Meet T. rex’s Surprising Cousin: Slim, Snouted, and Overlooked for 50 Years

A rediscovered fossil from Mongolia changes what we thought we knew about tyrannosaur evolution

This weekend, I found myself sitting on a picnic blanket surrounded by half-eaten cupcakes, inflatable dinosaurs, and at least a dozen five-year-olds hyped up on sugar and Dinosaur facts. 

It was a birthday party (dinosaur-themed, of course), and I had just started chatting with a few of the kids about their favorite prehistoric creatures.

“Did you know that T. rex had really short arms but the strongest bite of any land animal ever?” one of them asked, eyes wide.

I nodded and threw in a few fun facts of my own, trying not to overwhelm them with too much science. But then, mid-sentence, one of the moms leaned over and announced, “She’s actually a paleontologist.”

There was a pause. Then an audible gasp.

The kids turned slowly toward my son, one of them exclaiming, “You never told us?!” The look on my son’s face was pure mischief, like he’d been keeping a secret weapon in his back pocket. He just shrugged, barely suppressing a grin.

I promised them then and there that I’d inform them of any new dinosaur discoveries, especially anything related to T. rex. So, to honor my word, here’s one for the books.

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A study published this month in Nature reveals the discovery of a new dinosaur species from Mongolia that rewrites what we thought we knew about the ancestry of Tyrannosaurus rex. The species is called Khankhuuluu mongoliensis, which translates loosely to “dragon prince of Mongolia,” and it’s already shaking up the tyrannosaur family tree.

This isn’t a flashy new fossil pulled fresh from the ground. It’s a case of rediscovery: two partial skeletons, dug up in the 1970s, were sitting in drawers at the Institute of Paleontology in Ulaanbaatar, labeled as belonging to another species, Alectrosaurus. But when Canadian PhD student Jared Voris got a chance to examine them, he noticed the bones didn’t quite fit that mold.

“It is quite possible that discoveries like this are sitting in other museums that just have not been recognized,” said co-author Dr. Darla Zelenitsky, a paleontologist at the University of Calgary.

Turns out, the bones belonged to an entirely new species; one that provides a crucial piece of the puzzle between smaller, early tyrannosaur relatives and the massive apex predators that would come to dominate the Late Cretaceous (i.e. T. rex).

To get to the bottom of Khankhuuluu’s identity, researchers used a mix of old-school anatomy and high-tech phylogenetic analysis. They compared the fossils’ features, like skull shape, leg proportions, and spinal structure, to dozens of known tyrannosaur species. Then they ran models to figure out where Khankhuuluu fits in the evolutionary tree.

The result? Khankhuuluu is the closest known cousin to the group of “true” tyrannosaurs, formally called Eutyrannosauria, without being one of them. Think of it as the evolutionary equivalent of standing on the doorstep of the T. rex family home.

At about four meters long and weighing 750 kilos, it wasn’t small, but it also wasn’t the kind of dinosaur that could crush a car like T. rex. More interesting, though, is its appearance. Khankhuuluu had a shallow snout, a light build, and other traits we usually associate with juvenile tyrannosaurs. Except this one was an adult.

That finding throws a wrench into long-held assumptions. For years, paleontologists thought dinosaurs with those juvenile-like features were earlier, more primitive forms. But Khankhuuluu was a fully grown adult, meaning those traits weren’t just leftovers from early evolution; they were deliberately retained. In evolutionary terms, we call this paedomorphosis, where a species keeps some of its youthful traits on purpose.

That stands in contrast to what we see in the lineage that leads to T. rex, which went the opposite direction. Those dinosaurs bulked up, developed giant heads, deeper snouts, thicker bones, and swapped their long legs for more compact, muscular builds. That’s peramorphosis: growth on overdrive.

Even more intriguing: this new study suggests that the so-called “Pinocchio rex” types (long-snouted tyrannosaurs from Asia) weren’t early offshoots like we thought. They were late bloomers in evolutionary terms, closely related to T. rex but shaped by different pressures. While T. rex became the bulldozer of the dinosaur world, the Alioramini (that’s the group “Pinocchio rex” belongs to) stayed agile and lightweight.

The bigger story here, for me, is what it says about evolution as a process. It’s not a straight line toward being big and scary. It’s more like a crowded family reunion where some cousins become bodybuilders, others stay slim runners, and a few just want to stay young forever. Nature doesn’t pick favorites; it just picks survivors that fit their niche.

It also reminds me why I love paleontology in the first place. Sometimes, the most exciting discoveries come from revisiting what we thought we already knew. These fossils sat in drawers for 50 years, mislabeled and ignored. And yet, they turned out to hold one of the keys to understanding how T. rex came to be.

That’s a message I want the cupcake-fueled five-year-olds to hear. Science isn’t just about big, flashy moments; it’s about being curious, paying attention, and knowing that every bone, every data point, every drawer might be hiding something incredible.

So yes, kids. There’s a new discovery connected to T. rex. And now you’re the first to know.

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I’m thrilled you’re here. Stay curious, and thank you for sharing this journey with me!

Best,

Sílvia P-M, PhD Climate Ages