National Geographic

All Hail the “Gore King”

Tyrannosaurus rex truly was the king of the tyrant dinosaurs. No other member of the carnivorous family was as large or looked quite so menacing. But where did this apex of Cretaceous rapacity come from? For decades, the origins of our cherished Tyrannosaurus seemed to lay in the north – a culmination of an evolutionary trend within tyrannosaurs towards predators with deep, wide, bone-crushing skulls. But a newly-named, roughly 80 million year old tyrannosaur from southern Utah complicates this transitional tale and hints at a new origin for one of the most fearsome carnivores ever to have walked the Earth.

Skeletal reconstructions of Lythronax, the yellow bones at top showing known elements. Art by Scott Hartman, courtesy Mark Loewen.

Skeletal reconstructions of Lythronax, the yellow bones at top showing known elements. Art by Scott Hartman, courtesy Mark Loewen.

Named Lythronax argestes in a PLoS One study by Natural History Museum of Utah paleontologist Mark Loewen and colleagues, the 23 foot long tyrannosaur lived about 12 million years before Tyrannosaurus and trod a part of southern Utah that laid near the coast of a long-lost subcontinent called Laramidia. The pieces of skull and postcrania were discovered within the vast wilderness of Grand Staircase-Escalante National Monument. Yet, despite the distance in space and time, the skull of Lythronax holds a remarkable resemblance to Tyrannosaurus.

Rather than having a narrow, streamlined skull typical of later tyrannosaurid dinosaurs found further to the north – such as Gorgosaurus and DaspletosaurusLythronax shared with Tyrannosaurus a skull that widened towards the back, giving these carnivores extra room for powerful jaw muscles and having the added benefit of situating their eyes to the side far enough to allow for binocular vision. Lythronax was one of the few predatory dinosaurs that could have stared you down.

But it would be a mistake to call Lythronax an ancestor of Tyrannosaurus. As study co-author and NHMU paleontology curator Randall Irmis commented at a press conference about the dinosaur this morning, Lythronax was more of a “great uncle” to Tyrannosaurus than an ancestor. Loewen put it another way – the ancestor of Tyrannosaurus is also the ancestor of Lythronax, meaning that these two predators represent close lineages that split from an even earlier common ancestor. This is why Lythrnoax is so strange. The 80 million year old dinosaur was among the first of the famous tyrannosaurid group, yet it looks very much like one of the very last of the tyrannosaurids. This suggests that are even more tyrannosaurs waiting to be found.

An evolutionary tree showing the relationship of Lythronax to other tyrannosaurs. Courtesy Mark Loewen.

An evolutionary tree showing the relationship of Lythronax to other tyrannosaurs. Courtesy Mark Loewen.

In the new evolutionary scenario suggested by Loewen and colleagues, Lythronax and two other recently-named southern forms – Teratophoneus and Bistahieversor – formed a group of relatively short-snouted, deep-skulled tyrannosaurids that closely resembled the later Tyrannosaurus. Meanwhile, between 80 and 74 million years ago, a different lineage of narrow, shallow-skulled tyrannosaurids such as Gorgosaurus roamed the habitats in the northern stretches of the same continent. The fact that Lythronax represents the wide, deep-skulled form at 80 million years ago suggests that the two tyrannosaur lines split even earlier in time. Loewen and coauthors expect that the bones documenting this divergence are waiting to be found in rocks about 90 to 82 million years old.

A restoration of Lythronax by Lukas Panzarin, courtesy Mark Loewen.

A restoration of Lythronax by Lukas Panzarin, courtesy Mark Loewen.

Sea level changes and the rise of mountains might have spurred the split. Between 100 and 95 million years ago a warm, shallow sea spread over ancient North America, dividing the subcontinent of Laramidia to the west from Appalachia to the east. The great mountains of the west were also being pushed up during this time, creating further barriers on the landmass. All of this created virtual islands on the continent – pockets where dinosaurs evolved in different ways in their isolation. Lythrnoax hints that tyrannosaurids evolved as a result of all these environmental changes, undergoing an evolutionary radiation that eventually spread to Asia when sea levels later began to fall.

The task ahead of paleontologists is to find these dinosaurs and track their evolution and movements throughout Laramidia. There is much more to the story than anyone presently knows. And researchers could certainly use more of Lythronax. While enough elements of the dinosaur’s skull and postcrania were uncovered to identify Lythronax as new to science, most of the tyrannosaur’s skeleton has not been found yet.

A sculpted restoration of Lythronax by Gary Staab, image courtesy Mark Loewen.

A sculpted restoration of Lythronax by Gary Staab, image courtesy Mark Loewen.

It’s not surprising that Lythronax is an elusive dinosaur. Not only were tyrannosaurs usually rare in the habitats they haunted, but dinosaurs in the Wahweap Formation are much harder to find than the abundant skeletons in the younger, overlying Kaiparowits Formation. Nevertheless, as Bureau of Land Management paleontologist Alan Titus said at this morning’s press conference, fossil experts have only searched about ten percent of the fossil-bearing rocks of Grand Staircase-Escalante National Monument. Decades of trodding over badlands in search of dinosaurs lay ahead, and who knows what the next field season might yield?

Paleontologists will need all the bits and pieces they can find. A dinosaur skeleton is a time capsule that speaks to ancient life and the grandeur of transmutation. Lythronax is more than another fearsome tyrannosaur. The carnivore is another piece in an evolutionary epic that is just coming into focus.

[Full disclosure: I am a paleontology volunteer at the Natural History Museum of Utah, although I had no involvement in the fieldwork, preparation, or study of this dinosaur.]

Reference:

Loewen, M., Irmis, R., Sertich, J., Currie, P., Sampson, S. 2013. Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans. PLoS ONE 8, 11: e79420. doi:10.1371/journal.pone.0079420

There are 16 Comments. Add Yours.

  1. Jamotorpal
    November 6, 2013

    Where is the dinosaur going in the first picture…?

  2. Zach Miller
    November 6, 2013

    A wonderful new tyrannosaur! Any chance there’s a larger version of the banner image? That would make a pretty kick-ass desktop background.

  3. Scott Bickard
    November 6, 2013

    Nice article!

  4. J.L.
    November 7, 2013

    “No other member of the carnivorous family was as large or looked quite so menacing”. At least change “or” to “and” so you’ll have a subjective defence, because Spinosaurus is the largest carnivorous dinosaur found.

  5. lion
    November 7, 2013
  6. 220mya
    November 7, 2013

    Its going towards the mosasaur carcass in the distance!

  7. Shea
    November 7, 2013

    Nice article, Brian! Yet another awesome creature. I really am enjoying all the PloS One and open source publishing that is going on…hopefully a trend that will continue to strengthen!

  8. Jon
    November 7, 2013

    Why do all these super predators have short arms? Being that tall, the arms would be 100% useless and serve no evolutionary purpose. It couldn’t grab anything and hold onto it while it killed it, or hold the meat while it chews a bite.
    When you stop and think about it, those arms are ridiculous. There is just no possible way that an animal would evolve to grow that large and have two useless arms. What other dinosaurs have useless limbs growing off of it, and why would these dinosaurs evolve to have useless limbs? It just seems like some paleontologists thought it would make the T-Rex and its comrades look cool so they put it up on two feet and gave it small useless arms.

  9. Zach Miller
    November 8, 2013

    Compared to abelisaurs, tyrannosaurs have LONG arms!

  10. Deb
    November 8, 2013

    @Jon: You ask the exact right question about the “useless forelimbs”: “Why would these dinosaurs evolve to have useless limbs?” There must have been a niche available at this point in time for a large predator that used only its massive, large-toothed head to kill and consume its prey, or to scavenge, maybe with an assist from its massive feet. T rex had what was needed to survive/thrive, and these things worked for its time/place/ecosystem. It’s interesting to speculate on what conditions might have worked on them to evolve a longer and/or more useful reach.

  11. steve cohen
    November 11, 2013

    Jack Horner once said the short arms were perfect for scratching its belly after eating.

  12. cam
    November 13, 2013

    @jon, have yp seen modern crocodillians? Their arms are used exclusively for locomotion, as they rely on their powerful jaws for hunting. The tyranosaurs essentially have vestigial forelimbs, most likely used for mating, the same way whales use their pectoral fins

  13. Mona Albano
    December 4, 2013

    It’s a reasonable hypothesis that the arms helped the dinosaur to rise to its feet efficiently, by bracing it so it didn’t slide forward instead.

  14. Marcos K. Pinheiro
    January 7, 2014

    I agree with that hypothesis too, Mona Albano! Even my father, hahaha. Studies of reconstructing the muscles of those small arms were made (like one from Smith and Carpenter) have shown that those were really powerful! Take Sue for instance, if I was holding on her left arm, she’ll take me up without breaking her arm! Because I don’t weigh as much as 190 or 200 kg (according to some studies)! Those arms could’ve been used for mating too! AND, in WWD 3D encyclopedia by Steve Brusatte, he tells that the juvenile tyrannosaurids had longer and stronger arms, longer leg bones and blade-like teeth not ready to crush but to cut or slice flesh! To me make sense, the youngsters hunts fast and hard catching prey like dromaeosaurs, baby dinosaurs and even ornithomimids (if they catch one)! Those arms certainly were needed! Who knows if those arms were less feathered or not when young and as they grew those arms became feathered (with quills like the cassowary or attractive feathers liek the ostricth) or not?! Who knows if that adults really dared to use those arms and la their chest on top of the back of ceratopsians or hadrosaurs and then bitedown crushing the back or neck vertebrae, like the ambushing Jaguar (stong jaws that can even crush small turtle shells and human skulls and is heavier and more robust than leopards)?! But I agree that those arms weren’t useless!

    When I read this news I became surprised and very happy! We have a new cool member in Tyrannosauridae! And yet, new and more complete specimen of Teratophoneus was revealed to public too! Keep finding more of them and I wish someone of some expedition to find a “mummy” tyrannosaurid!

  15. Marcos K. Pinheiro
    March 18, 2014

    In some websites about this news, it says that Tarbosaurus too had a wide rear at the back of its huge skull, despite being slender than the T. rex’s! Still, I wonder about that 2007 study about its braincase results. It resulted that specimen had keen senses of smell and hearing but its vision or eyesight was poor. But I doubt that, if that Tarbosaurus had blindness? And if the adult Tarbosauruses had wide rear at the back of their skulls, they probably had binocular vision at some degree! To me it’ll make sense even more that Tarbo is close cousin with T. rex and Lythronax and could hunt with its sharp vision against Saurolophus (by ambushing it), Therizinosaurus and Tarchia (a fight between life and death). Anybody else think of this matter?

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