Fish’s big mistake preserved an unusual fossil for us

Some extinct species left copious fossil remnants of their existence. Ammonites—an extinct type of cephalopod—are one such example. From the Devonian through the Paleocene, wherever ancient seas once covered Earth, one can usually find their coiled shells. So one more exquisitely preserved ammonite isn’t necessarily a big deal.

With the exception, perhaps, of one intact example found in the Posidonienschiefer Formation in Germany, where most ammonite shells are flattened and fragmentary. Now, decades after its original discovery, scientists have taken a more careful look at the well-preserved ammonite and the fossil fish it was seemingly nestled against. What they found surprised them: the fish had actually swallowed the large ammonite—something we’ve never seen before, even in fossils of much larger marine species that we know attempted to feed on ammonites.

It didn’t work out well for the fish. The size of the ammonite may have caused the fish to drown, or it may have blocked its digestive tract, causing internal bleeding. Drifting down to the seafloor, the fish was eventually buried and fossilized, preserving that ammonite—along with information about the ecosystem it and the fish inhabited—for over 170 million years.

The diet of an ancient fish

This fossil was uncovered in 1977 in the Fischer Quarry in Zell unter Aichelberg, Germany, and has remained in the collection of the State Museum of Natural History Stuttgart (SMNS) ever since. This July, scientists described the fish and its fatal meal in a paper published in Geological Magazine.

The two authors on this paper are both employed at that museum. Samuel Cooper is a paleontologist studying for his PhD, and Erin Maxwell is the curator.

“We have been aware of the specimen for some time,” Cooper explained in an email to Ars, noting that Maxwell has been at the museum longer than he has and thus had previous knowledge of its existence, “but we were initially skeptical whether or not the shell was actually inside of the fish’s gut. It wasn’t until we began re-examining the fossilized stomach contents of Pachycormus in the SMNS collection [that] we decided to take a closer look at this specimen.”

Pachycormus macropterus, the species of fish described, is a pachycormid: an extinct kind of marine ray-finned fish that could be anywhere from 0.3 to 15 meters (almost a foot to about 50 feet) long. With some species comparable in size to today’s tuna or swordfish, these were not tiny. This particular Pachycormus, at 850 mm (almost 3 feet), was an adult, but it was not quite fully grown, according to Cooper, who said that “the largest examples of this [species] rarely exceed 1 meter.”

Pachycormid diets haven’t been firmly established, which is why the researchers became interested. Gut contents in a few isolated examples indicate that they preyed upon soft-bodied cephalopods and smaller fish. “From what we have been able to determine, Pachycormus usually ate soft-bodied squids as an adult fish, but young Pachycormus seem to have eaten primarily other fishes,” Maxwell added. Based on this work, ammonites don’t appear to be on the menu for this species.

Adiel Klompmaker, curator of paleontology at the University of Alabama Museums, was not involved in this research. Noting that what Cooper and Maxwell have uncovered about the Pachycormus diet is a “remarkable result,” he wrote that he particularly liked how they “showed that the fish Pachycormus had a different diet depending on their size! Young individuals preyed upon fishes, but adults ate non-shelly cephalopods called coleoids. Shifts in diet are [rarely] reported in the marine fossil record. Such evidence helps us to reconstruct ancient food webs more accurately.”

Given what they’ve ascertained about Pachycormus’ diet, Maxwell asserted that “the ammonite appears to have been an unfortunate mistake.”

There is one example of another bony fish fossil from the same time period (Saurostomus esocinus) that has gut contents of a tiny ammonite larva, but because it also contains larger prey, it’s thought that swallowing the ammonite larva was an accident. In the case of this new fossil, that may be true of swallowing the whole ammonite, as well.