A Tyrannosaurus with THREE fingers?

My wife merely considers it a quirk a living with a paleontologically-oriented husband; whenever I feel like my efforts are futile, I hold my hands up against my chest with two fingers extended to represent the "useless forelimbs" of Tyrannosaurus. The evolutionary narrative of how Tyrannosaurus came to have such a massive head and such small limbs is a familiar one, physical constraints determining that the development of a large, terrible head would cause a reduction in limb size. This doesn't make the relatively minuscule limbs of Tyrannosaurus and its close relatives any less puzzling, however, and over the years numerous explanations (from holding on to mates to pushing the dinosaur off the ground when it was getting up from a nap) have been proposed in order to find some sort of function for the arms.

Part of the problem surrounding the arms of Tyrannosaurus was that no one knew for sure what they looked like until the discovery of lower limb bones in 1989. The initial discoveries, two skeletons found by Barnum Brown at the beginning of the 20th century, did not preserve the radius, ulna, metacarpals, or digits, and so some of the earliest reconstructions envisioned Tyrannosaurus with three fingers like the Jurassic Allosaurus. Relatives of Tyrannosaurus discovered just before or some time after Brown's significant finds (like Lawrence Lambe's discovery of Gorgosaurus) showed that tyrannosaurids possessed two fingers, not three. It should be noted that such scientific considerations didn't hold back Walt Disney when he was planning his epic film Fantasia, though; scientific understanding to the contrary, Disney's Tyrannosaurus had three fingers as he felt it looked better that way.

• A proposed mount for Barnum Brown's two Tyrannosaurus specimens (note the three-fingered hands). This idea was scrapped, and fear that housing both skeletons in one place would be dangerous if the museum was bombed during WWII (such an event destroying the only known skeleton of Spinosaurus at the time in Germany), led the museum to sell the less complete holotype to the Carnegie Museum in Pittsburgh, PA in 1941. Such a design will get a second lease on life starting this November, however; the new "Dinosaurs in Their Time" exhibit at the Carnegie will feature two Tyrannosaurus mounts squabbling over a carcass just like Osborn's idea, but with updated postures. Photo from Osborn, H.F., and Brown, B. "Tyrannosaurus : restoration and model of the skeleton." Bulletin of the AMNH ; v. 32, article 4.

Although I used terms like "useless" and "minuscule" to initially open this post, I used such language to contrast the popular understanding of the small arms of Tyrannosaurus with the scientific one. Limb strength is not merely all about size, and it appears that the forelimbs of Tyrannosaurus were very powerful. Indeed, some have likened them to meat-hooks, holding prey in place while the heavy head did most of the damage. Hypothetically it makes sense, but being that we cannot watch a Tyrannosaurus take down a hadrosaur or ceratopsian today it's difficult to imagine how the arms would have worked during an attack (if they were used at all).

Such controversy about what the arms were used "for" pales in comparison to news that Tyrannosaurus might have had three fingers after all. An abstract from this year's upcoming GSA meeting entitled "ANATOMY AND FUNCTION OF DIGIT III OF THE TYRANNOSAURUS REX MANUS" is causing a considerable stir, but I have to wonder if this find is significant for all tyrannosaurids. The section of the abstract concerning the fingers is as follows;

In northeastern Montana's Hell Creek formation, a Tyrannosaurus rex has been unearthed with three fingers on each hand. The existence of digit III had previously surmised because of a joint surface on the ulnar aspect of the base of metacarpal II. The phalanges of digit III and its metacarpal are fused into one slightly flexed unit, with proximal and radial joint surfaces. Sturdy dorsoulnar and volar muscle attachments are present on the base of the metacarpal.

It's not much to go on, but I'm sure we'll be hearing a lot about it after the presentation is given in a few weeks. Still, if this particular specimen of Tyrannosaurus has three fingers, does that mean that all of them did? Not necessarily. Of course, a reexamination of existing Tyrannosaurus, Albertosaurus, Gorgosaurus, Daspletosaurus, Tarbosaurus, etc. material would have to be undertaken to determine if there were osteological signs of a third digit present, but it seems that for tyrannosaurids the number of digits has been well established as two. I have a cast of one specimen with a very small part of the third digit and have seen similar structures in tyrannosaur skeletons on display at the AMNH, and so I have to wonder if the hypothetical third digit is really an atavism.

Perhaps I'm wrong and there is some sweeping taphonomic feature that biases tyrannosaurid skeletons from preserving the third digit and associated bones in the wrist, but it seems more likely to me (at least from my position of relative ignorance being that I haven't read the paper or seen the presentation) that the third finger is something that was expressed as a result of development, not a feature common to tyrannosaurids. If correct, this would show that the number of digits on the hand of Tyrannosaurus was constrained by developmental factors, but every now and then a three-fingered specimen could show up again. This would make this fossil even more valuable as it would be fossil evidence of an atavism, as Tyrannosaurus and its kin were likely derived from three-fingered ancestors.

Hopefully someone more qualified to judge the significance of the study will be present at GSA and will report back on it, as I have the feeling that the findings could cause a good deal of misunderstanding. Just because this one specimen has a preserved third digit does not necessarily mean its the rule for Tyrannosaurus or its relatives, and careful study will be required to determine whether the long-lost third finger is a throwback or a genuine shared character of tyrannosaurs. Either way, it's definitely exciting news, and hopefully we'll hear more about it in the near future.

[Hat-tip to When Pigs Fly Returns]