A fossil found in China is in such good shape that scientists identified its belly button. The fossil belongs to a Psittacosaurus, a bipedal dinosaur that lived between 100 and 120 million years ago! Now wait, you may think, don't belly buttons belong to placental mammals? Yes, but they also happen in egg-laying species. The embryos of birds and retiles have a cord attached to the yolk sack within the egg, which can leave a scar. Most modern egg-laying animals lose the scar soon after hatching. However a few species, such as alligators, keep their "umbilical scar" throughout their lives.
The Psittacosaurus fossil was unearthed in 2002. A new laser imaging technique called Laser-Stimulated Fluorescence (LSF) was used to map the fine details of its skin, and revealed the umbilical scar. It also revealed scales, wrinkles, and skin patterns. With this technology, we can see dinosaurs as they existed in life. This is the oldest example of a belly button in a non-avian dinosaur more than 66 million years old. Read more about this research at Smithsonian.
(Image credit: Jagged Fang Designs)
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