Every fossil has many stories to tell. From the most spectacular dinosaur skeleton to the most common little foram shell, every fossil fragment and trace contains clues about prehistoric lives, ancient ecologies, and worlds that vanished so long before we were born that we can’t even fully comprehend the full span of time between ourselves and those vestiges. Even the most mundane turtle shell fragment or brachiopod is a time capsule which testifies to the intricacies of prehistoric lives. And, every now and then, paleontologists are fortunate enough to find rare hints of prehistoric behaviors among the collected bits of shell, tooth, and bone.
In the case of a fossil described last week by paleontologists Adiël Klompmaker of Kent State University, Ohio, and René Fraaije from Oertijdmuseum De Groene Poort, the Netherlands, fortuitous conditions of fossilization created a literal window inside the coiled shell of a Jurassic ammonoid. The shelled cephalopod specimen in question belonged to the species Harpoceras falciferum, and, as in other mollusks, this 180 million-year-old ammonoid’s shell was coated in a thin outer layer called the periostracum. That organic patina was all that remained of the ammonoid — a translucent fossilized layer which allowed Klompmaker and Fraaije to look inside the creature’s body cavity. There were three minuscule lobsters inside.
The lobsters were not quite like those you might see on the dinner table. These small crustaceans belonged to an entirely extinct group of lobsters called the Eryonidae — a group which had compressed, circular bodies with spikes around the rim. That’s as specifically as Klompmaker and Fraaije were able to identify the decapods. But the fact that the three lobsters were preserved together, more than halfway inside the coiled shell of the ammonoid, hinted that the minuscule lobsters had taken up residence inside the ammonoid’s empty shell.
It’s amazing that anyone was able to see the lobsters. The fossils, each only about an inch long, look like little discolored smudges inside the ammonoid. Yet, when viewed close up, their claws and body segments stand out from the grey and tan of the ammonoid fossil. The question is how the crustaceans entered the cephalopod shell. It would not be unreasonable if the fossils were the molted shells of lobsters which were washed inside an empty shell and then preserved. Just because the lobsters were found inside the ammonoid shell does not automatically mean that the fossil is a snapshot of ancient behavior.
Klompmaker and Fraaije considered several explanations for the fossil juxtaposition. The lobsters were certainly not ammonoid food — if the cephalopod had eaten them, the lobsters would have been mashed up into chunklets and preserved in the location of the ammonoid’s digestive tract. Nor is there any evidence that the lobsters were scattered by currents, and the fossils appear to represent actual lobsters rather than just molted shells. (Molted shells would be expected to show a characteristic split down the midline, among other clues.) And the positioning of the lobsters in relation to each other is another indication that the crustaceans were alive when entombed. The three lobsters form a semicircle facing away from each other, with their tails centered around the same point. This kind of coreography hints at some kind of behavior rather than just a random assemblage of flotsam which washed inside a vacated shell.
Lobsters weren’t the only creatures to utilize empty mollusk shells. Nor were they the first. Trilobites sometimes congregated in cephalopod shells, and small groups of fish have been discovered inside the shells of huge inoceramid clams. Even small ammonoids have been found inside the shells of larger ammonoids. Empty shells have been prime real estate for marine organisms for millions upon millions of years. What remains uncertain is why so many different species used empty mollusk shells.
Why the lobsters congregated inside the ammonoid shell is a mystery. There is more than one possible answer. The lobsters may have entered the spacious shell to molt, in search of protection from predators, because there were some tasty ammonoid bits left to scavenge, or because the shell was a suitable place to store collected food and otherwise be a cozy home. At the moment, there’s no way to tell which of these hypotheses is correct.
Whatever the reason for their occupancy, though, the lobsters appear to have had a close relationship with ammonoids. “These particular, small lobsters have only been found in ammonoid body chambers so far,” Klompmaker and Fraaije write, and point out “Not a single specimen [of this lobster species] is known that was not associated with an ammonoid shell” after fifty years of collecting at the fossil site the new specimen came from. The coincidence could be attributable to the ammonoid shells — perhaps the lobsters were so small and fragile that they were always destroyed unless protected inside the shell of another creature. Then again, Klompmaker and Fraaije explain, other fossil crustaceans have been found inside ammonoid shells and as isolated specimens in the same deposits. The fact that the lobsters Klompmaker and Fraaije examined have only been found in ammonoids, by contrast, might be a true signal that these lobsters specialized in making homes out of empty shells. Perhaps, with the description of additional specimens, these little lobsters can contribute a unique story to the ever-growing book of prehistory.
Read on — Paleontologists recently discovered that a very different kind of creature also lived in ammonoid shells. Parasites infested ammonoid shells while the cephalopods were still alive, and the tentacled mollusks retaliated by creating pearls.
Klompmaker, A., & Fraaije, R. (2012). Animal Behavior Frozen in Time: Gregarious Behavior of Early Jurassic Lobsters within an Ammonoid Body Chamber PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0031893