Fossilized vestiges of a freshwater shark that lived 270 million years ago in what is now the municipality of São Gabriel in the Brazilian state of Rio Grande do Sul, 320 kilometers west of Porto Alegre, may be the oldest record of infestation of a vertebrate by a type of tapeworm or cestode. A cluster of 93 oval-shaped microstructures was found inside a coprolite (petrified feces) of the fish and interpreted as being eggs of that intestinal parasite. Most of the eggs were filled by pyrite, an iron disulfide known as fool’s gold, and appeared to have been preserved before the worms had a chance to break through them. One of them stood out from the rest. “That egg contains a probable developing larva of the parasite,” says paleontologist Paula C. Dentzien-Dias of the Federal University of Rio Grande (Furg), the principal scientist responsible for the discovery. An analysis of the content of the rare coprolite, recovered in rocks from the Permian period in the Rio do Rasto geological formation, was published on January 30 in the scientific journal PLoS One.
The worms were hidden inside a coprolite with a spiral shape, a distinctive characteristic of shark excrement, that measured five centimeters long by two centimeters in diameter. The coprolite was “sliced” lengthwise to obtain a section thin enough to view in an optical microscope. The objective was to look inside the feces for organic fragments that would give an indication of the animal’s diet. Several slices from this coprolite, and another 13 obtained in the region, revealed the presence of scales and teeth of other fish. One slice, however, offered a major surprise: nearly a hundred small oval structures inside.
Initially, the researchers hypothesized that it could be some kind of structure of inorganic origin, produced during the fossilization process. But a more detailed observation of the slice led them to a different conclusion. It was a cluster of tapeworm eggs, nearly all having the same dimensions: 145–155 micrometers long and 88–100 micrometers wide. The presence of pyrite in the coprolite is an indication that the material was exposed to conditions with little or no oxygen, which are favorable to fossil preservation. This mineral is known to form only in the absence of oxygen.
Because it was identified in fossilized feces of a freshwater fish, this ancient form of cestode suggests that the worm’s first habitats were predominantly lakes and rivers. Its first hosts were probably aquatic animals such as the paleosharks in São Gabriel. “The new fossilized tapeworm eggs show that these parasites existed at least 270 million years ago, but they must have appeared long before that. The problem lies in finding preserved vestiges of these worms,” Dentzien-Dias says.
Today different species of tapeworms can be found in many animals, such as swine, bovines and fish. If infested with the worm, poorly washed food and undercooked meats can transmit two diseases to humans: taeniasis and cysticercosis. In more serious cases, the latter disease can be fatal. Although it has not been possible to determine which species of tapeworm appears to have infested the ancient shark, the vestiges of the parasite are reminiscent of the eggs produced by worms of the order Tetraphyllidea. About 540 species of parasites of this order can be found presently in shark intestines.
With its abundance of sedimentary rocks from the Middle and Upper Permian (270-250 million years ago), the São Gabriel region is rich in fossils from vertebrates, invertebrates and plants. In this soil characterized by sandstones, siltstones and mudstones, special conditions prevailing throughout millions of years preserved the fossilized feces, a kind of organic vestige of the past that tends to be effaced by the action of the environment. A bit of luck and eyes trained to differentiate simple rock from petrified excrement were essential to making this finding.
On a field expedition in 2010, Dentzien-Dias and other paleontologists from the state of Rio Grande do Sul discovered an area 100 meters long by 30 meters wide—slightly smaller than the size of an official soccer field—with a concentration of over 500 coprolites, most of them from sharks. Some were buried in the soil, and others had come to the surface. The size of the feces varied from 0.6 to 11 centimeters in length. “There were so many coprolites that people were even tripping over them,” quips paleontologist Cesar Schultz of the Federal University of Rio Grande do Sul (UFRGS). Schultz did not participate in the expedition but coordinates the research project and is a co-author of the scientific article. The project is funded by the Brazilian National Council for Scientific and Technological Development (CNPq). The small area containing so many fossilized feces was nicknamed Coprolandia.
The odd concentration of coprolites produced by freshwater fish indicates that there had been a pond in that area some 270 million years ago. But how did such an enormous quantity of organic feces end up preserved in a corner of this now-vanished body of water, even creating the illusion that there may have existed a place where the animals preferred to tend to their bodily needs? The researchers believe that there was a sudden intense drought in the region during the Permian period and that much of the ancient pond evaporated quickly. In order to survive, the animals had to gather in places where water still remained. A movement of that kind would naturally produce a concentration of feces in the refuge where the fish were confined. “We think the drought was temporary and did not cause the fish to die,” Schultz comments. “We did not find any fossilized animal bones alongside the coprolites.”
Of the approximately 500 petrified feces recovered in São Gabriel, 14 have been analyzed. So far, the coprolite with tapeworm eggs has produced the most exciting data, but there may be other discoveries to be made in this collection of organic impressions of the distant past.
DENTZIEN-DIAS, P.C. et al. Tapeworm eggs In a 270 million-year-old shark coprolite. PLoS One. 30 Jan. 2013.