In October 2025, Sereno and several other paleontologists published a paper on the newly discovered “mummy zone” in western North America. It’s an unusually thick section of the Lance Formation that was predisposed to pristinely preserving dinosaur bodies

Rex Junior is a mummified dinosaur skeleton. It was found at the top of the mummy zone that also produced other mummified dinosaurs from eastern Wyoming, including several Edmontosaurus from the Zerbst Ranch.

However, simply calling Rex Junior “mummified” isn’t doing the specimen justice, from Sereno’s perspective.

What immediately stood out to him was that Rex Junior was three-dimensionally preserved. None of the articulated bones are squashed, distorted, or scattered.

It was like the dinosaur went to sleep, completely undisturbed.

“It’s a particularly interesting case because (the body) does not look dried,” Sereno said. “The mummies that we described in the paper look dried, like they’d been exposed for several days or weeks before being covered by a flood. (Rex Junior) doesn’t look like that.”

Sereno noted that Rex Junior isn’t just three-dimensionally preserved – it’s perfectly three-dimensionally preserved. The articulated arm, in particular, is preserved in what he called “a lifelike position.”

This is one instance where Sereno said “lifelike” is unnatural.

“When you die as a carcass, there’s skin sagging. The hands are pulled up in a deathlike pose. The body dries out like beef jerky. I think the position of the hands and ribs is too natural. It’s preserved in a lifelike position.”

That’s exactly what Sereno speculates might have happened. Rex Junior couldn’t be buried as a carcass, because the dinosaur wasn’t dead when it was buried.

“I personally believe that it was buried alive in a flood,” he said. “I don’t think it was a carcass. This looks like it was breathing when it was suddenly buried, like it didn’t have a chance to escape.”