The function of the tiny arms of T. rex?

Increasingly, Tyrannosaurs are being reconstructed with feathers:


(“It’s not your father’s tyrannosaur: Yutyrannus huali, a newly discovered ancestor of Tyrannosaurus rex, was covered from head to tail in downy feathers.” Wired Magazine.)

The puzzle of the function of the comparatively tiny forelimbs of T. rex might be solved if they were used in social display, perhaps a threat display of feathered forelimbs:

A descendant of the dinosaurs, a modern bird, does a threat display with its forelimbs:

A skeleton of a pigeon. The upper limbs look less impressive without feathers:


((((((((((((((((((((((((((((((7899English Dragoon Pigeon 1

The “tiny arms” of T. rex might have looked a bit more impressive covered in feathers too.


This chart seems to reconstruct the forelimbs of Tyrannosaurs with feathers:


29 responses to this post.

  1. Posted by evilwhitemalempire on December 14, 2013 at 7:09 am

    The puzzle of the function of the comparatively tiny forelimbs of T. rex might be solved if they were used in social display, perhaps a threat display of feathered forelimbs
    T rex was a secondarily flightless bird.

    When T rex’s [bird sized] ancestors became flightless, the forelimbs, too specialized for flying to be any good for grasping prey, became reduced in size to minimize their burden in weight and energy consumption.

    In fact nearly all the dinosaurs were neoflightless birds from back in the day when all the birds had teeth and tails like archaeopteryx.

    T rex is a Cretaceous dinosaur, by then the flying birds had forelimbs too specialized in flight to be of any other use.

    But Jurassic dinos like Allosaurus became flightless back when the forelimbs of flying birds still retained enough digits and articulation to be useful for other things.


  2. But why bother with an evolutionary middle ground that is not a dominant trait? Either wings or longer gripping arms, or strong running legs are all better solutions filling evolutionary demands that would be beneficial in making a species dominant. The tiny arms are obviously proof that God has a sense of humor.


    • If the “tiny arms” had feathers on them, in accordance with the new thinking that tends to the opinion that T. rex and its relatives were at least partly covered with feathers, they might not have seemed so tiny. Even bird wings look relatively small in skeletons too, because the feathers are missing.

      Evolution works with what it has, and the use of upper limbs for signalling is known in modern reptiles (lizards). I shall try to add a video to this post.

      Here is the abstract of a paper arguing that lizard threat displays are honest signals of endurance and presumably fighting ability. Threat displays help to avoid fights.


  3. Posted by alcestiseshtemoa on December 18, 2013 at 5:29 pm

    What is your favorite Dinosaur themed movies?


  4. Heh. In the photo, it looks like the T-Rexes are chasing Cassowaries.

    I’m a bit disappointed the director of the new Jurassic Park sequel said there won’t be any feathered dinos in the movie. Not that the film series ever really aimed for scientific accuracy, but feathers sure would make the dinos look pretty. Velociraptor’s were like big aggressive chickens! *girlish giggle*


    • I don’t think that is actually T. rex. A related species. I should clarify that.

      I have been harassed by emus and scrub turkeys. Sounds funny but not nice at all.


      • Haven’t people been eviscerated by Emus or is that an urban legend?

        Talking about dangerous Australian wildlife I once saw a cryptozoology “documentary” that claimed a species of T-Rexes was roaming out in Far North Queensland.

      • Cassowaries are very dangerous, supposedly. They are relatives of emus. Cassowaries have sharp dagger-like growths on their feet. I have read that they are used as daggers by Papuans. These flightless birds live in New Guinea and North Queensland.

        I have been to Far North Queensland to meet Negritos and measure UV
        light in the rainforest. It is strange country up there, but dinosaurs are not in the local fauna!

      • Like most crypto-beasts, the Far North Queensland T-Rex came across as an urban legend being propagated by some tourist board. Remote Northern Japan has its own menagerie of implausible creatures.

        At best, I can see the “T-Rex” sightings being a misidentified species of monitor lizards. Or an abnormally large crocodile. Apparently some crocodiles can grow up to nearly 7 meters long!

        These flightless birds live in New Guinea and North Queensland.

        I’ve seen a few Cassowaries here in zoos. Along with Tazmanian Devils (who look so adorable, even when they hiss and show off their teeth!).

        In my area, we don’t have many flightless birds; aside from the occasional turkey. My backyard trees are populated mostly by Barn Owls and Blue Jays, much to my poor cat’s dismay. Blue Jays are nature’s answer to Tweety Bird – nasty ‘lil fellows that seem to delight in torturing their predators, not to mention bullying other birds that encroach on their territory. In “Who Killed the Cock Robin?” I’m sure the Raven was framed for the murder by some Blue Jays.

        Honestly, the thought of giant predatory birds roaming around the Earth with behavior similar to modern bird species, is terrifying enough without the extra “reptile” angle added in!

  5. […] wrote previously about the puzzle of the function of the comparatively small forearms of tyrannosaurs such as T. […]


  6. Posted by Julian O'Dea on April 19, 2015 at 11:57 am

    “Qianzhousaurus (right, illustrated battling other dinosaurs) may have sported colorful feathers.”



    “Prehistoric plumage patterns: Ornithomimus dinosaur with preserved tail feathers and skin tightens linkages between dinosaurs and birds.”


  8. Posted by Julian O'Dea on November 22, 2015 at 4:50 am

    “A research team led by a University of Kansas alumnus has identified a new giant raptor, the largest specimen ever found with wing feathers.

    Named Dakotaraptor, the fossil from the Hell Creek Formation in South Dakota is thought to be about 17 feet long, making it among the largest raptors in the world.

    “This new predatory dinosaur also fills the body size gap between smaller theropods and large tyrannosaurs that lived at this time,” KU Paleontologist and co-author David Burnham said.”


  9. Posted by Jim on December 13, 2015 at 3:19 am

    As a kid I always wondered why the T. Rex had such tiny arms.


  10. […] More at my blog here. […]


  11. Posted by Julian O'Dea on June 7, 2017 at 11:58 am

    “Study casts doubt on the idea of ‘big fluffy T. rex'”


  12. Posted by Julian O'Dea on June 8, 2017 at 1:15 am

    “Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution”


  13. Posted by Julian O'Dea on December 15, 2017 at 11:30 pm

    Extinct bird that used its wings as clubs:


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