This fossilized fish skull is filled with feces

Enlarge / View of the fossilized skull of an extinct species of stargazer fish, showing preserved fecal pellets in the brain. (credit: 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 skull is the first 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. Together, the researchers proposed that tiny scavenging worms ate their way into the dead fish's skull and pooped out the pellets.

Around 1824, British fossil hunter Mary Anning (recently portrayed by Kate Winslet in the 2020 film Ammonite) was the first to notice the presence of so-called "bezoar stones" in the abdomens of ichthyosaur skeletons. 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 and suggested that the stones were actually fossilized feces. He dubbed them coprolites.

Coprolites aren't quite the same as paleofeces, which retain 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 Coprolites are fossils, so most organic components have been replaced by mineral deposits like silicate and calcium carbonates.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 coprolites typically boast spiral or annular markings and, as Anning discovered, often contain undigested fragments of food.

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