Study: Palaeocampa anthrax, an armored freshwater lobopodian with chemical defenses from the Carboniferous (DOI: doi.org/10.1038/s42003-025-08483-0)
A fossil specimen misidentified since its discovery during the Civil War and correctly identified by a University of Michigan researcher reveals an evolutionary leap: It marks the first time this line of sea creatures is known to have left the ocean.
U-M postdoctoral fellow Richard J. Knecht found that the specimen, identified as a seaworm for well over a century, is actually a kind of invertebrate called a lobopod. Lobopods are soft-bodied invertebrates that closely resemble a "worm with legs," according to Knecht, who led the work as a researcher at Harvard University.
Lobopods are considered the ancestor to all arthropods-all insects, spiders and crustaceans-and scientists have long thought that lobopods, which first appeared in the fossil record more than 500 million years ago, only lived in marine environments.
Now, Knecht suggests that the specimen, named Palaeocampa anthrax, likely lived in a freshwater environment. Knecht also found the specimen likely had chemical defenses that suggest it may have spent at least part of its time on land. The study, which was supported by the National Science Foundation, was published in Communications Biology.
"This is the first evidence that we have that this group actually did leave the ocean. Transitioning from a saltwater environment, even to a freshwater environment, is a major evolutionary step," Knecht said. "Any time you find a group that made this transition, it's not just insightful, it's really important evolutionarily to try to understand how this happened."
Knecht says his study also shows the importance of revisiting fossil collections, even fossils that have been previously identified, with fresh perspectives and updated techniques. Since its discovery, this specimen has been variously misidentified as a caterpillar, a marine worm, and a millipede-organisms from very different evolutionary lineages. Even recent studies cited photographs of the fossil without recognizing its true identity.
"When the specimen was first discovered, lobopods didn't even exist as a group. This is an example of why revisiting older specimens can be of real value," said Knecht, a researcher in the U-M Department of Ecology and Evolutionary Biology and Museum of Paleontology. "Even specimens that are iconic and super well known-provide an opportunity to make big discoveries just by viewing them with new techniques and fresh perspectives."
Palaeocampa anthrax and new techniques
Knecht first encountered the specimen while looking at a collection of millipedes in the Invertebrate Paleontology Department of the Museum of Comparative Zoology at Harvard University. He immediately recognized it as a species of lobopod-not a millipede as it was labeled in the drawer.
Paleozoic lobopods can be difficult to pin down, Knecht said. This species looks superficially like woolly bear caterpillars, with papillae and bristles of spikes growing off the trunks of their bodies. Some lobopods from this era had plates of armor, some had forward-facing arms prodruding from their heads. One species' body is absent of features and just looks like a straw with smooth straw-like legs.
But when Knecht looked at the Palaeocampa specimen, he noticed several noticeable differences from other lobopodian species. For example, most lobopodian spines exhibit a cone-in-cone growth pattern like a stack of traffic cones coming to a point at the tip. Instead, Palaeocampa's spines look like a series of rigatoni growing out of its trunk, each segmented like bamboo, with a top that looks like a castle turret, Knecht said.
Then, the research team noticed orange halos at the tips of the preserved spines. The team wondered if there was a fluid or chemical that the lobopod may have emitted from the tips of its spines.
To investigate this, Knecht worked with Nanfang Yu, a physicist at Columbia University. Yu used a technique called FTIR spectroscopy and infrared light to determine that there were fossilized molecules present at the tips of the specimen's spines that weren't present anywhere else along the spine or in the rock. This suggests, Knecht says, that there was a chemical specifically emanating from the tip of the spine.
This characteristic is the most significant evidence indicating that the lobopod was likely amphibious, Knecht said. Most marine or aquatic organisms that have spines with chemical defense systems have spikes that they can use to inject predators. If their defense systems emanated from an open tip, their chemical defense would be less effective and simply dilute in the water.
"If I were to place a chip on the table, I'd say this organism was probably more terrestrial or amphibious, specifically because of that tip. It makes more sense in an aerial rather than a subaqueous environment," he said.
Statistical miracles
Lobopodians are rare in the fossil record especially after the Cambrian Period, Knecht said. Because invertebrates have no bones, their soft bodies degrade very quickly after death. An unusual set of circumstances has to exist in order for them to fossilize: They have to die and then quickly be buried before scavengers or decomposition takes hold, often in aquatic environments with fine grains or mud and little oxygen.
All known lobopodians come from Lagerstätten-extraordinary fossil sites with the rare geological conditions needed to preserve soft-bodied organisms in exceptional detail and abundance. Palaeocampa anthrax was discovered both at Mazon Creek, Illinois, and Montceau-les-Mines, France, both Lagerstätten renowned for their fossils.
This specimen is one of two lobopodians from the Carboniferous, the last period within the Paleozoic-and represents the youngest fossil lobopodian species yet discovered. It's also a transition fossil, an example of a species that represents an evolutionary leap.
Trying to find transition fossils is extraordinarily difficult, Knecht said. They often occur during a small snapshot of time, geologically speaking. It's even more difficult to discover transition fossils of invertebrates; their lack of bones means they don't preserve well in the fossil record. Finding one is a stroke of luck. To capture the Palaeocampa fossil in such detail, recording chemicals emanating from its tips, its preservation must have been almost instantaneous, Knecht said. It likely was buried alive in the mud.
"It's like winning the lottery not once, but three times. First, that the site existed at all; second, to have this level of preservation; and third, to then find it over 300 million years later," he said. "It's a series of statistical miracles."
