The fossils, unearthed in southern Brazil, date to the Paleozoic era (251 million to 542 million years ago), before dinosaurs roamed the Earth. This predates other known examples of intestinal parasites in vertebrates by 140 million years.
The eggs are each only about 150 microns long, or about one-and-a-half times the average width of a human hair. The researchers discovered the eggs by cutting coprolites into thin slices.
"Luckily in one of them, we found the eggs," researcher Paula Dentzien-Dias, a paleontologist at the Federal University of the Rio Grande in Brazil, told LiveScience. "The eggs were found in only one thin section."
This coprolite was found with more than 500 others at one site. The researchers suggest the area was once a freshwater pond where many fish got trapped together during a dry spell.
The mineral pyrite, also known as fool's gold, was found in the coprolite. This suggests its environment was depleted of oxygen, conditions that probably helped preserve the fossils for millions of years.