Re: Allosaurus forelimb range

[Tim Williams]

> Qingqing Liang, Lida Xing, Peter Lewis Falkingham, Chunlei Du, Kexiang Wen & Jin Lin 

> (2024)
> Forearm range of motion in Allosaurus fragilis (Dinosauria: Theropoda)
> Historical Biology (advance online publication)
> doi: https://doi.org/10.1080/08912963.2024.2403599

> https://www.tandfonline.com/doi/full/10.1080/08912963.2024.2403599

Above all, what this study indicates to me is that the forelimbs of _Allosaurus_ were not essential for predation.  The jaws did almost all the hard work, and the forelimbs only played a supporting role (at best).  The forelimbs couldn't go very far forward - the hands couldn't even reach the mouth.

Overall, remarkably similar to what has been found for _Dilophosaurus_ (Senter & Sullivan, 2019): "Forelimb morphology and ROM are consistent with two-handed prehension, clutching objects to the chest, manually hooking objects, and seizing prey beneath the predator’s chest or the base of its neck (but no further forward). Limited shoulder mobility suggests that the mouth, not the hands, made first contact with prey."

Therefore I doubt that having an impaired right forelimb (as in _Allosaurus_ specimen HS0387) would have had any impact on active predation.  The injuries had healed long before the animal died.  I'm skeptical of the authors' interpretation that the long-term reduced mobility of one forelimb contributed to the death of this predator due to "a shortage of food due to a change in the predation pattern".  

This _Allosaurus_ study reminds me of the study by Senter & Juengst (2016) of the _Dilophosaurus_ holotype (UCMP 37302).  The latter had some pretty serious fractures and deformities in its shoulders and forelimbs, but it just went on living and feeding.  So its injuries were not fatal, and probably had little if any effect on its predatory abilities.

The simplest explanation is that the forelimbs weren't very important for predation.  Little wonder that so many lineages of large theropods reduced their forelimbs to next-to-nothing (abelisaurids, carcharodontosaurids, tyrannosaurids).  Forelimbs were expendable in these kinds of predatory theropods - so get them out of the way.  According to Canale et al. (2022) (in their description of the short-armed carch _Meraxes_) there was a lower limit to how small the arms could get (0.4 forelimb/femur ratio), at least in non-avialan theropods.

[Richard W. Travsky]

 

What its neck? Was its neck flexible enough to reach something held in its hands? I'm looking at a picture of the specimen at the University of Wyoming

 

https://cowboystatedaily.com/2023/11/08/wyomings-famous-big-al-dinosaur-gets-sliced-up-for-its-life-story/

 

Hard for me to say if its neck tissue and such would have allowed it…

[Mike Habib]

It almost certainly cannot get its head to its hands, and in fact, there are rather few viable positions where it can *see* its hands, which further challenges the notion that the head and hands were used together in predation.

Personally, I take a stronger stance than the authors given their results: I don’t think the hands were used to capture prey at all. Full stop. All attempts to find a way that they could be that I have seen to-date amount to special pleading. 

In fact, I have yet to see a theropod that has anatomy that seems consistent with using the hands in prey capture. I suspect that old conventional wisdom is just wrong.

And now I await people ready to fight me over this… (lol)

Cheers,

—Mike

Michael B. Habib, MS PhD
Director of Data Visualization
Adjunct Associate Professor of Medicine
UCLA Cardiac Arrhythmia Center
Division of Cardiology
Vatche and Tamar Manoukian Medical Building
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MBH...@mednet.ucla.edu

Research Associate, Dinosaur Institute
Los Angeles County Museum of Natural History
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biology...@gmail.com
+1 (443) 280-0181

On Sep 27, 2024, at 8:03 PM, Richard W. Travsky <RTra...@uwyo.edu> wrote:



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[Mickey Mortimer]

So were the claws purely defensive in your view?  Because the hypertrophied claws and forelimb muscles of e.g. megalosauroids suggest they were doing something important with their arms, and unlike e.g. therizinosaurs or ornithomimosaurs their jaws would seem to be effective enough for defense especially when they're usually among the largest carnivores in their ecosystems.

Mickey Mortimer

[Adrian Boeye]

I think Mickey Mortimer's point is a pretty valid one, muscle tissue is heavy and expensive to maintain. If it is not being used, there would be active pressure to encourage more noticeable atrophy of the forelimbs, and pretty quickly a population would show significantly reduced arms. However that does not appear to be the case, and at least for individuals they seem to have pretty well defined muscle attachment points. A similar point can be made for the claws, although I think it also doubles down on the previous point, the bone cores are large and would be pretty sizeable when they would have their keratin sheathe in life. The arms definitely have some functional use as a weapon, perhaps they could tear into an animal restrained in the jaws? Just speculation though and I am unsure if this is feasible in every scenario. That said, I agree that the arms are probably auxiliary to the jaws when we are discussing predation. However, if there are other ideas on what they might be for I'm more than interested to hear them.

Something that I think should also be worth considering is some trends in derived allosauroids (looking specifically at carcharodontosaurids) and how this analysis plays into it. Previous studies have shown that as the role of the jaws in predation became more important, the size of the arms did not actually increase and became proportionally smaller as body size increased. Given that Allosaurus had some pretty clear functional limitations on the use of its arms even at a smaller size when compared to most derived carcharodontosaurids, what if we are starting to see a sort of point where the use of the arms begins to significantly fall off and the head becomes more and more important as overall body size increases (thus for later derived species the heads become proportionally bigger and the arms proportionally smaller)?

Best,

Adrian

[Michael Habib]

> On Sep 28, 2024, at 11:26 AM, Mickey Mortimer <therizino...@gmail.com> wrote:
>
> So were the claws purely defensive in your view? Because the hypertrophied claws and forelimb muscles of e.g. megalosauroids suggest they were doing something important with their arms, and unlike e.g. therizinosaurs or ornithomimosaurs their jaws would seem to be effective enough for defense especially when they're usually among the largest carnivores in their ecosystems.
>
> Mickey Mortimer

I find myself hard pressed to see how the forelimbs would even be of much use defensively in most theropods. The target basically has to be rammed into their chest, with the theropod clawing at them blindly. There are positions where the hands can be used in combat, but those positions are all quite contrived. Or, put another way, I’m much more struck by the short lengths and limited anterior range of motion than I am about muscle attachments and large claws.

> On Sep 28, 2024, at 1:17 PM, Adrian Boeye <aboe...@coa.edu> wrote:
>
> I think Mickey Mortimer's point is a pretty valid one, muscle tissue is heavy and expensive to maintain. If it is not being used, there would be active pressure to encourage more noticeable atrophy of the forelimbs, and pretty quickly a population would show significantly reduced arms.

To be fair, the arms *are* rather reduced, even in things like allosaurids where they are touted as more “robust”. And the forelimbs show a pattern of further (sometimes extreme) reduction in multiple lineages. The “energy expenditure” model is often overcooked - it does not predict rapid, complete atrophy across the board. There are too many developmental constraints for the patterns to consistently be that clean. There’s a broad pattern of reduction, which to my mind, is sufficient to suggest limited functionality (but perhaps not useless - see below).

> However that does not appear to be the case, and at least for individuals they seem to have pretty well defined muscle attachment points. A similar point can be made for the claws, although I think it also doubles down on the previous point, the bone cores are large and would be pretty sizeable when they would have their keratin sheathe in life.

With regard to the muscles, we should keep in mind that the muscles of the shoulder and brachium double as anterior trunk stabilizers in many vertebrates. Additionally, the muscle attachment areas of the pectoral girdle are shared between forelimb and cervical musculature. So so of the well defined muscle attachment morphology could well be related to stabilization and support of the neck and trunk. It doesn’t necessarily mean the forelimbs are doing a lot.

> The arms definitely have some functional use as a weapon, perhaps they could tear into an animal restrained in the jaws?

It’s remarkably difficult to find positions where the hands can reach an animal while the jaws are also engaged. Sure, you can find them, but it’s awkward and, once again, strikes me as special pleading. The arms do not, in fact, “definitely have some functional use as a weapon”. They look like weapons to us at first glance, but then making that hypothesis work gets very cumbersome. I propose that perhaps the right move is to take a step back and consider other options.

> Just speculation though and I am unsure if this is feasible in every scenario. That said, I agree that the arms are probably auxiliary to the jaws when we are discussing predation. However, if there are other ideas on what they might be for I'm more than interested to hear them.

Two functional options come to mind (I’m sure more exist):

1) Assisting with standing up. The trunk to hind limb length ratios in many theropods are such that, when standing up from a prone position, the animal would necessarily tend to pitch forward. There are a few ways this can be compensated for, but one of the most simple is that the forelimbs give an assist. The large claws are reinterpreted as traction structures in this model. And while that might seem weird, keep in mind that some of the most wicked looking claws in the modern world belong to animals that use them for gripping substrates. Credit for this idea goes to David Hone, who pointed it out to me years ago.


2) The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals. It doesn’t matter that it’s a bit awkward to get to it or see it if it’s dead. At the same time, you can carry much bigger chunks than in the jaws (partly because of maximum gape, but also because of moment arm advantages - suddenly, all those things that make the arms hard to use as weapons are useful - held close to the center of gravity, force-advantageous short limb segments, etc).

Why would a theropod need to carry food? Well, parental care, for a start. But also, if you’re not the biggest theropod on the block, then you might well have to get out in a hurry when something larger comes to steal your find/kill. If you can take some of it with you, then that’s a big deal. This jives with behaviors observed at carrion/kill sites in extant systems. It also aligns with your excellent observation that:

> Previous studies have shown that as the role of the jaws in predation became more important, the size of the arms did not actually increase and became proportionally smaller as body size increased. Given that Allosaurus had some pretty clear functional limitations on the use of its arms even at a smaller size when compared to most derived carcharodontosaurids, what if we are starting to see a sort of point where the use of the arms begins to significantly fall off and the head becomes more and more important as overall body size increases (thus for later derived species the heads become proportionally bigger and the arms proportionally smaller)?

We should also take note here that the muscle attachment tradeoff issue I remarked on earlier also predicts this trend - so there are multiple reasons (not mutually exclusive) why this pattern might emerge. But one of them could well be that the biggest, best armed theropods in their environments just don’t have to flee with food.

This is all off-the-cuff stuff, so I make no claim that it’s brilliant. But I do think we should try to think outside the weapons box with regard to theropod forelimbs. It requires less special pleading to do so.

Cheers!

—Mike

> To view this discussion on the web visit https://groups.google.com/d/msgid/DinosaurMailingGroup/95b84262-b483-45f4-a52c-fb9e8537e61fn%40googlegroups.com.

[Mickey Mortimer]

"I find myself hard pressed to see how the forelimbs would even be of much use defensively in most theropods. The target basically has to be rammed into their chest, with the theropod clawing at them blindly."

Here I was thinking of long-armed taxa like ornithomimosaurs, therizinosaurs, caenagnathids, Citipati-style oviraptorids, etc..  I think they would make a good deterrent from a predator pushing its face into the neck or underside.  For a modern equivalent, I was thinking of giant anteaters where the arms are about the same length compared to the trunk.

"Assisting with standing up. The trunk to hind limb length ratios in many theropods are such that, when standing up from a prone position, the animal would necessarily tend to pitch forward."

Maybe, but what about Limusaurus and abelisaurids with their long trunks?

"The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals."

That I could see.

Mickey Mortimer

[Michael Habib]

> On Sep 28, 2024, at 7:24 PM, Mickey Mortimer <therizino...@gmail.com> wrote:
>
> "I find myself hard pressed to see how the forelimbs would even be of much use defensively in most theropods. The target basically has to be rammed into their chest, with the theropod clawing at them blindly."
>
> Here I was thinking of long-armed taxa like ornithomimosaurs, therizinosaurs, caenagnathids, Citipati-style oviraptorids, etc.. I think they would make a good deterrent from a predator pushing its face into the neck or underside. For a modern equivalent, I was thinking of giant anteaters where the arms are about the same length compared to the trunk.

Oh, I see. Yes, that makes sense. They would have to “rear up” into a sub vertical position, but that’s not altogether dissimilar from modern birds that wing “slam” rivals and/or potential predators. Waterfowl come to mind. And, of course, for the animals you listed, it basically would be a wing slam - albeit with rather small wings and the added benefit of nasty claws that could rake eyes etc.

> "Assisting with standing up. The trunk to hind limb length ratios in many theropods are such that, when standing up from a prone position, the animal would necessarily tend to pitch forward."
>
> Maybe, but what about Limusaurus and abelisaurids with their long trunks?

Yes, I have thought a bit about abelisaurids in this context, at least (good point on Limusaurus), and one trait worth noticing is that massive chest with broad, strong pectoral elements. These were probably anchoring big ol’ neck muscles (which could explain the forelimb reduction), but it also means that the animals could probably just brute force it and push off with the chest. It’s not elegant, and it’s probably slower than using the forelimbs to assist, but it could get the job done and might be perfectly fine if you rarely have to get up in a hurry.

> "The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals."
>
> That I could see.
>
> Mickey Mortimer

Thanks Mickey. Yeah, that one seems to track pretty well so far, though it’s hardly at the “rigorously tested” stage. It even explains details like the orientation and hypertrophy of the “thumb” and associated claw in allosaurids.

> To view this discussion on the web visit https://groups.google.com/d/msgid/DinosaurMailingGroup/268c197d-42c5-48f6-b717-1adeb36481e1n%40googlegroups.com.

[Russell Engelman]

> The “energy expenditure” model is often overcooked - it does not predict rapid, complete atrophy across the board. 

Meanwhile a number of flightless neornithine lineages greatly reduce their forelimbs in...how many Ma? Moas go to a greater extreme than any non-avian theropod, though the sketchy NZ fossil record makes it unclear how rapid that was. Isn't the most recent research from St. Nathan's suggesting a much younger dispersal to NZ than previously thought? I know rails can lose well-developed forelimbs in a very short amount of time.

Also many of these further reductions in forelimb size are associated with specific ecological habits. Specifically gigantism. Yes you also have abelisauroids but the point is to actually get the forelimbs reduced beyond Allosaurus level it seems like you need a push associated with moving into some kind of specific adaptive zone...which implies some sort of selection to keep the forearms the size the previously were. And if they were that useless there should be no selection for longer forelimbs within coelurosaurs, because then selection has nothing to work on.

>The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals.

Why would that even be necessary? Hyenas and big cats have absolutely zero preventive ability in their forelimbs and they do just fine dragging things off with their mouths.

Also let's not forget the complete elephant in the room here with megaraptorans. If anything these forms seem to have hypertrophied forelimb claws.

[Michael Habib]

> On Sep 28, 2024, at 9:17 PM, Russell Engelman <neovena...@gmail.com> wrote:
>
> > The “energy expenditure” model is often overcooked - it does not predict rapid, complete atrophy across the board.
>
> Meanwhile a number of flightless neornithine lineages greatly reduce their forelimbs in...how many Ma? Moas go to a greater extreme than any non-avian theropod, though the sketchy NZ fossil record makes it unclear how rapid that was. Isn't the most recent research from St. Nathan's suggesting a much younger dispersal to NZ than previously thought? I know rails can lose well-developed forelimbs in a very short amount of time.

True. That said, this might well have something to do with being flighted to begin with, so we cannot necessarily expect similar rates of reduction in non-avian theropods. As it is, the reduction happens very differently: birds lose limb strength much faster than length when becoming flightless (see Habib and Ruff, 2008). Non-avian theropods seem to lose length faster than mechanical strength. And the “atrophied” states look very different: long, narrow splints in birds versus short blocky elements in things like abelisaurs.

> Also many of these further reductions in forelimb size are associated with specific ecological habits. Specifically gigantism. Yes you also have abelisauroids but the point is to actually get the forelimbs reduced beyond Allosaurus level it seems like you need a push associated with moving into some kind of specific adaptive zone...which implies some sort of selection to keep the forearms the size the previously were. And if they were that useless there should be no selection for longer forelimbs within coelurosaurs, because then selection has nothing to work on.

That may well be the case, but now we are speculating on the specifics of selection coefficients, which is dodgy territory. Besides, as you note, we *do* have abelisauroids. So the gigantism could be coupled with forelimb reduction, or it could be coincidental.

In any case, I didn’t argue that the forelimbs were useless, only that I do not buy their utility in predation. A predation model leads to testable predictions about what we could reasonably expect in terms of range of motion, length, and coevolution with the craniocervical anatomy - and it seems to me that it fails on all of them.

> >The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals.
>
> Why would that even be necessary? Hyenas and big cats have absolutely zero preventive ability in their forelimbs and they do just fine dragging things off with their mouths.

Hard to say. Maybe it’s not “necessary” but is a sizable advantage if you happen to be a biped. The critter that can carry more wins. Or perhaps it’s a moment arm problem - the ability of many theropods to carry large loads in their mouths was limited by the relatively forward position of the jaws relative to the center of mass. It could even just be the elevated position of the head - when cats and hyenas take away particularly large chunks of food they often drag (or partially drag) them.

We can speculate all day on why theropods would end up with a meat carrying adaptation that modern carnivorans lack. My point is simply that this hypothesis is consistent with the morphology of the limbs without failing on ROM and length tests. To my mind, that makes it better than a predation hypothesis - but there are probably even better ideas out there. My core point is that trying to force predation usage to fit because it’s intuitive is special pleading.

To put it another way: how do we detect if the predation usage model is wrong? I would think it would include: the hands can’t get anywhere near the mouth, the animal cannot see it’s own hands except in very awkward positions, the hands cannot reach prey except in very contrived positions, and/or the forelimbs are extensively reduced in multiple lineages.

> Also let's not forget the complete elephant in the room here with megaraptorans. If anything these forms seem to have hypertrophied forelimb claws.

I don’t see them as an elephant in the room. In fact, I think it’s the opposite: they have hypertrophied claws without any apparent “improvements” in reach or ROM that we might expect to see if the forelimbs were more “weaponized” than normal. In other words, we see lower covariance between forelimb traits than we would expect under a predation model.

> To view this discussion on the web visit https://groups.google.com/d/msgid/DinosaurMailingGroup/7d123fb2-eecb-4853-971d-51b4c7fbe39dn%40googlegroups.com.

[Moritz Dukatz]

Nonprofessional paleoartist here. I'm very excited to read this take because I found it basically impossible to depict predatory non-avialan theropods as employing their forelimbs in any meaningful fashion doing predation - especially so if they're supposed to come in contact with prey before a bite is employed - and I'm surprised nobody seems to have raised this issue before.

Biomechanical studies (Carpenter, 2002; Senter and Robins, 2005; Senter and Sullivan, 2019) all agree that the humeri could extend forward to a point they were held perpendicularly-ish to the entire animal's long axis. In the majority of non-maniraptoriform tetanurans, it is impossible to bring the very tips (!) of those perplexingly big-clawed, powerfully constructed, purportedly grasping tools any further in front of the animal's body than the atlas/skull junction.

We can speculate all day on why theropods would end up with a meat carrying adaptation that modern carnivorans lack. My point is simply that this hypothesis is consistent with the morphology of the limbs without failing on ROM and length tests. To my mind, that makes it better than a predation hypothesis - but there are probably even better ideas out there. My core point is that trying to force predation usage to fit because it’s intuitive is special pleading. To put it another way: how do we detect if the predation usage model is wrong? I would think it would include: the hands can’t get anywhere near the mouth, the animal cannot see it’s own hands except in very awkward positions, the hands cannot reach prey except in very contrived positions, and/or the forelimbs are extensively reduced in multiple lineages.

This! I'm not going to pretend I can add much to this discussion, but I'm fascinated to see people tackling this issue.

[Tim Williams]

Russell Engelman <neovena...@gmail.com> wrote:

> And if they were that useless there should be no selection for longer forelimbs within coelurosaurs, because then selection has nothing to work on.

I wonder if the driving force behind forelimb elongation in certain coelurosaurs was display - specifically, the use of the feathered forelimbs as display surfaces.  And (not mutually exclusively) use of the forelimbs as wings, in a subset of these coelurosaurs.  So in either case, nothing to do with prey capture. 

Small coelurosaurs capable of powered flight (_Microraptor_, _Archaeopteryx_, _Jeholornis_, _Confuciusornis_, and so on) still retained forelimb claws.  I doubt their forelimbs were used in prey capture.  (I don't regard the hypothesis that these claws were used in tree-climbing as plausible.)  Perhaps the claws were retained for intraspecific combat, just as many modern birds have wing-spikes and -spurs.

> Also let's not forget the complete elephant in the room here with megaraptorans. If anything these forms seem to have hypertrophied forelimb claws.

Again, the use of the arms and claws in intraspecific display and combat comes to mind.  Earlier Mike mentioned: "The target basically has to be rammed into their chest, with the theropod clawing at them blindly.  There are positions where the hands can be used in combat, but those positions are all quite contrived. Or, put another way, I’m much more struck by the short lengths and limited anterior range of motion than I am about muscle attachments and large claws."  

I fully concur with this.  This prompted me to think that large forelimb claws might have been used as part of a threat display between rivals, and possibly engaged at close quarters during combat.  Such contests might not have been very pretty, but the aim was not to disable or kill an opponent, just demonstrate superior strength.  So the relatively limited ROM of the forelimbs didn't matter.

I also like the hypothesis that the forelimbs were used as meat hooks, for transporting carcasees or pieces of carrion.

[Tim Williams]

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

> In fact, I have yet to see a theropod that has anatomy that seems consistent with using the hands in prey capture. I suspect that old conventional
> wisdom is just wrong.

One conventional wisdom is that dinosaurs became bipeds to free up the forelimbs for other functions - such as prey capture ("grasping/raking manus" of theropods, and all that).  I've never liked this explanation.  A more plausible explanation is that this shift from quadruped to biped was associated with locomotion (cursoriality).  

Outside of dinosaurs, shuvosaurids also became obligate bipeds, and the function (if any) of their little forelimbs isn't clear.  

[Adrian Boeye]

• One conventional wisdom is that dinosaurs became bipeds to free up the forelimbs for other functions - such as prey capture ("grasping/raking manus" of theropods, and all that).  I've never liked this explanation.  A more plausible explanation is that this shift from quadruped to biped was associated with locomotion (cursoriality).  

The locomotion hypothesis checks out pretty well and really should replace the older idea of freeing up the forelimbs for functional use. Work by Hutchinson and other bio-mechanists have generally shown the COM of archosaurs closely related to dinosaurs was moving further and further posteriorly which started to allow for a more effective form of bipedal running. It really is more about function of the legs and not use of the arms, although that may have been a nice bonus. There is also a general discussion on the musculature of sauropsids v synapsids with sauropsids generally having more powerful hindlimbs (a big part of that being the usually hypertrophied caudofemoralis) while synapsids generally have more powerful forelimbs (this will feed into some later thoughts I have). 

• 2) The big claws and robust (but short) arms are perfectly serviceable meat hooks that could carry chunks of carrion or dispatched prey animals. It doesn’t matter that it’s a bit awkward to get to it or see it if it’s dead. At the same time, you can carry much bigger chunks than in the jaws (partly because of maximum gape, but also because of moment arm advantages - suddenly, all those things that make the arms hard to use as weapons are useful - held close to the center of gravity, force-advantageous short limb segments, etc).

On the subject of meat hooks for carrying off food and what not- that is a fairly interesting idea and makes a fair amount of sense. If we are going with models of interaction where these animals might aggregate for komodo dragon or crocodile like "feeding frenzies" being able to carry off as much food as you can without getting mauled is not a bad thing. I do wonder if having such an enlarged thumb claw is necessary for carrying off something that is probably not struggling although  manipulation of a carcass or clutched object may be a good enough explanation.  

•  We should also take note here that the muscle attachment tradeoff issue I remarked on earlier also predicts this trend - so there are multiple reasons (not mutually exclusive) why this pattern might emerge. But one of them could well be that the biggest, best armed theropods in their environments just don’t have to flee with food.
• This is all off-the-cuff stuff, so I make no claim that it’s brilliant. But I do think we should try to think outside the weapons box with regard to theropod forelimbs. It requires less special pleading to do so.

Thinking outside the weapon box is also a very fair way to think about theropod arms. While I do favor some use of the arms as weapons that assisted in predation (even if infrequently and at best in a supporting role), there isn't a good reason to say they weren't doing something else completely such as intraspecific combat or a weird display instead of using them for predation (I know I already talked about this but the meat hooks idea is just awesome, the thought of a large predator carrying off pieces of its prey with large talons is perfect as we near October and Halloween). On the subject of intraspecific combat, this was previously mentioned with waterfowl but I think we could go a little further- a lot of monitor lizards will rear up and clutch each other with their forelimbs before trying to slam their opponent to the ground. What if something similar was going on where the competing animals would rear up and also try to slam the opponent to the ground while also using their claws to rake into the sides of their opponent? Largely speculation but it's something I think could be interesting. 

 Some food for thought is our modern analogues; most decently sized mammalian predators that target other large animals have reasonably large forelimbs that are comparatively more emphasized than those of most theropods. I do somewhat wonder if our perspective might be a bit warped due to how our perception of modern predators might affect our view of extinct ones since we don't really have any bipeds like extinct theropod dinosaurs. This is largely me speculating at this point, but while the function of extinct theropod forelimbs may not be as heavily emphasized, I do not think it is an unreasonable assumption that they could still perform various activities reasonably well even if said activities appear limited or weird when we try to draw comparisons to modern animals. At the very least, the pathologies that appear to be the results of stressful physical activity appear to suggest the arms were doing something interesting. Again though, I think in the larger context of the animal and the trends of later derived species it is pretty safe to say that the role of the forearms was reduced.

Best,

Adrian

[Jura]

My biggest complaint with most of these forelimb ROM studies is that they always view the forelimb as a solid unit apart from the rest of the body. So, they rarely take into account the cartilaginous aspect of the shoulder joint. Carpenter (2002) assumed relatively small cartilaginous caps on the glenohumeral and elbow joints based strictly off birds, and didn't tackle shoulder (coracosternal) joint excursions themselves. Liang et al. 2024 took essentially the same approach, albeit with some lip service toward the importance of considering articular cartilage. Joint articular cartilage makes a huge difference to ROM studies, as extensively reductive studies by the Hutsons have shown. So, adding some extra cartilage to those joints will likely increase the ROM of these theropods.

It still won't get them to protract very far, though. That's where the coracosternal joint likely comes in. At last year's ICVM meeting, Sarah Burch and I both commiserated over the rather bad job that Prehistoric Planet did with theropod forelimbs. They were all just useless set dressing save for the one species that should have had "useless" arms (Carnotaurus). Burch pointed out that theropod shoulder girdles are basically chameleon shoulder girdles. This wasn't the first time this observation has been made. Bakker (1975) originally compared the two based on Jane Peterson's seminal work on chameleon locomotion and shoulder anatomy (Peterson 1973, 1984; later confirmed by Fischer et al. 2010). Despite this distinct similarity, few paleontologists seem to have touched on it. Nicholls and Russell (1985) were probably the first, followed by Jasinoski et al. (2006) and then Burch (2014, among others). Somewhat relatedly, Baier and Gatesy (2013) also found that coracosternal mobility is important for increasing ROM in the forelimbs of alligators. When authors assume similar mobility for theropods, forelimb ROM increased pretty substantially, including a rostral (anterior) reach of the arms. So, that's one point.

The other thing to consider is how theropods were hunting. The discussion so far has focused on a very primate/feline view of predation, in which the arms are the first part to contact prey and that prey were tackled from a rostral-most position. Why should this be the case? 

Firstly, most theropods had eyes on the side of their rather laterally-compressed heads. This would afford more 360° view of their surroundings (barring several unknowns about foveal placement). Which means that, yes, Allosaurus could see it's arms just fine by just slightly twisting its head along its axis (similar to a lizard or bird that is viewing something). Single-eye information would be more important here, with depth judged more from motion paralax and other 2D cues. 

Secondly, I think we all have our views on theropod predation skewed by too much paleo-art showing large adults taking on large adults. If theropods acted more like modern predators, then they probably routinely went after animals that were significantly smaller than they were. That means they would probably be tackling prey from above or at some 45–60° angle. It would seem that arms would be much better placed for pinning prey at that point. This could still be a problem if the prey is too small to reach the mouth (I doubt Allosaurus was tossing a lot of animals into its mouth like popcorn), but there would still be several prey animals that could be pinned down by arms while being bitten by the jaws (many a juvenile sauropod comes to mind). 

Thirdly, an initial attack can still come from the jaws, while the claws could offer a secondary function in raking the prey animal's hide in the same way that the hindlimbs of cats are never the first weapons deployed during predation but can do one hell of a number on an animal after they have been pinned by both jaws and forelimbs. Varanids have been observed performing forelimb raking while biting and thrashing apart large prey. The pinning and raking of forelimbs mixed with a solid bite is pretty successful at eviscerating a prey animal.

Finally, this entire thread has focused on adult animals only (a common theme in dino paleo). The use of the forelimbs in predation could have followed an ontogenetic trajectory too, with relatively longer forelimbs in carnosaur juveniles playing a more prominent role in predation. More juvenile skeletons would help here. Dinosaurs grew through several ecological size classes from hatchlings to adults. I would think that many of the requirements to hunt insects and small lizards didn't carry over much to hunting juvenile sauropods and smaller theropods, much less subadult and adult animals.

Just to round things out, all of this was in relation to forelimb use for predation. As others have already noted, I'm sure the forelimbs could do a lot more than just "predator tools". Few body parts are single function.

My half dollar on the matter.

Jason

==============================================================

References

Baier, DB, Gatesy, SM. 2013. Three‐dimensional skeletal kinematics of the shoulder girdle and forelimb in walking Alligator. Journal of Anatomy, 223 (5), pp.462-473.

Bakker, R. 1975. Dinosaur renaissance. Scient. Am. 232, 58-78.

Burch, SH. 2014. Complete forelimb myology of the basal theropod dinosaur Tawa hallae based on a novel robust muscle reconstruction method. Journal of Anatomy, 225 ( 3), pp.271-297.

Carpenter, K. 2002. Forelimb biomechanics of nonavian theropod dinosaurs in predation. Senckenb Lethaea 82, 59–76.

Fischer MS, Krause C, Lilje KE. 2010. Evolution of chameleon locomotion, or how to become arboreal as a reptile. Zoology 113, 67–74.

Hutson JD, Hutson KN. 2012. Using the American alligator and a repeated-measures design to place constraints on in vivo shoulder joint range of motion in dinosaurs and other fossil archosaurs. J Exp Biol, 216, 275–284.

Jasinoski SC, Russell AP, Currie PJ. 2006. An integrative phylogenetic and extrapolatory approach to the reconstruction of dromaeosaur (Theropoda: Eumaniraptora) shoulder musculature. Zool J Linn Soc 146, 301–344.

Liang, Q., Xing, L., Falkingham, P.L., Du, C., Wen, K. and Lin, J., 2024. Forearm range of motion in Allosaurus fragilis (Dinosauria: Theropoda). Historical Biology, pp.1-12.

Nicholls EL, Russell AP. 1985. Structure and function of the pectoral girdle and forelimb of Struthiomimus altus (Theropoda: Ornithomimidae). Palaeontology 28, 643–677.

Peterson, JA. 1973. Adaptation for Arboreal Locomotion in the Shoulder Region of Lizards. PhD Thesis, University of Chicago.

____________  1984. The locomotion of Chamaeleo (Reptilia: Sauria) with particular reference to the forelimb. J Zool 202, 1–42.

[Gregory Paul]

Concerning scapula  mobility, this was present in most dinosaurs as originally restored by Dr Bob based in part on the chameleon analogy -- the latter have remarkably dinosaur like scap-coracoids unlike other lizards (so does the huge therapsid Lisowicia), although normal lizards have some mobility as demonstrated by Jenkins and the Harvard bunch back in the day. Crocs also have mobility, albeit very divergent in form. An item that has gone almost entirely ignored is that as predicted by the Bakker model Iguanodon had an anterior sternal element between the coracoids that was usually cartilage (Fig. 56 in Norman's 1980 description of I. bern.). The coracoids glided fore and aft in that structures lateral grooves as per lizards, they did not directly articulate with the paired sternal plates to the immediate rear (see the hadrosaur mummies). Scapular mobility is common in amniotes and is possible because clavicles are either absent or not rigid, as per we people. And exception are avepods which have on ossified furcula  (which may be a modified interclavicle according to a recent paper) that locks the shoulder girdle up. Presumably because the arms are no longer being used for locomotion. Pterosaurs also had little or no shoulder mobility. 

Manipulating bird carcasses I found that sometimes joints were more mobile than the bones alone indicated, other cases less. Dinosaurs having thick cartilage joint caps like birds until the ossify up after completing growth means that restoring joint mobility is problematic. 

Among the dinosaurs with heavy necks that hindered pointing the head down and back and had forward facing eyes and dinky arms were tyrannosaurids. 

GSPaul

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[Mike Habib]

It’s a good half dollar, Jason! Some thoughts in response:

- Yes, including non-bone tissues in ROM studies is important, and I heartedly second your lament in this regard. That said, taking into account cartilage and ligaments can also *reduce* ROM compared to bone-only estimates (Manafzadeh and Padian, 2018 comes to mind).

- The mobile pectoral girdle is a great point, and it could add some reach - but probably not enough. If someone puts in the work on this and finds otherwise, then I would change my mind. But, so far as I can tell eyeballing it, it seems that  forelimbs are just far too short in something like Allosaurus for pectoral mobility to make forward grasping work.

- While we are on the subject of the mobile girdle, keep in mind that in crocodilians, the forelimb musculature appears to reverse origin/insertion to stabilize that girdle for the neck muscles when striking with the jaws. This could explain the robust muscle insertion points in the forelimbs of theropods. 

- To be clear, I am not assuming large prey. In fact, I’m a fairly outspoken proponent of the “theropods munching juveniles” model. The thing is, once we are into the realm of “well, if we imagine the size ratio like this and the angle like this…” we are rapidly spiraling into special pleading. A predation function model doesn’t have special standing. We might be able to “make it work”, but why would we do that? 

- On that same note, one of my gripes with the forelimb grappling/pinning concepts is that, by and large, one good bite from a large theropod would be game over for something 20-30% the mass of the predator. The results of theropod bite studies vary, of course, but they all pretty much agree that it varied from “extremely nasty” to “devastating”. 

- I don’t like getting into the weeds on scenario building, but I have a hard time seeing a situation in which the solution isn’t “keep moving and bite it again”. 

- This is a bit circumstantial, but theropods were fairly brittle animals. That creates a mismatch with a grappling hypothesis.

- I take issue with the assertion that Allosaurus could see its hands “just fine”. Yes, it could see them with long axis rotation of the neck, but it takes a fair bit of rotation and the forelimbs are still in the periphery of the estimated field of view. That’s perfectly fine for picking stuff up, or checking out a small wound, but to my thinking, it doesn’t really jive with the forelimbs-as-weapons model.

- The possibility of an ontogenetic shift in limb use had occurred to me, also. It may be that the forelimbs had more functionality in feeding when theropods were very young. 

- To be clear, I’m not saying that I don’t think a large theropod never ever raked anything with its hands or ever grabbed at an animal it was eating. Rather, I propose that these were incidental and rare sorts of behaviors, and that the morphology of the forelimbs in theropods was primarily “shaped” by something other than selection for performance in a predation context. And, on any given day, I suspect that the forelimbs of something like Allosaurus didn’t do a whole lot. 

Cheers,

—Mike

Michael B. Habib, MS PhD
Director of Data Visualization
Adjunct Associate Professor of Medicine
UCLA Cardiac Arrhythmia Center
Division of Cardiology
Vatche and Tamar Manoukian Medical Building
100 Medical Plaza, Suite 660
Los Angeles, CA 90095
MBH...@mednet.ucla.edu

Research Associate, Dinosaur Institute
Los Angeles County Museum of Natural History
900 W Exposition Blvd. Los Angeles, 90007

biology...@gmail.com
+1 (443) 280-0181

On Sep 29, 2024, at 11:06 PM, Jura <arch...@gmail.com> wrote:



[Tim Williams]

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

> To be clear, I am not assuming large prey. In fact, I’m a fairly outspoken proponent of the “theropods munching juveniles” model.

There are famous examples of predatory theropods that did target large prey.  But these don't necessarily contradict your model.

In the case of the "fighting dinosaurs" _Velociraptor_ vs _Protoceratops_, they both ended up dead; so not a success from the POV of the putative predator _Velociraptor_.  

In the case of  a _Tyrannosaurus_ tooth found embedded in the tailbone of an adult hadrosaur, the intended prey got away (and the wound healed).  

> The possibility of an ontogenetic shift in limb use had occurred to me, also. It may be that the forelimbs had more functionality in feeding
> when theropods were very young.

If the small theropods called 'compsognathids' are indeed juveniles of larger theropods (basal tetanurans), then this would appear not to be the case for these predators.

> And, on any given day, I suspect that the forelimbs of something like Allosaurus didn’t do a whole lot. 

No argument from me.