The butt orifice of a dinosaur was described for the first time back in 2021 thanks to a particularly well-preserved Psittacosaurus specimen. Now the world is fixated on a different hole (of sorts) found on the same animal: its belly button.

If you’re thinking, “Wait, what? I didn’t know dinosaurs had belly buttons,” then join the club. The discovery marks the first evidence of such a mark on a pre-Cenozoic amniote, the group of animals that contains mammals, birds, or reptiles. The research is published in BMC Biology.

The Psittacosaurus specimen’s unique preservation was crucial to uncovering the belly buttons of dinosaurs. First discovered in the Early Cretaceous Jehol Group of Liaoning Province, China, it’s now kept at the Senckenberg Research Institute and Natural History Museum Frankfurt, where the authors of the paper were able to use special equipment to closer inspect its abdomen.

“The specimen is an individual of the early ceratopsian dinosaur Psittacosaurus, an early relative of Triceratops,” study author Michael Pittman told IFLScience.

“The fossil was first described in 2002, but it wasn’t until we had high detail imaging produced by myself and collaborator Thomas G Kaye using lasers that we had the data needed to identify the belly button.”

The exceptionally-preserved soft tissues of the SMF R 4970 specimen, which is positioned on its back, have yielded epidermal scales, a keratinous jugal “horn,” a long plume of tail bristles, a cloaca, and even skin pigmentation patterns showing whole-body countershading. Now, SMF R 4970 can add a belly button to its inventory.

Many may be surprised to learn that modern birds and reptiles have belly buttons, too. However, the witness mark of a hole that once connected the developing embryo to the extraembryonic equipment that kept it fed and oxygenated usually disappears within days to weeks after hatching.

As the oldest preserved umbilical scar known to science, the belly button of Psittacosaurus is also an intriguing find because the preserved specimen on which it was found was not a few days to weeks old when it died. It’s believed to have been close to sexual maturity, if not already there, indicating that the “umbilicus” (as these scars are also known, though we prefer belly button) endured throughout the dinosaurs’ life.

An enduring belly button is not unheard of in the animal kingdom. There are many examples of mammals (including humans) that keep their belly buttons throughout their life. The authors reference the rock pigeon, Columba livia, as a species of bird that sometimes retains its umbilicus to a later stage.

Some reptiles such as snakes and lizards also maintain a notable scar from the good old days in the egg. For snakes, this sometimes presents as a crease, while for lizards it can alter scale morphology.

The belly button of Psittacosaurus is most similar to that of a lizard, presenting as a line of paired scales that are larger and misshapen compared to those around it, stretching along the midline of the abdominal wall. So, why did Psittacosaurus’s umbilicus scar linger on?

“The umbilical scar varies in extant reptiles. Thus, seeing one in a Psittacosaurus dinosaur specimen that is just shy of sexual maturity does not necessarily mean that they will persist with age in other non-avian dinosaurs,” said Pittman. “We’ll need to discover more dinosaur belly buttons to find out.”

BRB, going prehistoric belly button hunting.