NEW YORK (NYT NEWS SERVICE ).- They say you should trust your gut, which is what Emmy Smith did when she went hunting for fossils in 2016. Smith, a field geologist, had a hunch she would find something interesting at a site north of Pahrump, Nevada, and she did. But what her gut hadn’t told her was that some of those fossils would turn out to contain the oldest known animal guts on the planet.

“It was just really lucky,” said Smith, who works at Johns Hopkins University in Baltimore and is part of the team that reported the find Friday in Nature Communications.

The guts are those of an extinct animal called Cloudina, which looked like a worm made of a stack of ice cream cones and lived about 550 million years ago, just after a period in Earth’s history when the entire planet was encased in ice.

Smith and a doctoral student in her lab wrapped the Cloudina fossils they found in toilet paper, put them in paint buckets and hauled them back to their field car, a Ford Ranger nicknamed Kitty. Later, Smith shipped the fossils to Tara Selly and James Schiffbauer, paleontologists at the University of Missouri, for further study.

Schiffbauer and Selly specialize in the group of fossils that Cloudina is a member of: the Ediacara biota. The group includes Earth’s oldest known animals, which means that if a researcher wants to figure out what the dawn of the animal kingdom looked like — and find out when animals developed intestines — studying animal fossils like Cloudina is a good place to start.

In their lab, the duo shined X-rays on Cloudina’s remains, building 3D images of the fossils’ insides. “The first one we were looking at, we found a gut,” said Selly, who spotted the digestive system in the lab while Schiffbauer was in his office.

“She texted me and said, ‘Hey, found something really cool; you have to come look at this,’” Schiffbauer said. When he got to the lab and saw the X-ray images, he knew exactly what they had on their hands.

“This is a gut,” he recalled saying.

The tubular guts are only about as wide as a cocktail straw. They run through Cloudina’s entire length, meaning they passed all the way through from the front end to the back end. Not every animal has a digestive system that ends in a different place from where it begins. But that setup has been common in everything from humans to insects to dinosaurs. Cloudina’s guts, then, are the first known example of our particular kind of digestive tract in the history of animal life.

“Finding that we had a tubular structure inside this skeletal tube tells us that it had a distinct mouth and a distinct anus,” Schiffbauer said. In other animals, like corals, the gut is a simple sac, and the only way into that sac is through the mouth, which also serves as the anus. But with the evolution of a through-going gut, animals no longer had to wait for their food to digest before regurgitating the waste so they could keep eating.

This made eating a lot more efficient, and it opened the door for other kinds of animals with through-going guts to evolve later on, said Lidya Tarhan, a paleontologist at Yale University who was not involved in the new research.

“I think this gets at the heart of some of the most outstanding questions about the evolution of complex life on our planet,” she said.

© 2020 The New York Times Company