First Known Dinosaur Navel Found in Fossil

Rendering of a recumbent Psittacosaurus, with insert showing the umbilicus.

Rendering of a recumbent Psittacosaurus, with insert showing the umbilicus.
Illustration: Cut-out Fang Drawings

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

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

For mammals, belly buttons are the result of an umbilical cord that is detached at birth. But reptiles and birds, whose reproductive method is to lay eggs, do not have this cord. Inside an egg, the embryo’s abdomen is connected to a yolk sac and other membranes. Scarring occurs when the embryo detaches from these membranes directly before or when it hatches from the egg. Known as an umbilical scar, it is the non-mammalian form of a belly button. And that’s exactly what the international team of scientists claims to have found in this fossil.

Psittacosaurusa bipedal dinosaur that lived during the early Cretaceous, is an early form of ceratopsian, a type of beaked herbivore that, later in this same geological period, would include Triceratops. Perhaps the most stunning fossil of the species ever found remains frozen in time, lying on its back, complete with fur and tail bristles. Its preservation, approximately 130 million years old, is breathtaking. And although released to the public in 2002, it continues to break new ground.

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

This “subtle scar,” as Pittman described it in an email, was found using LSF. And it’s thanks to the LSF that the team was able to study the skin’s scales – their patterns, wrinkles and any scarring – in excellent relief. For help working with the skin, the team turned to Phil Bell, a dinosaur paleontologist at the University of New England’s Paleoscience Research Center in Australia, who has considerable experience in the subject. Bell is the lead author of the new article.

“LSF brings out the details spectacularly,” Bell said in a video interview. “It really feels like the animal can get up and walk away. You can see every little wrinkle and bump on 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 main goals of my work.”

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

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

The team found evidence of wrinkled skin, but not on the abdomen, where the umbilicus is located. Healed lesions would exhibit regenerative tissue; there would be a clear break in the scale patterns, with smooth granulation tissue over the injured area.

Instead, explained Pittman, “[t]Umbilical scales have regular sizes, smooth margins, and are arranged along the midline of the Psittacosaurus. This suggests that the scar was not the result of an injury.”

To determine the age of the dinosaur, most would cut through the bone. The extreme rarity of this fossil means that researchers want to avoid such destructive analysis. So the team compared the length of her femur with those of others. Psittacosaurus specimens and estimated that this particular animal was about 6 or 7 years old. In other words, this dinosaur was approaching sexual maturity.

Not every reptile or bird that lives today retains an umbilical scar into adulthood. The authors note that a specific exception is the American alligator (mississippiensis alligator). Also, some scars are a result of yolk sac infections in birds or crocodiles raised in poor conditions. With all these variables, it is not certain that all dinosaurs – or even all Psittacosaurus— would have an umbilical scar.

Psittacosaurus

Psittacosaurus
Illustration: Julius T. Cstonyi

Pittman described how he and Kaye “gathered a huge library of LSF data from the Psittacosaurus specimen in 2016”, which they are still scouring and studying. “It led to a paper that year in an observed counter-shading camouflage pattern, the first identified in a dinosaur. We plan to further analyze the LSF data because our images provided a lot of extra information about the skin.”

“We are finalizing a detailed description of the skin of Psittacosaurus,” he added. “This required us to look at every square inch of the fossil.” And that’s how this discovery of the umbilical scar took place.

Looking at preserved skin in such detail is Bell’s area of ​​expertise. He explained that few scientists focus on fossil skin, thus making any research prone to exciting discoveries. Furthermore, he said, in speaking to the general public, he finds that they are often surprised to hear that fossil skin exists, let alone what it reveals. Even within paleontology, he says, the main focus remains on bones.

I think the bottom line is that scaly reptiles are interesting,” Bell said. He hopes that both the public and the scientific community at large will realize how much we still have to learn about dinosaur skin and its biological function. Noting that “the skin is the largest organ in the body,” he noted how, for example, scales protect modern reptiles from dehydration and UV rays. Bell wants to change the perception that scales are less exciting than feathers.

“It’s an absolutely impressive specimen,” Bell commented of the Psittacosaurus fossil. “And the fact that it is still yielding surprises 20 years [from the time] was first announced to the public is extraordinary, and this is due to the development of these new imaging techniques.”

These surprises – the knowledge we have gained so far – would not have been possible had the fossil remained in private hands. this beautiful Psittacosaurus specimen have a controversial story. Its exact provenance is unknown, as it changed from one private collector to another prior to its purchase by Senckenberg. Then, as now, there are those who expect the fossil to be repatriated to China. At the end of their article, the authors write: “There is an ongoing debate over the legal ownership of this specimen and efforts to repatriate it to China have not been successful. Our international team of Australian, Belgian, British, Chinese and American members expect and support an amicable solution to this ongoing debate. We think it is important to note that the specimen was acquired by the Senckenberg Museum to prevent its sale into private hands and to ensure its availability for scientific study.”

Jeanne Timmons (@mostlymammoths) is a New Hampshire-based freelance writer who writes about paleontology and archeology in mostly mammoths.wordpress.com.

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