Dinosaurs indeed had belly buttons

The oldest belly button ever found in reptiles and mammals.


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Dinosaurs, unlike humans, had no umbilical cord because they deposited eggs. Dinosaur yolk sacs were connected to the body by a slit-like aperture, also discovered in other egg-laying terrestrial animals. This is the opening that is sealed up about when the animal hatches, leaving a long umbilical scar.

While the egg-laying nature of dinosaurs predicts a long belly button scar, a new study is the first to support this hypothesis with fossil evidence.

Using a high-tech laser imaging technology, scientists from The Chinese University of Hong Kong (CUHK) and worldwide have revealed the finest details of a 125-million-year-old dinosaur fossil found in China 20 years ago. Their analysis confirmed that the dinosaurs indeed had belly buttons. What’s more, they even set a new record for the oldest belly button ever found in reptiles and mammals.

Early Cretaceous horned dinosaur Psittacosaurus
The Early Cretaceous horned dinosaur Psittacosaurus was discovered in northeast China and is a distant relative of Triceratops. Image credit: Julius T. Csotonyi.

The team applied the Laser-Stimulated Fluorescence (LSF) technique to a fossilized skin specimen of Psittacosaurus. This two-meter-long and two-legged plant-eater lived in China during the Cretaceous period. They identified distinctive scales surrounding a long umbilical scar in the Psittacosaurus specimen, similar to certain living lizards and crocodiles.

Dr. Michael Pittman, Assistant Professor of CUHK’s School of Life Sciences and joint-corresponding author of the study, said, “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 due to its exceptional state of preservation.”

“While this beautiful specimen has been a sensation since it was described in 2002, we have been able to study it in a new light using novel laser fluorescence imaging, which reveals the scales in incredible detail.”

Laser-stimulated fluorescence
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). Image credit: Bell et al. 2022.

Dr. Phil R. Bell from the University of New England in Armidale, Australia, the study’s lead and joint-corresponding author, commented“This Psittacosaurus specimen is probably the most important fossil we have for studying dinosaur skin. But it continues to yield surprises that we can bring to life with new technology like laser imaging.”

The research team includes Dr. Christophe Hendrickx of the Unidad Ejecutora Lillo in San Miguel de Tucumán, Argentina, and Mr. Thomas G. Kaye of the Foundation for Scientific Advancement in Arizona, USA.

Journal Reference:

  1. Bell, P.R., Hendrickx, C., Pittman, M. et al. Oldest preserved umbilical scar reveals dinosaurs had ‘belly buttons.’ BMC Biol 20, 132 (2022). DOI: 10.1186/s12915-022-01329-9


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