, the passing of millions and millions of ammonites have been preserved from their hard shell. Despite their ubiquity, little is known about their soft body. Based on their evolutionary relationship to modern cephalopods, they have sometimes reconstructed resembling squids or the modern nautilus. A study published recently in the Swiss Journal of Paleontology describes an extraordinary fossil - an ammonite from the Late Jurassic preserved outside its shell and showing traces of its internal organs.

The fossil was discovered in the limestone of the Solnhofen-Eichstätt region of southern Germany. Around 150 million years ago, this area was a lagoon surrounded by an archipelago of islands. Cut off from marine currents, an oxygen-deprived lower layer of water formed at the bottom of the lagoon. These conditions allowed dead creatures to sink into the mud unscathed by predators or bacteria, according to Christian Klug, a paleontologist at the University of Zurich in Switzerland and the first author of the paper.

At first, the fossil appears like a series of non-descriptive blobs. However, observed under ultraviolet light, traces of organic material preserved in the limestone start to glow, revealing finer details. Klug first identified the animals mouth thanks to the aptychus, a sort of curved shelly plate, now understood to be part of the jaw mechanism of ammonites. Behind the jaws, he found a chitinous tube, likely the esophagus, leading to the stomach and then a lump that suggested a digestive tract with fecal matter fossilized still within the intestine (a so-called cololite). The central ganglion, part of the nervous system, surrounds the esophagus and is connected to the animal's two eyes.

To explain how the ammonite came to be separated from its shell, the authors offer various scenarios. Maybe the animal slipped out when the tissue connecting the body to the spiral-form shell began to decay long after its death. Another explanation imagines an attack breaking the ammonite’s shell. The dying animal, as the shell likely also played an important role in maintaining buoyancy in the water thanks to chambers filled with gas, then quickly sunk to the bottom of the sea. The best explanation, according to Klug, is a predator pulling the entire animal out of its shell, but losing grip on its prey at the last moment. This would explain why the fossilized ammonite is missing its arms. The rest of the animal then quickly dropped into the layer of oxygen-free water, outside the reach of the unknown attacker.