Science

First Known Dinosaur Belly Button Found in Fossil

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Rendering of a reclining Psittacosaurus, with insert showing the umbilical scar.

Rendering of a reclining Psittacosaurus, with insert showing the umbilical scar.
Illustration: Jagged Fang Designs

Forget dinosaurs engaged in vicious combat. Put aside terrifying fangs and claws. Scientists have discovered a softer side to dinosaurs: the reptilian equivalent of a belly button.

For the first time ever, scientists have identified an umbilical scar on a non-avian dinosaur. The paper announcing this find is published in BMC Biology, and it’s yet another exciting discovery from a particularly rare and well-preserved Psittacosaurus fossil from China. (Other delights from this same specimen include a cloaca and countershading camouflage.)

For mammals, belly buttons are the result of a detached umbilical cord at birth. But reptiles and birds, whose reproductive method is to lay eggs, have no such cord. Inside an egg, the embryo’s abdomen is connected to a yolk sac and other membranes. The scar occurs when the embryo detaches from those membranes directly before or as it hatches from the egg. Known as an umbilical scar, it is the non-mammalian form of a belly button. And that is exactly what the international team of scientists claims to have found on this fossil.

Psittacosaurusa bipedal dinosaur that lived during the beginning of the Cretaceous, is an early form of ceratopsian, a type of beaked herbivore that would, later in that same geologic period, include Triceratops. Perhaps the most dazzling fossil of the species yet found remains frozen in time, lying on its back, complete with skin and tail bristles. Its preservation, at approximately 130 million years of age, is breathtaking. And although made known to the public in 2002, it continues to break new and unique ground.

Michael Pittman has studied this particular fossil in detail. He’s a paleobiologist, assistant professor at the Chinese University of Hong Kong, and co-author of the new paper. He and co-author Thomas G. Kaye, from the Foundation for Scientific Advancement, were able to visit the fossil in Germany in 2016 at the Senckenberg Research Institute and Natural History Museum Frankfurt. The two scientists invented Laser-Stimulated Fluorescence (LSF), a relatively new imaging technique. With this non-destructive method, they have been able to reveal details in fossils that might otherwise remain unseen.

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This “subtle scar,” as Pittman described it in an email, was found using LSF. And it is thanks to LSF that the team could study the scales of the skin — their patterns, wrinkles, and any scarring — in exquisite relief. For help working on the skin, the team turned to Phil Bell, a dinosaur paleontologist at the Palaeoscience Research Center, University of New England in Australia, who has considerable expertise on the subject. Bell is the lead author of the new paper.

“LSF brings out the detail in spectacular fashion,” Bell said in a video interview. “It really looks like though the animal could get up and walk away. You can see every little wrinkle and bump in the skin. It looks so fresh. Imagining these animals as living, breathing entities, rather than just dead skeletons, is what fascinates me. Bringing them to life is one of the major goals of my work. ”

Laser-stimulated fluorescence (LSF) image of the Psittacosaurus specimen showing the umbilical scar and scales.

Laser-stimulated fluorescence (LSF) image of the Psittacosaurus specimen showing the umbilical scar and scales.
Image: Bell et al. 2022

The team did find evidence of wrinkled skin, but not in the abdomen where the umbilical scar is located. Healed injuries would display regenerative tissue; there would be a distinct break in scale patterns, with smooth granulation tissue over the injured area.

Instead, explained Pittman, “

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