Forget about dinosaurs involved in cruel fighting. Set aside your terrifying fangs and claws. Scientists have discovered a softer side of dinosaurs: the reptile equivalent of a navel.
For the first time, scientists have identified an umbilical scar in a non-avian dinosaur. The paper announcing this finding is published in BMC Biology, and is another exciting discovery of a particularly rare and well-preserved Psittacosaurus fossil from China. (Other delights of this specimen include a sewer and a counter-shadow camouflage.)
For mammals, the navels are the result of a detachment of the umbilical cord at birth. But reptiles and birds, whose reproductive method is to lay eggs, do not have this cord. Inside an egg, the abdomen of the embryo is connected to a yolk sac and other membranes. Scarring occurs when the embryo comes off these membranes directly before or when it hatches from the egg. Known as an umbilical scar, it is the non-mammalian form of a navel. And this is exactly what the international team of scientists claims to have found in this fossil.
Psittacosaurus, a bipedal dinosaur that lived during the early Cretaceous, is an early form of ceratopsian, a type of beaked herbivore that later in the same geological period would include Triceratops. Perhaps the most dazzling fossil of the species ever found remains frozen in time, lying on its back, with its bristles on its skin and tail. Its conservation, at about 130 million years old, is impressive. And although it was introduced to the public in 2002, it continues to open new and unique avenues.
Michael Pittman has studied this particular fossil in detail. He is a paleobiologist, assistant professor at the Chinese University of Hong Kong and co-author of the new article. 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 Museum of Natural History. 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 go unseen.
This “subtle scar,” as Pittman described it in an email, was found with LSF. And it is thanks to LSF that the team was able to study the scales on the skin (its patterns, wrinkles and any scars) with exquisite relief. For help working the skin, the team turned to Phil Bell, a dinosaur paleontologist at the University of New England Paleoscience Research Center in Australia, who has considerable experience in the subject. Bell is the lead author of the new paper.
“LSF brings out the detail dramatically,” Bell said in a video interview. “It looks like the animal could get up and walk away. You can see all the little wrinkles and bumps on the skin. It looks so cool. What fascinates me is to imagine these animals as living, breathing entities, instead of only dead skeletons. Bringing them to life is one of the main goals of my work. “
Laser-stimulated fluorescence (LSF) image of the Psittacosaurus specimen showing the scar and umbilical scales. (Image: Bell et al. 2022)
The team found evidence of wrinkled skin, but not in the abdomen where the umbilical scar is located. Healed lesions would show regenerative tissue; there would be a clear rupture in the scale patterns, with smooth-grained tissue over the injured area.
Instead, Pittman explained, “[t]The umbilical scales are of regular size, smooth margins, and are arranged along the midline of Psittacosaurus. This suggests that the scar was not the result of an injury. “
In order to determine the age of the dinosaur, most would cut the bone. The extreme rarity of this fossil means that researchers want to avoid any such destructive analysis. Thus, the team compared the length of its femur with that of other Psittacosaurus specimens and estimated that this particular animal was about 6 or 7 years old. In other words, this dinosaur was about to reach sexual maturity.
Not all reptiles or birds that live today maintain an umbilical scar during adulthood. The authors point out that a particular exception is the American alligator (Alligator mississippiensis). In addition, some scars are the result of yolk sac infections in poorly bred birds or crocodiles. With all these variables, it is not a fact that all dinosaurs, or even all Psittacosaurus, had an umbilical scar.
Psittacosaurus (Illustration: Julius T. Csotonyi)
Pittman described how he and Kaye “collected a large library of LSF data from the Psittacosaurus specimen in 2016,” which they are still reviewing and studying. “This led to an article that year about an observed anti-shadow camouflage pattern, the first one identified in a dinosaur. We planned to further analyze the LSF data because our images provided a lot of additional information about the skin.”
“We are currently finalizing a detailed description of Psittacosaurus skin,” he added. “That forced us to look at every square inch of the fossil.” And that’s how this discovery of the umbilical scar came about.
Looking at preserved skin in such detail is Bell’s area of expertise. He explained that few scientists focus on fossil skin, making any research prone to exciting discoveries. Also, he said, when he talks to the general public, he discovers 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 the bones.
“I think the thing to take away is that flaky reptiles are interesting,” Bell said. He hopes that both the public and the wider scientific community 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 referred to 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 said of the Psittacosaurus fossil. “And the fact that it continues to surprise 20 years [from the time] which was first announced to the public is extraordinary, and this is due to the development of these new imaging techniques. “
Those surprises, the knowledge we have gained so far, would not have been possible if the fossil had been left in private hands. This magnificent specimen of Psittacosaurus has a controversial history. Its exact origin is unknown, as it was moved from one private collector to another prior to its purchase by Senckenberg. Then, like 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 about the legal ownership of this specimen and efforts to repatriate it to China have been unsuccessful. Our international team of Australian, Belgian, British, Chinese and Americans look forward to and support a friendly solution to this ongoing debate.We believe it is important to note that the copy was purchased by the Senckenberg Museum to prevent its sale to private hands and to ensure its availability. for scientific study “.
Jeanne Timmons (@mostlymammoths) is a New Hampshire-based freelance writer who publishes a blog on paleontology and archeology at mostlymammoths.wordpress.com.