Oldest Known Macroscopic Skeletal Organism Was Masquerading as Fossilized Feces

Sometimes in science, your findings can look pretty crappy—but here’s something that might make you give your work a second look before you flush it all away. As Stephanie Pappas at Live Science reports, researchers are now unveiling the oldest skeletal remains ever discovered. Before the fossils were properly identified, however, the remains of Palaeopascichnus linearis, a little marine creature that may have resembled a globular amoeba, were believed to be very old pieces of poop.

P. linearis fossils resemble a series of closely packed spheres, which is why some researchers initially dismissed them as teeny turds from a bygone era, Pappas reports. As their fossils are found in rocks around the world, scientists speculated that perhaps they were footprints from a hungry creature shuffling along the ocean floor, or the remains of algae or another lifeform. Then, of course, someone posited fossilized feces.

It wasn’t until Anton V. Kolesnikov, a paleontologist at Russia’s Trofimuk Institute of Petroleum Geology and Geophysics of Siberian Branch Russian Academy of Sciences, and an international cohort of colleagues stepped in that the debate was finally resolved. A new study, published this month in the journal Precambrian Research, details their findings.

Many of the regions that house fossils of P. linearis are protected—but the researchers struck gold in northeastern Siberia, where they harvested hundreds of new specimens. Sliced open and studied under the microscope, the fossils finally revealed their true, not-so-fecal nature. When the researchers performed the same analyses on fossils collected from other parts of the world, they found a global consensus: the existence of a true skeleton.

As it turns out, P. linearis used materials from their undersea environment to construct their own exoskeletons. That’s pretty impressive—especially considering how laborious this must have been with no hands and only bits of sand at the ready. A chain of little ovoids, measuring between about 0.04 and 0.2 inches in diameter, comprised each fossil, giving the appearance of a string of uneven beads. The petrified pearls can stretch a few inches in length—hence the linearis.

P. linearis wasn’t the first life form to bony up. Earlier organisms with similar exoskeletons did exist, dating back almost 750 million years ago, the researchers detail in the study. However, these organisms were a lot smaller—probably microscopic, Kolesnikov explains in an interview with Pappas of Live Science. The researchers estimate P. linearis was around as far back as 613 million years ago, making it “the oldest known macroscopic skeletal organism.”

That means P. linearis predates the Paleozoic Era, which started around 540 million years ago and is when scientists previously believed critters big enough to be visible to the naked eye first evolved skeletons. Instead, P. linearis’ appearance comes at the cusp end of the Proterozoic Era—and the researchers think these hardy little guys may have outlived many of their peers that were felled during one of Earth’s first mass extinctions, which occurred just before the beginning of the Paleozoic.

Shortly after, though, the end came too for P. linearis. But it might have a modern doppelganger in an amoeba called a xenophyophore: a single-celled resident of the ocean floor. Though they are separated by hundreds of millions of years, both of these amorphous creatures have the ability to scoop up surrounding sediment and glue the grains to their bodies, anchoring themselves in place with a DIY skeleton. In theory, it’s possible the two groups are related—but much of the evidence has probably been lost to time, alongside the years P. linearis spent in fecal anonymity.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.