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Belly buttons are, typically, a human's first scar. A sign that you used to feed through an umbilical cord that connected your tummy to a placenta. But it turns out you don't have to feed from a placenta to get a similar scar. It might just be a little hard to find.

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I don’t know about you,  but I love my belly button.

It’s an adorable reminder that I spent  about nine months inside another human, leeching off their body until  I was big and strong enough to get nutrients down my own mouth hole. You know, instead of the fleshy  straw sticking out of my tummy.

But while human belly  buttons are pretty prominent, we’re not the only animals to have them. And they’ve been around a lot  longer than you might think. In fact, the oldest one we’ve  found belongs to a creature you might not associate with  belly buttons at all: a dinosaur. [♪ INTRO] Usually, your belly button  is your very first scar.

It marks the place where your umbilical  cord attached you to your parent’s placenta. The placenta is an organ that only forms  during pregnancy, and it has the very important job of keeping you alive without  you needing to eat, drink, or breathe… at least until you leave the uterus. And after you’re born, someone severs  your connection to the placenta by cutting the umbilical cord and tying off  whatever’s left sticking out of your abdomen.

Eventually, that leftover bit  of cord dries up and falls off, leaving behind a belly button. But this is basically true for any species  that grows a placenta, not just humans. Although fewer medical professionals  tend to be involved in those final steps.

Placentas are made by most mammals. And if you’ve got a furry friend at home,  you can see the remnants for yourself. In dogs, the belly button is just below  the rib cage, in the middle of the belly.

It looks like a tiny scar, a ridge  in the skin, or a whorl of fur. And if you have a cat, it’s in the same place. And you might even be able to feel  raised ridges on your kitty’s belly.

But remember, when it comes to cats and  belly rubs, please proceed with caution. As for smaller pets like rabbits,  guinea pigs, and hamsters, they may be harder to find. But I promise you the belly  button is definitely there.

Here’s the thing, though. Umbilical scars aren’t unique to mammals,  let alone mammals that grow placentas. For example, viviparous sharks are  sharks that give birth to live young.

Their babies start out attached to a yolk sac, which unlike a placenta is a  finite nutritional resource. And sometimes, that resource runs out  before the babies are ready to be born. In some species, this is solved  with fratricide and cannibalism.

But in others, like hammerheads or lemon  sharks, the babies attach themselves to their mother’s uterine wall using a  quasi-umbilical cord until they’re done cooking. The resulting belly button fades over time, but you can see one if you  find a young enough shark. And this is also true of many animals  that don’t give birth to live young.

Because if you’re a fetus chilling inside an egg, you’ve still gotta have some way to get  nutrients into your body and get waste out. Yes, I’m suggesting birds and  reptiles have belly buttons, too. Just like mammals, birds  and reptiles are amniotes, which means their embryos develop  inside an amnion, or sac of fluid.

And inside that amnion, a  developing reptile or bird embryo is connected to a yolk sac through what’s  basically a hole in their abdominal wall. Just before hatching, the embryo  slurps any remaining bit of yolk sac through that hole, which eventually  closes up and leaves a scar behind. Scientists generally don’t  call it a belly button, though.

They call it an umbilicus. And in many species, it vanishes  over the course of days or weeks. But in others, like rock pigeons and  alligators, it sticks around until adulthood.

The umbilicus might be  hidden by feathers or scales. And in some animals, the way  those scales are arranged relative to one another can clue  you into what’s hiding underneath. That's even true of egg-laying amniotes that haven’t walked the  Earth in millions of years.

That’s right. I’m talking about the non-avian dinosaurs. Back in 2022, a paper published in BMC  Biology reported evidence of a 130 million year old umbilicus, preserved in the  fossil of a dino named Psittacosaurus.

Which sounds incredible, right? Well this particular specimen, bearing  the rather unimportant-sounding moniker SMF R 4970, was way better preserved  than most dinosaur fossils. It’s so complete, scientists have  been able to study its soft tissues, including its butthole.

I’m sorry, it’s “cloaca”. And we can even tell what  color pattern its skin had. But that level of preservation  isn’t the only thing that’s important when hunting  for a fossilized belly button.

It also helps that this animal was  striking a very particular pose, giving researchers a great view of its abdomen. Still, we’re talking about  a long dead, petrified dino. It’s hard enough to find an  umbilicus on modern dinosaurs.

Better known as birds. So how did the research team do it? Well, this sample is so unique, and  so important to the fossil record, that they wanted to be as hands off as possible.

And what’s more hands off than lasers? The imaging technique this team used goes by the name laser-stimulated fluorescence. Fluorescence is a kind of glow created  by atoms after they get excited.

Basically, one or more of their  electrons has absorbed some extra energy that it doesn’t want to hold onto forever. And to shed that energy, the electron emits light. As the name “laser-stimulated fluorescence”  suggests, the research team excited the atoms on the surface of their dinosaur  fossil by hitting them with a laser beam.

By using a laser, instead of  a less directed light source, they could produce an image with finer detail. But the rest of the experimental  setup was a bit less SciFi. They captured all that fluorescence  using a regular ol’ DSLR camera, plus slapped a filter on the front  to block some light they didn’t want.

And after a little bit of processing in Photoshop, they had their image of the dino’s belly. Taking a closer look at the final product, they found a raised ridge right  where the umbilicus would be. That might not be enough evidence by  itself, so they used a similar method to take images of umbilical scars in  present-day birds and reptiles, too.

Turns out, they looked very similar  to the scar on Psittacosaurus. So it’s very likely we really are  looking at a dino belly button, here. And because this creature  lived 130 million years ago, it’s the oldest one we’ve ever seen.

But that’s not all. Since this particular Psittacosaurus  specimen was almost an adult, it could mean that just like you and I, the whole species kept their belly  buttons throughout their lives. While this doesn’t mean that every  adult dinosaur was rocking an umbilicus, it’s fun to think about what different  dinosaurs might do if they had one.

I for one, sure hope that T. rex didn’t have a belly button. Just imagine the agony they’d go  through whenever it started itching, and they couldn’t scratch it  with those tiny little arms! Thanks for watching this episode of SciShow.

If learning this fun fact has made your  brain rewind back to being a six-year-old obsessed with dinosaurs, that six-year-old brain may want to learn more from  our other channel SciShow Kids. We’ve got plenty of fun episodes about dinosaurs, so you should get started by heading  over to [♪ OUTRO]