March 4, 2026
2 min read
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This may be the oldest ‘butthole’ imprint on Earth
Fossils show exceptionally rare evidence of a cloacal vent—the slit that most vertebrates use to excrete, have sex and lay egg—which could shed light on the evolution of the orifice
Fossil impression of the hind limb, tail and cloacal vent of Cabarzichnus pulchrus, a small lizard-like reptile.
Around 299 million years ago volcanic eruptions buried a patch of mud in what is today central Germany. Amid impressions of scales, tails and feet that fossilized in the patch was something else: Earth’s oldest known “butthole” imprint.
“Finding an impression of [an] animal squatting in the mud and preserved with such fidelity is quite a scoop,” says Jakob Vinther, a paleontologist at the University of Bristol in England, who was not involved with a new study describing the find in Current Biology. “The animal literally cemented itself—and its nether regions—into eternal history like the movie stars on the Hollywood Walk of Fame.”
The structure imprinted into the fossilized mud is a slit that is technically called a cloaca. Unlike marsupials and placental mammals that split their “business elements” into separate orifices, “most other animals have the Swiss Army knife equivalent of a rear opening,” Vinther says. This gives them “one opening for everything—pooping, peeing, sex and laying eggs.”
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Lead author Lorenzo Marchetti, a paleontologist at the Natural History Museum of Berlin, discovered the vent while scrutinizing rare diagonal and hexagonal scale imprints in the fossil. “I noticed something unusual, and after a comparison with modern animals, it became clear what it was,” he says.
A closer look at the fossilized impression of the cloacal vent.
Cloacal vents vary in size and shape across the reptile world, but a lack of fossil preservation has kept their evolution mysterious.
“Only two examples of this structure are currently known in fossil reptiles,” Marchetti says. They are this one and that of a 130-million-year-old ceratopsian called Psittacosaurus, which Vinther and his colleagues reported in 2021. Radioisotopes of ash within the new fossil suggest its cheeky impressions were planted about 170 million years before the Psittacosaurus fossil.
The scale imprints and footprints in the ancient muck helped Marchetti and his colleagues to determine that the derriere belonged to a never-before-named species. The animal, which the researchers named Cabarzichnus pulchrus, was a small lizardlike reptile that had likely been lounging in the mud to cool off, Marchetti says.
“It’s pretty remarkable to see the fine details preserved of such a small animal,” says Phil Bell, a paleontologist at the University of New England in Australia, who was not involved with the research. “The consistency of the mud has to be absolutely perfect for such an imprint to occur.”
Fossil cloacae “are as rare as hens’ teeth,” Vinther says, “and finding another one is exciting to say the least.”
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