An international team of paleontologists has recently discovered the oldest belly button known to science in a 125-million-year-old dinosaur fossil found in China two decades ago. The fossil belongs to a member of the genus Psittacosaurus, a group of bipedal, horned dinosaurs which roamed the Earth during the Early Cretaceous period (about 145 to 100.5 million years ago).
By applying the Laser-Stimulated Fluorescence (LSF) technique to a fossilized skin specimen of this two-meter-long herbivorous creature, the scientists observed a barely perceptible navel mark.
“Using LSF imaging, we identified distinctive scales that surrounded a long umbilical scar in the Psittacosaurus specimen, similar to certain living lizards and crocodiles,” explained study co-author Michael Pittmann, an assistant professor of Paleobiology at the Chinese University of Hong Kong (CUHK). “We call this kind of scar a belly button, and it is smaller in humans. This specimen is the first dinosaur fossil to preserve a belly button, which is due to its exceptional state of preservation.”
Unlike humans and other mammals, dinosaurs did not have an umbilical cord, because they laid eggs. Instead, the yolk sac of dinosaurs was directly attached to their bodies through a slit-like opening that was also discovered in other egg-laying land animals. At the time the animals hatch, this opening seals up, leaving a distinctive long umbilical scar. Although scientists assumed for a long time that dinosaurs must have had such belly buttons, this study is the first to support this hypothesis with fossil evidence.
“This Psittacosaurus specimen is probably the most important fossil we have for studying dinosaur skin,” said study lead author Phil R. Bell, a vertebrate paleontologist at the University of New England in Australia. “But it continues to yield surprises that we can bring to life with new technology like laser imaging.”
“Unlike most extant reptiles and birds that lose this scar within days to weeks after hatching, the umbilicus of Psittacosaurus persisted at least until sexual maturity, similar to some lizards and crocodylians with which it shares the closest morphological resemblance,” wrote the study authors.
“This discovery is the oldest record of an amniote umbilicus and the first in a non-avian dinosaur. However, given the variability of this structure in extant reptilian analogues, a persistent umbilical scar may not have been present in all non-avian dinosaurs.”
The study is published in the journal BMC Biology.