At Least Some Dinosaurs Had Bellies, An Exquisite Fossil Has Just Been Revealed

The oldest “belly button” known to science has been found on a beautifully preserved horned dinosaur fossil from China.

The slit-like scar does not come from an umbilical cord, as it does in mammals, but from the yolk sac of the egg-laying creature that lived around 130 million years ago.

Many living snakes and birds today shed their so-called umbilical scar within days or weeks after hatching from an egg. Others, like adult alligators, keep them for life.

The umbilical scar found on the Psittacosaurus The dinosaur is more like alligators and lasts at least until sexual maturity and possibly long after.

It is the first example of a navel in a non-avian dinosaur, predating the arrival of “recent life” in the Cenozoic, around 66 million years ago.

The finding doesn’t mean that all land dinosaurs sported the same lingering umbilical scar, but it does leave the possibility open.

“This Psittacosaurus specimen is probably the most important fossil we have for studying dinosaur skin,” says vertebrate paleontologist Phil Bell from the University of New England in Australia.

“But it continues to yield surprises that we can bring new technologies like laser imaging to life.”

The dinosaur fossil Psittacosaurus The genus, known as SMF R 4970, was found in China in 2002. It is beautifully preserved, showing individual scales, long plumes of tail hairs, and the first cesspool ever seen in a non-avian dinosaur.

Lying on its back, the individual presents practically all its details to the scientists.

3D reconstruction of a reclining chair Psittacosaurus showing the long umbilical scar. (Designs of jagged fangs)

Using detailed laser imaging, the researchers have now been able to identify a change in the pattern of skin and scales where the dinosaur’s belly button would be, representing where the yolk sac was reabsorbed by a young dinosaur.

Similar to the umbilical cord, the yolk sac is what supplies the growing embryo inside the egg with oxygen and nutrients; there is another connected transparent sac, the allantois, which collects the waste inside an egg. Before an animal hatches, these connections seal, leaving a long scar.

Researchers have long speculated that egg-laying dinosaurs possessed such a scar, but this is the first evidence to support the idea.

Soft tissue fossils are rarely preserved in dinosaurs, but SMF R 4970 gives us unprecedented insight into a thin column of paired scales on an ancient abdomen.

The regular size of the scales and their smooth margins suggest that the scar was not caused by physical trauma or disease.

Instead, its similarity to alligators strongly suggests we’re looking at a navel.

2Laser image of the Psittacosaurus umbilical scar, with close-ups of its scales. (Bell et al., BMC Biology2022)

“Using LSF imaging, we identified distinctive scales that surrounded a long umbilical scar in the Psittacosaurus specimen, similar to some living lizards and crocodiles,” says paleontologist Michael Pittman from the Chinese University of Hong Kong.

“We call this kind of scar a navel, and it’s smaller in humans. This specimen is the first dinosaur fossil to retain a navel, which is due to its exceptional state of preservation.”

From a full-fledged asshole to a clear navel, the SMF R 4970 Fossil really showed off its most intimate parts.

The study was published in BMC Biology.

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