Paleontology Newsflash: Tyrannomimus fukuiensis

Sure, this thing looks exactly like Tyrannosaurus rex, but don’t be fooled: it’s only a tyrannosaur mimic! It’s not closely related, it’s not a new rival, it’s not shedding new light on the media’s . . . er, everyone’s favorite dinosaur. It’s just a clever impersonation, like the paleontological equivalent of Rich Little. Totally not a clone. Nope. But anyone could be forgiven for mistaking it for T. rex, absolutely!

Japan scores another new dinosaur, and this one has been a long time coming. For years, the prolific dinosaur sites in Fukui prefecture have produced skeletons of ostrich-like ornithomimosaur dinosaurs, and scientists have finally accumulated enough data from these scattered remains to establish a new taxon based upon them. Here’s the lowdown on what they’ve discovered about Tyrannomimus fukuiensis:

  • This particular species of ornithomimosaur harks from the Early Cretaceous and therefore represents the earliest known definitive member of the group’s two main branches, though a surprising conclusion from this study suggests it’s not the absolute oldest member (more on that later). It belongs to the deinocheirid group of ornithomimosaurs, making it a forerunner of the bizarre Mongolian dinosaur Deinocheirus, which looks like a duckbill dinosaur attempting to dress like Spinosaurus for Halloween and failing miserably. Tyrannomimus, though, as far as the current data goes does not in itself look much like Deinocheirus. Instead, in general it seems built more like the fleet-footed ornithomimid archetype, with some interesting details that hint at the weirdness of its later relatives.
  • Despite the jocularity of this post’s picture caption, the scientists describing this animal dubbed it “Tyrannomimus” with good reason. Most other members of the ornithomimosaur—literally “bird mimic reptiles”—group tack the suffix mimus onto some sort of bird name: Pelicanimimus, Gallimimus (chicken mimic), or even mythical examples like Garudimimus. This animal’s name, however, labels it as a tyrannosaur mimic in a direct inversion of the tradition, in reference to certain features of the ilium hip bone which had previously only occurred within the tyrannosauroid group. Specifically, a ridge previously found mainly in tyrannosaurs divides Tyrannomimus’ ilium into two sections, marking attachment points for large leg muscles. Other features throughout Tyrannomimus’ skeleton, including the overall shape of the ilium and its attachments to other hip bones, clearly mark it as an ornithomimosaur–it just happens to have a few tyrannosauroid design features.
  • This isn’t the first time tyrannosaur features have turned up among the ostrich mimic dinosaurs. They and some other theropods built for running share a toe bone design, called an arctometatarsus, which clusters the upper toe bones together for added strength. This design foreshadows the full fusion version found in birds, called a tarsometatarsus–compare the Gastornis skeleton with the tyrannosaurs in the Stewart Museum for a good look at how this works. In the mid-90s, Thomas Holtz, a tyrannosaur expert, proposed that this feature indicated a common evolutionary origin for all dinosaurs sporting this feature. Before the end of the decade, however, multiple lines of evidence showed that the arctometatarsus by itself does not mark evolutionary relationships. Instead, it popped up independently among various groups when they adopted a high-speed running style. Note that among the giant Late Cretaceous tyrannosaurids this feature appears to have proven equally useful for dealing with extreme mass. Strange as it may seem, most tyrannosauroids sported light skeletons built for speed, much like ornithomimosaurs. We’re just more familiar with the exceptional giants like T. rex. Tyrannomimus seems to provide extra support for early members of the group closely resembling tyrannosauroids through convergent evolution, not so much through shared ancestry.
  • On that note, though Tyrannomimus represents the earliest definite deinocheirid, an animal from the Late Jurassic of Europe might turn out to claim that title if we get more fossils of it. Previous studies have found that this rival for the title, Aviatyrannis, belongs to the tyrannosauroid group, due in part to that vertical ridge on the ilum. The Tyrannomimus study presents a case which throws that hypothesis into doubt and places Aviatyrannis within the ornithomimosaur group. If future discoveries bear this hypothesis out, it would fill in a huge gap between the hypothesized Middle Jurassic origin for ornithomimosaurs and the currently oldest known members from the Early Cretaceous. Such a discovery could in turn answer such questions as why ornithomimosaurs developed as runners, why they lost their teeth, and what their diet really was. It could likewise help fill in a gigantic hole in our knowledge regarding Middle Jurassic theropods and especially their evolutionary history.
  • One other factoid on the Tyrannomimus name: some have joked that since so many dinosaurs found in this region bear its name, Fukui, with a suffix tacked on (e.g. Fukuiraptor, Fukuititan, Fukuivenator), future discoveries would follow suit, leading to such unoriginal monikers as Fukuipelta, Fukuiceratops, Fukuihadros, and even Fukuimimus! Any one of those possibilities remains in play (unfortunately), but it’s nice to get a break in the pattern . . . even though the study authors did sneak Fukui into the species name anyway. (I totally understand the patriotic enthusiasm for the Fukui region, don’t get me wrong, but would it KILL anyone to go for names with a little more Japanese panache, like maybe Kaijutitan, Oniraptor, Obakesaurus, or Ishiryu? At least Kamuysaurus scratches that itch, and Tyrannomimus has some style.)
  • Some features of the holotype–the collection of specimens which forms the basis of the taxon–suggest that it came from a juvenile animal. Hattori and colleagues did consider whether that might make it a young Fukuiraptor, but found that the shapes of the bones just didn’t fit with the diagnosis for Fukuiraptor.
  • Interestingly enough, dinosaurs and even cockroaches from Fukui more closely resemble those found from coeval Mongolia than Northeastern China. This suggests some interesting geography in the area at the time preventing animals from moving between China and Japan, but allowing such faunal interchange between Mongolia and Japan. This could make Early Cretaceous Mongolia a hypothetical template for Fukui’s ecology, and vice versa, lending scientists more ideas to test and fleshing out our knowledge of both regions more effectively. Such a correlation could also help geologists refine the timing of Japan’s separation from the Chinese mainland. Tyrannomimus and its Mongolian counterpart Harpymimus add another line of evidence supporting this link.

In short (too late?), we now know that a small, emu-like dinosaur ran around in Early Cretaceous Japan, and it shared design features for its speediness with tyrannosaurs despite their relatively distant evolutionary relationship. Whether it would make good yaki-tori or karaage is still anyone’s guess.

* Hattori, S., Shibata, M., Kawabe, S. et al. New theropod dinosaur from the Lower Cretaceous of Japan provides critical implications for the early evolution of ornithomimosaurs. Sci Rep 13, 13842 (2023).
* Holtz, Thomas R. “The phylogenetic position of the Tyrannosauridae: implications for theropod systematics.” Journal of Paleontology 68.5 (1994): 1100-1117.
* Holtz, T. R. (1998). A new phylogeny of the carnivorous dinosaurs. GAIA: revista de geociências, (15), 5.
* Rauhut, O. W. (2003). A tyrannosauroid dinosaur from the Upper Jurassic of Portugal. Palaeontology, 46(5), 903-910.