Dinosaurs had 'belly buttons,' new research reveals

In egg-laying amniotes (reptiles, birds and monotremes), the developing embryo is tethered to a number of the extraembryonic membranes that deliver oxygen and nutrients and remove metabolic waste products throughout embryonic development. Prior to, or soon after hatching, these membranes detach from the animal leaving a temporary or permanent umbilical scar equivalent to the navel or ‘belly button’ in some placental mammals, including humans. Now, paleontologists report the oldest preserved umbilical scar in a fossil amniote from Psittacosaurus, a species of ceratopsian dinosaur that lived some 130 million years ago.

“Amniotes are characterized by the presence of a number of extraembryonic membranes (allantois, amnion, chorion and yolk sac) and a semipermeable eggshell that provide a stable environment and nourishment for the developing embryo,” said University of New England paleontologist Phil Bell and his colleagues.

“Two of these membranes in particular — the allantois and yolk sac — are intimately connected to the embryo via vitelline and allantoic blood vessels that penetrate the abdominal wall and, in placental mammals, are enclosed within a long umbilical cord.”

“Immediately prior to or soon after hatching, this communication is severed and the yolk sac is internalized, although the opening in the abdominal wall may take several days to weeks to fully close, leaving an umbilical scar or umbilicus.”

“Detachment of the placental mammal umbilical cord after birth results in the characteristic navel or ‘belly button,’ which is the topographic and developmental equivalent of the umbilicus in reptiles and birds.”

Laser-stimulated fluorescence (LSF) image of the whole Psittacosaurus specimen showing the location of the umbilical scar. Insets show the umbilical scar close-up, including the distinctive scales surrounding it (highlighted in blue in the line drawing). Credit: Bell et al. 2022

In the study, the paleontologists examined the 125 million-year-old fossilized skin of Psittacosaurus , a 2-m-long, two-legged plant eater from the Jehol Group of Liaoning Province, China.

“While it’s common for land animals to have umbilical scars for at least part of their life, until now, no evidence of a belly button had been found on any dinosaur,” Dr. Bell said.

“The specimen is a superbly preserved skeleton that was found lying on its back, entirely covered in fossilized skin.”

“The same specimen made news in 2021, when paleontologists revealed the appearance of its cloaca — the common opening for the genitals and digestive tract, which it shares with birds and reptiles.”

“Using the laser-stimulated fluorescence technique, we identified distinctive scales that surrounded a long umbilical scar in the Psittacosaurus specimen, similar to certain living lizards and crocodiles,” said Dr. Michael Pittman, a researcher at the Chinese University of Hong Kong.

“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.”

The findings appear in the journal BMC Biology.


Phil R. Bell et al, Oldest preserved umbilical scar reveals dinosaurs had 'belly buttons', BMC Biology (2022). DOI: 10.1186/s12915-022-01329-9

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