[♪ INTRO] When you picture a fossil, the first thing that comes to mind is probably a dinosaur bone.

We’ve all seen those terrifyingly enormous skeletons that go all the way up to the museum’s ceiling. But what about their brains?

And their blood vessels, and skin, and other organs? Most of the super old fossils we’ve found are just bones because it’s a lot easier to preserve bone than soft tissue for millions of years. But we have found some fossilized soft tissue from dinosaurs, as well as other animals that lived millions of years ago, including a fossilized piece of dinosaur brain.

It’s just really rare. Before any part of an animal could start being fossilized, it first needed to, you know, die. And then, for most types of fossils, it needed to be buried.

For the most part, that didn’t happen right away. Instead, the bodies would just sit out there in the open. Perfect targets for scavengers and microbes to come pick off and decay the soft tissue, so that by the time the animal was buried, only the bones were left.

Over time, minerals would seep into where the bones were buried and harden them into the fossils we find today. Paleontologists can sometimes use these fossilized bones to indirectly study soft tissues using what are known as trace fossils: things like skin patterns left in the mud. But very, very rarely, they’ll find an animal fossil that includes actual fossilized soft tissue, because the animal was buried quickly enough for its soft tissue to fossilize before it decayed.

Some of the most important examples of this are fossils from the Cambrian Explosion, a period of time about 550 million years ago that saw one of the most drastic increases in the number of species on Earth. But a lot of those species were soft-bodied, meaning that they had no bones. So normally, we wouldn’t expect to find fossils of them.

Unfortunately for them, but lucky for us, some of these animals were buried almost instantly in events like underwater mudslides, so fossilization could start right away. Without the soft tissue fossils we’ve found from the Cambrian Explosion, there’s a lot we’d never know about one of the most exciting times in the history of life. We’ve also occasionally found soft tissue from a few other animals, including dinosaurs.

In 2005, for example, researchers at North Carolina State University claimed to have found actual blood vessels and other soft tissues preserved in a T. rex fossil, meaning that they weren’t fossilized. They’d just been kept from decaying. The team suggested that the tissues were preserved because the iron in the dinosaur’s blood turned into formaldehyde, but the findings are still disputed because direct preservation of organic matter over millions of years really shouldn’t be possible.

With or without formaldehyde. There have been other, less controversial findings, though. In late 2016, paleontologists found a 99-million-year-old dinosaur tail perfectly preserved in amber.

And we’ve found a few dinosaur fossils that contain fossilized skin and feathers, like an Ornithomimus fossil discovered in 2009. We already knew that Ornithomimus was weirdly similar to the modern ostrich, but the new fossil showed us that it also had the same arrangement of feathers on its body and bare skin on its legs, probably to help regulate its body temperature. And, yes, we’ve found fossilized dinosaur brain.

In 2016, researchers announced that they’d found a pebble-sized fossil that contained brain tissue. They don’t know exactly which dinosaur it came from, but it’s about 133 million years old. That’s the only dinosaur brain tissue we’ve ever found, but there are plenty more fossils waiting to be discovered.

So who knows what we’ll learn about the softer parts of dinosaurs someday. Thanks for watching this episode of SciShow! If you’re interested in learning more about dinosaurs and the history of life on Earth in general, you can check out our new sister show, Eons, over at youtube.com/eons. [♪ OUTRO]