Microraptor forelimb feathering and soft tissues (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

Maxime Grosmougin, Xiaoli Wang, Xiaoting Zheng, Thomas G. Kaye, Matthieu Chotard, Luke A. Barlow, T. Alexander Deccechi, Michael B. Habib, Juned Zariwala, Scott A. Hartman, Xing Xu & Michael Pittman (2025)
Forelimb feathering, soft tissues, and skeleton of the flying dromaeosaurid Microraptor
BMC Ecology and Evolution 25: 65
doi: https://doi.org/10.1186/s12862-025-02397-5
https://link.springer.com/article/10.1186/s12862-025-02397-5



Background

Microraptor is an essential animal for understanding the evolution of flight in birds and their closest relatives. Recent studies have uncovered evidence of its powered flight potential and details of its diet and ecology. However, we are still missing a thorough description of the anatomy of Microraptor connecting feathers, soft tissues, and osteology together. Here we focus on the forelimbs of ten new Microraptor specimens from the Shandong Tianyu Museum of Nature studied under Laser-Stimulated Fluorescence. We compared our results with extensively studied existing specimens (e.g., IVPP V13352 and BMNHC PH881), other key early paravians (e.g., Anchiornis, Archaeopteryx and Confuciusornis), as well as modern birds to expand what we know about flight origins, and early diverging paravian theropods more generally.

Results

Plumage was previously only minimally known. Reconstruction of the forewings relied on brief descriptions of the primary and secondary feathers. With the new specimens studied here, we uncovered the whole shape of the wing from the tip of the digits to the proximal end of the ulna, the different layers of feathers, and the number as well as characteristics of each feather type. Skeletal features of the forelimb remain mostly unchanged from previous descriptions, but we bring new information regarding wrist bones and functional implications of humerus and radius features. The most significant advances have been recovered in preserved soft tissues including those of the shoulder, propatagium and postpatagium. In particular, the new specimens of Microraptor help us to understand the impact of the soft tissues on lift generation and cohesiveness of the forewing.

Conclusions

This study permitted us to recreate the most accurate forewing of Microraptor to date. Taken together, new information on the forelimb anatomy shows that Microraptor shares many of the forewing characteristics of early avialans and modern birds, and helps us to better understand the flight behaviour and ecology of this iconic and unique ‘four-winged’ animal along with its role in flight evolution. These results serve as a starting point to conduct more precise and integrative analyses (e.g., including hindwings and/or tail) on the locomotor behaviours of Microraptor.

[Tim Williams]

Although not specifically mentioned, this excellent paper is the final nail in the coffin for the "arboreal _Microraptor_" hypothesis.  This study finds that manual digits II and III of _Microraptor_ were bound together by soft tissue, just like _Anchiornis_ and enantiornithines.  These two digits were absorbed into the wing of _Microraptor_, with digit III almost totally embedded in the postpatagium.  So the hand of _Microraptor_ was likely functionally didactyl.  All of which made the hands useless for grasping branches.

There is some suggestion that the strong biceps muscle of _Microraptor_ might have been useful for trunk-climbing.  But the authors don't seem very enthusiastic about this idea.  Rather, this feature was likely flight-related, including rapidly changing wing positions during aerial pursuit predation.  Cool stuff.

So the idea that _Microraptor_ could clamber through tree branches like a juvenile hoatzin is absurd.

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[Stephen Poropat]

Hoatzin chicks only have two claws per hand, yet manage to climb… not saying that that undercuts your argument about Microraptor specifically, but it does render functional didactyly as a poor indicator against climbing ability.

Dr Stephen F. Poropat

Deputy Director

Western Australian Organic and Isotope Geochemistry Centre

School of Earth and Planetary Sciences

Curtin University

Bentley, Western Australia

Australia 6102

https://stephenporopat.weebly.com/

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[Tim Williams]

 Stephen Poropat <stephen...@gmail.com> wrote:

> Hoatzin chicks only have two claws per hand, yet manage to climb… not saying that that undercuts your argument about Microraptor specifically, 

> but it does render functional didactyly as a poor indicator against climbing ability.

True.  It wasn't so much the didactyly per se that rendered the _Microraptor_ hand so ineffective for grasping branches, it's that digits II and III appear to have been bound together, with the latter embeedded in the postpatagium.  In hoatzin chicks (which can't yet fly) the manual digits are free and flexible.  As Bakker put it in 'The Dinosaur Heresies': "an ugly little chick that climbs through the vegetation by grasping with its three-fingered, claw-tipped hands".

[Stephen Poropat]

Ah - thank you for clarifying.

Dr Stephen F. Poropat

Deputy Director

Western Australian Organic and Isotope Geochemistry Centre

School of Earth and Planetary Sciences

Curtin University

Bentley, Western Australia

Australia 6102

https://stephenporopat.weebly.com/

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[Michael Habib]

Thanks Tim, glad you liked it!

And yes, you are correct - we are not particularly keen on the trunk-grasping model, but we can’t exclude it, per se, so we included it for the sake of giving every idea its fair shake. This study is not really a specialized kinematic or ROM study, even though the work has implications for those sorts of questions. I rather suspect that if we really sat down and tried to work out moment arms for the biceps brachii, we would find that (like in birds) its capacity to pull the forearms/hands against a tree was quite weak, and that its primary action was adjusting span and (to a lesser degree) span wise twist of the wing.

Cheers,

—Mike Habib

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[Gregory Paul]

Hoatzins are very adept quad climbers (Abourachid A et al. 2019 Hoatzin nestling locomotion. Acquisition of quadrupedal limb coordination in birds. Sci Adv 5:0787) that do so as nestlings and fledglings (Mullner A 2004 Breeding ecology and related life-history traits of the hoatztn, Opistohcomus hoazin, in a primary rainforest habitat. PhD Dissertation, Bayerischen Julious-Maximilians-Universitat Wurzberg) until they can fly. The thumb is not flexible other than at its base. Is there actual documentation that the central finger is flexible along its joints for grasping branches during it entire climbing period? If so please provide documentation. 

GSPaul 

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[Tim Williams]

To quote from Abourachid et al. (2019)'s work on nestling hoatzins...  

"In a more complex environment with branches, hoatzin nestlings used a quadrupedal walking coordination, but due to the irregularity of the substrate, the coordination was far less regular than on our experimental substrate. The head was also used as a hook: It was flexed so that the base of the beak was positioned on the branch, the neck appearing to pull the body upward and helping the wings. The claws on the fingers were actively moved independent of the movements of the rest of the hand skeleton."

This behavior of juvenile hoatzins is completely incompatible with _Microraptor_.

In short, Abourachid et al. (2019) demonstrated that in a three-dimensional environment like tree branches, juvenile hoatzins use the beak for climbing, not just the four limbs (including the grasping, arboreal-adapted feet).  The head and neck play a major role in pulling the juvenile hoatzin upward through branches.  Strict quadrupedal climbing (four limbs only) by juvenile hoatzins was only observed by Abourachid et al. when climbing on inclined surfaces, not branches (where the head/neck acted as a 'fifth' limb).  This is the part of Abourachid et al.'s study that people forget to mention when they claim the juvenile hoatzin is an analog for "arboreality" in _Microraptor_.

Plus, I'll note these important observations from Grosmougin et al. (2025):

"...the highly recurved and relatively slender claws of _Microraptor_ do not match the rather broad and less recurved claw of juvenile hoatzin birds. This suggests that the claws of _Microraptor_ are potentially a characteristic
more of their dromaeosaurid heritage rather than an arboreality adaptation."

and

"Also, a range of motion study would give insight into the forearm positions available to _Microraptor_, helping to distinguish whether climbing motions or hanging postures were possible, while keeping in mind the potential movement restrictions necessary to limit damage to the long feathers of the forewing and hindwing."

The above is something that hoatzin chicks don't need to worry about, because they can't yet fly.

[Tim Williams]

Michael Habib <biology...@gmail.com> wrote:

> And yes, you are correct - we are not particularly keen on the trunk-grasping model, but we can’t exclude it, 

> per se, so we included it for the sake of giving every idea its fair shake. This study is not really a specialized 

> kinematic or ROM study, even though the work has implications for those sorts of questions. I rather suspect

> that if we really sat down and tried to work out moment arms for the biceps brachii, we would find that (like in 

> birds) its capacity to pull the forearms/hands against a tree was quite weak, and that its primary action was

> adjusting span and (to a lesser degree) span wise twist of the wing.

Yes, a flight-related function rather than a climbing-related function seems far more likely for _Microraptor_.  

On the topic of climbing ability, one very minor point regarding inferred climbing ability in _Velociraptor_: it's mentioned (p.18) that: "For example, FEA of a manus claw of _Velociraptor_ suggested it was able to support its own weight while climbing [126]."  Reference [126] is Manning et al. (2009 doi: 10.1002/ar.20986).  Fowler et al. (2011 doi.org/10.1371/journal.pone.0028964) were sharply critical of Manning et al. (2009).  

Manning et al. (2009) proposed that the claws of dromaeosaurids were “well-adapted for climbing as they would have been resistant to forces acting in a single (longitudinal) plane, in this case due to gravity.”  Fowler et al. disagreed, and found the conclusions of Manning et al. were due to "methodological error and misinterpretation of data".  As Fowler et al. put it: "Much of the evidence supporting a climbing function for the D-II ungual depends upon erroneous interpretation of claw curvature".  Manning et al. also failed to make any comparisons to birds of prey (a basic methodological error also made by Feduccia 1993).  Instead, Fowler et al. interpreted the hypertrophied D-II claw of dromaeosaurids as being used to maintain grip on relatively large prey (like modern eagles) - part of their Raptor Prey Restraint (RPR, or “ripper”) model.

[Gregory Paul]

There is nothing in the below that contradicts quad climbing microraptors. 

Of course baby hoatzins use their beaks to help climb. Why not, it's there. Does not mean they cannot climb using limbs alone. Maybe microraptors also used their jaws. 

The latter could also flex their claws on their fingers. Apparently there is no data showing that hoatzins can flex the central finger to actually grasp branches. May be they can, hoatzing climbing is not as well studied as one would wish. In any case quad climbing theropods are using the claws as hooks to pull themselves up with. Central finger flexion is not vital. 

Hoatzin claws are poorly developed in size and curvature. Probably because they are temporary features that will be cast off. Could be causing them to use their beaks a lot. Permanent microraptor claws are much better developed hooks that would have further boosted climbing performance. That they evolved originally for terrestrial purposes in no way prevents them from also being deployed for climbing. Think cat claws. 

Likewise the enormous flight muscles -- I have been contending micros were powered fliers since the wings were found -- were also great for vertical climbing. 

You have small animals with big hooked claws on strong, powerful arms and clawed, flex toed legs. Of course they could climb. I have already detailed why some but not all of the protobirds incl micros were scansorial/arboreal on the list, and that is in the dinoguides. 

GSPaul

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[Tim Williams]

Domestic cats aren't arboreal.  Not scansorial either (unless you use a very loose definition of scansorial).  Heck, I climbed trees when I was a kid (though not very tall ones).

There's no evidence that _Microraptor_ was arboreal.  Probably not even scansorial.  Ditto for all "protobirds" - _Anchiornis_ and _Archaeopteryx_ included.  All had a very limited range of motion at the wrist, hip, and ankle - just what you *don't* need if you want to climb up a trunk or clamber through a tree crown. 

Not sure why they would *want* to climb a tree - they could probably all fly up there (except _Anchiornis_, at least as adults).  Certain "protobirds" could perch (and maybe _Microraptor_ too, if the branch was thick enough).  So what was the point of climbing?  Tree-climbing would risk damaging the wings, even if they were capable of it.  

Plus (as mentioned) in _Microraptor_ and certain "protobirds" (though apparently not _Archaeopteryx_) digits II and III are bound together, which would have reduced the hand's prehensile abilities.

It takes special pleading to put these critters up in trees.  Though I know certain folks are keen on the idea (e.g., GSP, A. Feduccia).

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[Ethan Schoales]

Were there any arboreal dinosaurs in the Mesozoic at all?

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[Tim Williams]

Ethan Schoales <ethan.s...@gmail.com> wrote:

> Were there any arboreal dinosaurs in the Mesozoic at all?

Certainly there were arboreal dinosaurs in the Mesozoic.  Sapeornithids look like decent fliers and perchers.  Maybe confuciusornithids spent time in trees - though it's doubtful they climbed, as some have suggested - they too likely would have flown up and perched.  Most enantiornithines and some ornithuromorphs (euornithines) appear adapted for arboreality.

Maybe some scansoriopterygids were arboreal - based on the fact that they had patagia inferred to have been used for parachuting/gliding.  

[Ethan Schoales]

Most if not all of those you mention that might have lived in trees could also fly or glide. To the best of our knowledge, few if any lived in trees but lacked flying or gliding ability…

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[Tim Williams]

Ethan Schoales <ethan.s...@gmail.com> wrote:

> Most if not all of those you mention that might have lived in trees could also fly or glide. 

Yes.

> To the best of our knowledge, few if any lived in trees but lacked flying or gliding ability…

I'd say none at all.  So far.

[Ethan Schoales]

Do we know why? 

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[Jura]

On Monday, July 7, 2025 at 10:25:34 PM UTC-5 Tim Williams wrote:

Domestic cats aren't arboreal.  Not scansorial either (unless you use a very loose definition of scansorial).  Heck, I climbed trees when I was a kid (though not very tall ones).

Wait, what? Of course cats are scansorial. Scansorial animals are just animals that regularly climb things. Yes, you as a human being are also scansorial. I've never seen the definition of scansorial restricted beyond this. I mean, how are you even separating scansoriality from arboreality at that point? 

[Tim Williams]

Yes I guess cats are scansorial, as is the wild cat (e.g. Pallandre et al. 2020 doi.org/10.1016/j.zool.2019.125714).  But I wouldn't have said humans are.

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[Thomas Yazbek]

For cats, scansoriality is an innate behavior that is more or less essential to their obtaining food. For humans, it is more cultural or learned, although there seems to be an ontogenetic aspect, since kids are much better climbers than adults. Humans are definitely descended from arboreal/scansorial organisms and are preadapted to such behavior even if it's not 'natural' (whatever that means, culture is such a strong factor that it's hard to define), which to me allows us to be a scansorial species under some definitions of the word. Cats definitely are - it is part of their innate behavioral repertoire.

T. Yazbeck

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[Tim Williams]

Ethan Schoales <ethan.s...@gmail.com> wrote:

> Do we know why?

Some light reading:

Dececchi TA, Larsson HC. (2011) Assessing arboreal adaptations of bird antecedents: testing the ecological setting of the origin of the avian flight stroke.  PLoS One. 6:e22292. doi: 10.1371/journal.pone.0022292.

Dececchi TA, Larsson HCE, Habib MB. (2016) The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents.  PeerJ. e2159.  DOI: 10.7717/peerj.2159

Manafzadeh AR, Padian K. (2018) ROM mapping of ligamentous constraints on avian hip mobility: implications for extinct ornithodirans.  Proc Biol Sci. 285:20180727. doi: 10.1098/rspb.2018.0727  

Pei R, Pittman M, Goloboff PA, Dececchi TA, Habib MB, Kaye TG, Larsson HCE, Norell MA, Brusatte SL, Xu X. (2020) Potential for powered flight neared by most close avialan relatives, but few crossed its thresholds. Curr Biol. 30:4033. doi.org/10.1016/j.cub.2020.06.105

[Ethan Schoales]

Would hip mobility constraints have prevented climbing from evolving or anything like that?

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[Tim Williams]

Ethan Schoales <ethan.s...@gmail.com> wrote:

> Would hip mobility constraints have prevented climbing from evolving or anything like that?

Maybe not prevented - but the restricted femoral abduction would have been very unhelpful to the evolution of scansoriality or arboreality (in the absence of powered flight).  

Same goes for the hinge-like knee primitive for theropods - although birds are no longer as constrained, with enhanced/extreme long-axis rotation at the knee due to fibular reduction (Manafzadeh et al. 2024 doi.org/10.1038/s41586-024-08251-w).  

BTW, I'm not saying that _Microraptor_ or _Archaeopteryx_ or _Anchiornis_ NEVER EVER climbed trees.  But the skeletal and soft-tissue anatomies of these cursorial bipeds doesn't lend any support to the hypothesis that they regularly climbed in a quadrupedal fashion.  Dinosaurs are built very differently to mammals, so citing cats (or any other mammal) as templates for proposed theropod climbing abilities is inapt.  Having curved claws and a small body size is not enough to make a theropod an expert climber or an arborealist.

[Gregory Paul]

I said cats. Not jut domestic. Like leopards, jaguars, and all of them which can use sharp tipped hook claws that evolved for predation also for climbing. And do not need supple fingers because the claws act as hooks. Why the hook claws of theropod fingers evolved is not critical, it is whether they were well suited for climbing, which they were. Better than the small, bluntish claws of climbing hoatzins. 

GSPaul

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[Tim Williams]

'Gregory Paul' via Dinosaur Mailing Group <DinosaurMa...@googlegroups.com> wrote:

> I said cats. Not jut domestic. Like leopards, jaguars, and all of them which can use sharp tipped hook claws that
> evolved for predation also for climbing. And do not need supple fingers because the claws act as hooks.

I had forgotten that domestic cats are scansorial.  The ones in my neighborhood don't tend to climb trees - probably because they are lazy and overfed moggies.

In general, felids have a low center of mass, flexible backbone, mobile elbows and wrists, and sharp claws - all of which are advantageous for climbing.

> Why the hook claws of theropod fingers evolved is not critical, it is whether they were well suited for climbing,
> which they were. 

Claw curvature values can be similar for trunk-climbing and predatory birds (the range of values overlap).  

But was the rest of the theropod "well suited for climbing", that's where the problem lies.

> Better than the small, bluntish claws of climbing hoatzins.

Hoatzin chicks have quite large feet that are specialized for grasping - so maybe wing-claws are less important.  Also the head/neck pulls the chick upwards, based on Abourachid &c's work 

> Apparently there is no data showing that hoatzins can flex the central finger to actually grasp branches. May be
> they can, hoatzing climbing is not as well studied as one would wish.

Yes.  Apparently clambering behavior using wing-claws is not unique to hoatzin chicks - young turacos and mousebirds are reported to do it too.