Oldest Known Guts Discovered in 550 Million-Year-Old Fossil, Shedding Light on Animal Evolution

Researchers have discovered 550 million-year-old fossils in the Nevada desert featuring what may well be the earliest known digestive tracts.

According to a study published in the journal Nature Communications, the digestive tracts are one of the oldest known fossilized anatomical structures, shedding new light on the evolution of animal life.

Between around 580 and 540 million years ago, toward the end of the Ediacaran Period, a group of bizarre, soft-bodied creatures emerged in Earth's oceans, representing the first major group of complex, multicellular life on the planet.

Ediacaran organisms represent an important milestone in the evolution of life on Earth as they immediately precede the Cambrian Explosion—a rapid increase in biodiversity after 540 million years ago when most of the major animal groups we know today appear in the fossil record.

But it is unclear how the Ediacaran biota—as they are referred to—relate to modern animals, because they largely disappeared right around the time when the Cambrian animals became dominant.

As such, the placement of Ediacaran organisms on the tree of life is contentious. Some are most probably true animals. But many of the larger examples are quite distinct from later life-forms: Among them are large, flat, quilt-like creatures; tube-shaped organisms; and plant-like, leaf-shaped critters.

In the latest paper, researchers identified possible digestive tracts in an extinct worm-like creature known as Cloudina—which formed part of a larger group which spanned the boundary between the Ediacaran and Cambrian.

"These fossils fit within a very recognizable group of organisms—the cloudinids—that scientists use to identify the last 10 to 15 million years of the Ediacaran Period, or the period of time just before the Cambrian Explosion," Jim Schiffbauer, an author of the study from the University of Missouri, said in a statement. "We can now say that their anatomical structure appears much more worm-like than coral-like."

ancient digestive tract
A three-dimensional image of a 550-million-year-old fossilized tube (left, in red) with internal digestive tract (gold, left and right.) University of Missouri

"Not only are these structures the oldest guts yet discovered, but they also help to resolve the long-debated evolutionary positioning of this important fossil group," Schiffbauer said.

The researchers identified the digestive tracts using a special CT-imaging technique which allowed them to create a digital 3D reconstruction of the fossil, helping them to see what was inside.

"With CT imaging, we can quickly assess key internal features and then analyze the entire fossil without potentially damaging it," Tara Selly, a co-author of the study, said in a statement.