Troodon forelimb reconstruction and motion range (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

Michael Serio, Ashley Heers, David Varricchio & Han Hu (2026)
First forelimb reconstruction and range of motion assessment of the Late Cretaceous dinosaur Troodon formosus.
PeerJ 14:e20987
doi: https://doi.org/10.7717/peerj.20987
https://peerj.com/articles/20987/



Troodon formosus (T. formosus) is a theropod dinosaur from the Late Cretaceous of North America. T. formosus, like many theropods, are speculated to have used their forelimbs to hunt, and the complexity of their preserved egg clutches suggests T. formosus may have also used its forelimbs to move its eggs. Understanding the morphology and range of movement of T. formosus’s forelimb could help shed light onto these hypotheses. However, no complete forelimb material has yet been found for T. formosus, and a 3D reconstruction and range of motion (ROM) estimate has not been attempted. This study aims to address this gap by leveraging digital modelling technology to create the first forelimb reconstruction and ROM for T. formosus. Surface scans from multiple T. formosus fossils housed in the Museum of the Rockies (Bozeman, Montana) were digitally combined to reconstruct a nearly complete forelimb. Digital articulations based on this assembled model were compared with physical ROM using 3D printed copies. Results show higher ranges of flexion than extension in T. formosus’s joints, consistent with closely related species. However, T. formosus shows higher manual extension than close relatives. The humerus also shows anatomy convergent with more basal theropod species. These differences may imply a divergent morphology and function of the manus, as well as a deviation from avian ancestor forelimb morphology. ROM results cannot confirm whether T. formosus was able to grasp objects single-handed, but two-handed apprehension of objects, including eggs, remains feasible.

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[Tim Williams]

So far this year there've been two starkly different interpretations of _Troodon_'s predatory abilities.  Fan et al. (doi.org/10.1002/brv.70145) propose that _Troodon, was probably omnivorous, and consumed plants and small animals - with "mammals and baby dinosaurs (hatched or unhatched)" being favored prey.

At the other end of the spectrum, this study by Serio et al. proposes that _Troodon_ was "using its forelimbs to grasp and hold onto potentially struggling or larger prey items, perhaps even making use of its forelimbs as primary hunting tools, similar to megaraptorids".  I'm highly skeptical of this hypothesis that _Troodon_ (or any troodontid) was capable of grasping and holding large prey with its forelimbs.

There is potential accord in Serio &c's suggestion that _Troodon was capable of "two-handed apprehension of objects, including eggs".  The authors had in mind _Troodon_ manipulating and shifting its own eggs.  But this action could conceivably be applied to _Troodon_ as a predator, using its forelimbs to run off with eggs from other nests (assuming the eggs were of the right size to be grasped).

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[Sean McKelvey]

The comparison to Megaraptorids is confusing to me because the arms of Troodontids don't appear to be structured similarly?

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[Jaime Headden]

I would tend to agree more with a composite than a "singular anatomical system" approach, when it comes to assessing predatory capability. There's no doubt troodontids were *capable* of predation, and a lot of literature still seems to argue that ambi-manual "clapping" a la Ostrom's "path to flight" theory is a useful mechanism to explain these large limbs. Seeing how oviraptorids have the same forelimbs and are now more or less considered almost exclusively herbivores, there's a lot of speculation that forelimb functionality is *relevant*. But it must have been useful, if not for climbing, digging, brooding, nesting, or perhaps even for defense (using claws will inflict a great deal more damage than engorged knuckles or wrists as in screamers and other galloanserans), than for something like ... prehension.

Multivariate approaches tend to have a better picture, but diet is chiefly assessed through principle processing, not necessarily acquisition method, and this seems to lean closer to the dental assessment of Fan et al., which strongly favors dental and cranial anatomy than single-unit systems, and that data suggests something not unlike an herbivore, though I'm cautious about this given that troodontids don't have many other features assessing them as even plant-dominant herbivores other than denticle morphology and isotopes (which, as I would suggest, are more compelling again than single-unit pronouncements).

Could troodontids (in general, not a specific species) be omnivores, catching prey and eating tough-fibrous plant matter alike? There's hardly any conclusive evidence for the lack of this faculative omnivory, so I fear this new paper does little to deflect from that view.

And really, we need to either reassign the holotype of Troodon formosus or please, please: stop calling it "Troodon formosus."

Cheers,

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Jaime A. Headden

The Bite Stuff: http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)

[Tim Williams]

Jaime Headden <jaimeh...@gmail.com> wrote:

>Seeing how oviraptorids have the same forelimbs and are now more or less considered almost exclusively

> herbivores, there's a lot of speculation that forelimb functionality is *relevant*.

Yes, exactly.  Oviraptorid forelimbs were strong and tipped with sharp, vicious-looking claws.  But their owners were herbivores.  This makes me think that their powerful, feathered forelimbs were used for intraspecific combat and display.  Same for troodontids, and other early-diverging pennaraptorans.  (And perhaps defense too, as a last resort.)

Screamers (which you mention) and the extinct _Xenicibis_ may be the best analogs we have for what the forelimbs were used for.

> But it must have been useful, if not for climbing, digging, brooding, nesting, or perhaps even for defense

> (using claws will inflict a great deal more damage than engorged knuckles or wrists as in screamers and other galloanserans), than for

> something like ... prehension.

I'm not a fan of the climbing hypothesis.  The theropod forelimb was so deficient in prehension, especially one-handed grasping.  How did they grab branches?  I know there is a view that some small theropods could climb trunks, but where did they put their legs?  Did they tuck them under the body like treecreepers?  It's not like they could stick their legs out the side, in the way scansorial mammals do.

> And really, we need to either reassign the holotype of Troodon formosus or please, please: stop

> calling it "Troodon formosus."

Hopefully this issue will be resolved by the designation of a neotype (in the works ~ Varricchio et al., 2025).