This fossilized fish skull is filled with feces


View of the fossilized skull of an extinct species of stargazer fish, showing preserved fecal pellets in the brain.
Enlarge / View of the fossilized skull of an extinct species of stargazer fish, showing preserved fecal pellets in the brain.

Calvert Marine Museum

A fossilized cranium of an extinct species of stargazer fish was stuffed with tiny fecal pellets known as coprolites, according to a recent paper published in the journal Rivista Italiana di Paleontologia e Stratigrafia—the first known skull in the fossil record to be completely filled with fecal pellets. This is a joint study by paleontologists at the University of Pisa in Italy, and the Calvert Marine Museum in Maryland, who proposed that tiny scavenging worms ate their way into the dead fish’s skull and pooped out the pellets.

It was a 19th century British fossil hunter named Mary Anning (recently portrayed by Kate Winslet in the 2020 film Ammonite) who first noticed the presence of so-called “bezoar stones” in the abdomens of ichthyosaur skeletons around 1824. When she broke open the stones, she often found the fossilized remains of fish bones and scales. A geologist named William Buckland took note of Anning’s observations five years later, suggesting that the stones were actually fossilized feces. He dubbed them coprolites.

Coprolites aren’t quite the same as paleofeces, which retains a lot of organic components that can be reconstituted and analyzed for chemical properties. Coprolites are fossils, so most organic components have been replaced by mineral deposits like silicate and calcium carbonates. It can be challenging to distinguish the smallest coprolites from eggs, for example, or other kinds of inorganic pellets, but they typically boast spiral or annular markings, and, as Anning discovered, often contain undigested fragments of food.

Scanning electron micrograph of a single fecal pellet (coprolite) found in the cranial cavity of a fossilized fish
Enlarge / Scanning electron micrograph of a single fecal pellet (coprolite) found in the cranial cavity of a fossilized fish

S.J. Godfrey et al., 2022

For archaeologists keen on learning more about the health and diet of past populations—as well as how certain parasites evolved in the evolutionary history of the microbiome—coprolites and paleofeces can be a veritable goldmine of information. For instance, last year we reported on an analysis of preserved paleo-poop revealing that the ancient Iron Age miners in what is now Austria were quite fond of beer and blue cheese.

In 2020, we reported on a new method (dubbed coproID) for determining whether fecal samples are human or were produced by other animals, particularly dogs.  (Dog poo bears a strikingly close resemblance to human feces in both size and shape, is frequently found at the same archaeological sites, and has a similar composition). The method combines host DNA and gut microbiome analysis with open source machine-learning software.

Samples of fecal pellets (coprolites) found in various fossils collected from the Calvert Cliffs in Maryland.
Enlarge / Samples of fecal pellets (coprolites) found in various fossils collected from the Calvert Cliffs in Maryland.

S.J. Godfrey et al., 2022

If a coprolite contains bone fragments, chances are the animal who excreted it was a carnivore, and if there are tooth marks on those fragments, it can tell us something about how the animal may have eaten its prey. The size and shape of coprolites can also yield useful insights. If it’s spiral-shaped, for instance, the coprolite might have been excreted by an ancient shark, since some modern fish (like sharks) have spiral-shaped intestines.

This new joint study examined several fossil samples the museum’s collection containing coprolites. The fossils were recovered from the Calvert Cliffs in Maryland, with rocks formed from the sediment of the coastal ocean that once covered the region. The so-called Calvert Formation is a rich trove for fossil hunters, and while the cliffs are closed to the public, people regularly comb the beach for fossilized shark teeth, which are especially plentiful.

The most exciting of the fossils the scientists examined was the skull of an extinct species of stargazer fish called Astroscopus countermani, found in 2011 and dating back to the Miocene epoch. Today’s surviving Astroscopus species are venomous and can produce electric shocks. They hunt by camouflaging themselves and ambushing prey, and have been called “the meanest things in creation” by ichthyologist William Leo Smith.

SEM images of the neurocranium of an extinct species of stargazer fish, stuffed with fecal pellets (coprolites)
Enlarge / SEM images of the neurocranium of an extinct species of stargazer fish, stuffed with fecal pellets (coprolites)

S.J. Godfrey et al., 2022

The team identified two types of coprolites. The first were tiny micro-coprolites about 1/8th of an inch long and gray or brownish black in color. They were found in snail shells, clamshells, barnacles, and burrows, as well as the stargazer fish skull, usually stuffed into tiny spaces…



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