A Blog by Ed Yong

Raptorex shows that T.rex body plan evolved at 100th the size

Meet Raptorex, the “king of thieves”. It’s a new species of dinosaur that looks, for all intents and purposes¸ like the mighty Tyrannosaurus rex, complete with large, powerful skull and tiny, comical forearms. But there’s one very important difference – it’s 100 times smaller. Unlike the ever-shrinking world of music players and phones, it seems that evolution crafted tyrannosaur technology with much smaller specifications before enlarging the design into the giant predators of the late Cretaceous.

Raptorex is a new species of meat-eating dinosaur, discovered in northwest China by Paul Sereno from the University of Chicago. The specimen is a young adult, but it wouldn’t have grown to more than 3 metres in length. It stood about as tall as a human, and wouldn’t have weighed much more. And yet Raptorex looked very much like a scaled-down version of its giant future relatives. All the features that made tyrannosaurs so recognisable and such efficient killers (except their enormous size) were present in this animal.

It really is a beautiful transitional fossil. As Sereno says, “Raptorex really is a pivotal moment in the history of the group where most of the biologically meaningful features of tyrannosaurs came into being, and the surprising thing is that they came into being in such a small animal.” Raptorex clearly shows that natural selection initially honed the distinct body shape of these giant predators at a 1/100th scale. This design was then scaled up with remarkably few modifications.

It had a skull that had clearly been developed into the animal’s primary weapon.  It was unusually big for its body size (40% of its torso length), it was structurally reinforced against the stresses of heavy bites, it had large places where powerful jaw-closing muscles attached and it was armed with sharp teeth.

Its limbs also had classic T.rex proportions – strong hind legs that that were fit for running, but miniscule forearms. In contrast, other early tyrannosaurids, such as Guanlong, Dilong, Eotyrannus and Stokesosaurus, looked very different with arms that were long and useful, and proportionally smaller heads (just 30% of its torso length). Only a few distinctive parts of their skeleton mark them out as early tyrannosaurids.

Raptorex brain was also large for its size. For comparison, the Jurassic predator Allosaurus had a brain that was just 60% bigger, despite having a body that was 10 times heavier! Raptorex‘s sense of smell was particularly well-developed, just as Tyrannosaurus‘s was. A scan of its skull showed a large area for its olfactory bulbs – the parts of its brain devoted to smell. These bulbs take up a full 20% of the brain’s total volume, a proportion that exceeds that of all meat-eating dinosaurs except the giant tyrannosaurs.

Despite what many newspapers will assuredly tell you, Raptorex isn’t the ancestor of Tyrannosaurus although it probably looked very much like what this hypothetical animal would have done. It’s more like an early cousin, but one that’s clearly more closely related to T.rex and its giant kin than any of the other smaller species so far discovered.

Based on his new fossil, Sereno tells a three-act story of tyrannosaur evolution. Act One was set in the Jurassic and early Cretaceous periods, with a cast that included Eotyrannus and Dilong.  Their snouts had become stronger and their jaws more powerful, but they were typical of other predators of the time. It was only during Act Two, around 125 million years ago, that this dynasty of predators started to become truly specialised, enhancing the skull, lengthening the legs, and shrinking the forearms.

All of these features were present in Raptorex, setting the stage of the final act in tyrannosaur evolution – getting really big. The lineage grew in bulk by around 100 times. By the end of the Cretaceous, the meat-eating scene in the northern continents was dominated by tyrannosaurids – predators such as Albertasaurus, Gorgosaurus, Daspletosaurus and Tarvosaurus, each weighing in at 2.5 tons or more.

It would be fascinating to see if the same story could be told for other lineages of predators, if the abelisaurids, carcharodontosaurids and spinosaurids all had their own mini-prototypes.

Reference: Science 10.1126/science.1177428

Images: Reconstruction by Todd Marshall; other images from Science/AAAS

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19 thoughts on “Raptorex shows that T.rex body plan evolved at 100th the size

  1. Very nice work, Ed! This is the kind of thing that reminds me why I love paleontology.
    Just one minor quibble. You wrote that Raptorex was “discovered in northwest China by Paul Sereno from the University of Chicago.” As Sereno says in the video interview, though, the fossil was dug up by a local person and sold to a private collector. That collector then contacted Sereno, who was able to convince the owner to donate the skeleton to science. Scary as it is to think about, Raptorex could have remained in private hands for a long time had its owner not felt as generous.

  2. I’m having trouble seeing where this “100 times smaller” figure comes from. For most comparisons, we use length. This might be comparing volume or mass, which exaggerates such differences by a power of three.

  3. To answer Nathan’s question: yes, we are using mass as a comparison. Raptorex was about 60 kg, some estimates of Tyrannosaurus mass are 6000 kg. In terms of length, Raptorex is about 1/4 or 1/5 the length of T. rex. Mass is the most important comparison here, since we are talking about the evolution of various skeletal features in relation to colossal body size.
    Steve Brusatte

  4. Nitpick: If mass is what’s being referred to by “size”, why not just change the title to say “100th the mass”? And instead of “100 times smaller”, “100 times less massive”?
    — A humble pedant for clarity and precision in scientific language

  5. Steve: I’m sorry to say that smacks of special pleading. You could say the same of almost any comparison of the size of taxa, except where you were comparing reach. By my lights, all such comparisons should be by mass, but that’s not the way it’s done, which makes it misleading in this case.

  6. “That’s not the way its done”? Hah? That IS the way its done. See this nice refereed paper in one of the biggest journals. After searching on “Size differences animals” and looking at the books in the results shows that mass is often used as the criterion.

  7. Bill Parker and Nick Gardner sent me the paper. Wonderful animal, and a real surprise, too. I’m surprised that there’s such a large morphological gap between Raptorex and Dilong/Guanlong. I wish the paper included photos of the fossil in situ, though. This was a 3D fossil, right?

  8. I wonder which/how many dino fossils have been used to identify these shared derived (ancestral modified) characters in the 3 act story of ryrannosaur evolution.

  9. I wonder which/how many reptile fossils have been used as a comparison to identify these shared derived (ancestral modified) characters in the 3 act story of tyrannosaurus evolution

  10. I wonder which/how many fossil(s) have been used as an external comparison to identify these shared derived (ancestral modified) characters in the 3 act story of tyrannosaur evolution.

  11. So what does this do to the arguement that TREX was a scavenger ? Part of the arguement was the huge olfactory bulbs and also the size of trex made him able to drive on other predators. So we now have this small one that must have hunted ?

  12. The weight of T rex is greatly exaggerated – depends on its skull, it weighed some – tonnes.
    What is more important from an evolutionary point of view is that tyrannosaurs are quite primitive theropods, they still have amphicoelous dorsals. The thing is, once opisthocoely is evolved, it never turns back to amphicoely. Something that cladistics is impossible to solve. So Torvosaurus et al were more derived theropods.
    I do not understand why people bother with questions whether T rex was a scavenger or not when it is impossible to answer…
    Peter Mihalda

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