Could T-rex’s “useless forearms” be competing with head and neck musculature?

[Sightreader]

OK, some naive, pure speculation about those “useless forearms” on large theropods (T-rex, abelisaurs, etc…)

 

Could it be that, in theropods, arms muscles actually compete with head and neck muscles for attachment space on the shoulder girdle? 

 

I can’t answer this because I have no idea how the muscles are laid out for theropods.  Is there even such a thing as neck muscles that attach to the shoulder girdle, or are these completely unrelated muscle systems? Would reducing arm musculature provide any sort of advantage to the head and neck of theropods?

 

My thought is that, having the combination of having powerful jaws capable of securing a grip on massive victims, then having the necessary neck and head strength needed to either wrench around or yank big chunks off the dangerously powerful prey of the time might be such a key advantage that it would be worth giving up your arms for. This would not mean, then, that their forearms are necessarily “useless”, which might explain why some of these tiny arms remain as well-muscled as they can be (without sacrificing head or neck power).

 

In addition, I heard somewhere that weight and volume grow by the CUBE of size while muscle strength only grows by the SQUARE of size (that is, by the area of a muscle cross-section).  I don’t know if it’s really that simple, but it would mean that, as these theropod heads and necks get bigger and bigger, they would need proportionally a LOT more neck musculature to catch up with the faster growth in head weight and inertia.  

 

Of course, this would not explain why those tiny, wacky alvarezsaurids have small forearms… that would have to be a totally different selection pressure.

 

Does this make any sense at all or did I just make a complete fool of myself?

 

Be gentle… I’m obviously new at this, lol!

[Danielb]

I am no expert but this is definitely an interesting way to think about it. 

[Tidgy's Dad]

I dunno, but interesting, isn't it?

The wonderful thing about palaeontology is that it is a science but has a vast degree of speculation.

There is so much to which we will never have a definite answer

[Randyw]

Well I’m no expert on this. but I’ve read that the theory of the smaller and smaller arms is that as the weight of the head and neck increased the weight and size of the arms were being sacrificed to keep the balance point at the hips….

[Sightreader]

 

1 minute ago, Randyw said:

Well I’m no expert on this. but I’ve read that the theory of the smaller and smaller arms is that as the weight of the head and neck increased the weight and size of the arms were being sacrificed to keep the balance point at the hips….

Hmm.  Perhaps it might be a combination of the two?  That is, arms both make the dinosaur front-heavy and deprive the head and neck of additional attachment area for muscles?  

 

Usually, if animals get front-heavy, I think they just get themselves a meatier or longer tails.  However, simply being heavier makes everything slower, although I would think some good arms would more than make up for any extra weight they add…?

[Randyw]

34 minutes ago, Tidgy's Dad said:

The wonderful thing about palaeontology is that it is a science but has a vast degree of speculation.

I think this sentence sums it up best! Of course both could be possible. The way the theory dealt with increased tail length for counterbalancing was that at some point the tail gets unweildy and gets in the way when making sharp turns in more confined areas. It was a quite awhile back when I read it and I wish I could remember the name of the article. (My mind is full of things like that but I never remember the important parts sigh)

[Randyw]

I’ve actually seen 3 theories on the small arms. One was the balance theory. The second was that it protected the arms when they were feeding together so they didn’t get bit by others while feeding, and the third was that the short arms actually increased the slashing power of the claws on the arms. I’m not sure how much credence I give to the feeding one….

[North]

Interesting point of view.

Not based on anything, but I have been imagined it simply that hands were used less and less, so they got smaller.

Head became more important tool and hands were left behind. Like our tails most likely would not have cost much harm, but it was not needed, so it disappeared.

[BirdsAreDinosaurs]

4 hours ago, North said:

Interesting point of view.

Not based on anything, but I have been imagined it simply that hands were used less and less, so they got smaller.

Head became more important tool and hands were left behind. Like our tails most likely would not have cost much harm, but it was not needed, so it disappeared.

Usually, in evolution, complex body parts do not simply disappear when they are no longer used. But if a part of the body is not really beneficial to the animal, and some costs of keeping that part are involved, it can be selected to become smaller or even disappear.

 

And keeping a useless bodypart will in most cases bring some costs. In the case of arms, it could break when the animal falls, become infected, grabbed in battle by an opponent… In any case, there will always be some energetic costs of simply maintaining the body part, but I doubt that that will usually be enough to strongly select against the body part.

Edited December 20, 2023 by BirdsAreDinosaurs

[North]

53 minutes ago, BirdsAreDinosaurs said:

But if a part of the body is not really beneficial to the animal, and some costs of keeping that part are involved, it can be selected to become smaller or even disappear.

True. This was my thoughts too, but I guess I should have opened it bit better.

 

55 minutes ago, BirdsAreDinosaurs said:

And keeping a useless bodypart will in most cases bring some costs. In the case of arms, it could break when the animal falls, become infected, grabbed in battle by an opponent

Good point. I have not given thought about possible damage that could happen to the bodyparts.

[Al Dente]

Some modern dinosaurs have reduced arms and heads. Photo from Wikipedia.

 

 

 

 

[Mark Kmiecik]

16 hours ago, Randyw said:

Well I’m no expert on this. but I’ve read that the theory of the smaller and smaller arms is that as the weight of the head and neck increased the weight and size of the arms were being sacrificed to keep the balance point at the hips….

 

Why not just a longer and/or stouter tail?

[Sightreader]

Thank you guys for the feedback!

 

A question still remains for me: can head and neck musculature be increased at the cost of forearm musculature? If so, then there would be positive selective pressure to reduce arms size despite those arms still being in use. 

 

The reason I say this lies in the curious fact that what arms T-rex had were actually rather well-muscled. You would think that, if T-rex truly had no use for its arms, then there would be no point to muscling them at all - unless they were still serving some sort of vital function. 

 

This, then, leads me to speculate that T-rex forearms were actually not vestigial, but rather a lower priority compared to increasing head and neck musculature: that is, the arms remained important and useful, but additional shoulder girdle space was desperately needed to keep up with the bigger and bigger heads and increasingly powerful jaws. 

 

Of course, this theory makes absolutely zero sense if head and neck musculature doesn’t actually attach to the shoulder girdle...

 

Thanks!

Edited December 20, 2023 by Sightreader

[BirdsAreDinosaurs]

Interesting theory. I do not know about the musculature, so I cannot help with that. But your point that their arms were functional might be valid. On the other hand, it could also be that the arms were still in the process of becoming truly vestigial.
 

Assuming the arms were functional, their size was the result of a trade-off between different aspects. Arms, even relatively small ones, can have many functions, such as display (especially if they were feathered), thermoregulation or they might for instance be used to grab and hold a partner during mating. Small arms might in some cases be better suited for those functions than big ones. Who knows? And bigger arms could have disadvantages too, such as being vulnerable in battle.

 

So I believe your theory is one of many possible explanations. But it is a fun one and I hope someone here knows more about the musculature of these animals!

[Randyw]

4 hours ago, Mark Kmiecik said:

Why not just a longer and/or stouter tail?

The balance theory addressed that as the longer/bigger tail would have been a hinderence in turning/moving through more enclosed or wooded spaces….

as to muscular attachments of course it’s partially guesswork partially science. I have these images saved on my iPad from what I remember they were made with the advice of Scott Hartman. Of course take them with a grain of salt

[North]

Arms of tyrannosaurus were indeed well musculed and many believe that they had some purpose. What that purpose is might be harder to tell, mating is popular theory.

I have been also thinking arms might be  usefull for younger tyrannosaurs and changed purpose after animal matures.

[Sightreader]

Oops… ended up with a null post…

Edited December 21, 2023 by Sightreader

[Sightreader]

4 hours ago, Randyw said:

 

as to muscular attachments of course it’s partially guesswork partially science. I have these images saved on my iPad from what I remember they were made with the advice of Scott Hartman. Of course take them with a grain of salt

These muscle reconstructions of Trex are absolutely spectacular!
 

Certainly, at least according to what is speculated there, you could easily see how larger arm muscles would interfere with the space available for muscles that move the massive head and neck. As is currently reconstructed, we see only a bit of the bottom-rear of the shoulder girdle being used for the forearms along with some muscles wired in interior to the girdle, leaving the entire forward edge of the shoulder girdle for attachment of those powerful neck and head muscles.

 

Regarding juveniles, I wouldn’t be surprised if the purpose and use of the forearms change as the animal ages. Perhaps the arms might even be longer relative to the body length when these guys are adolescents, then hit a point where they stop growing while the rest of the animal grows around them. Still, I doubt even the young ones used their arms much in the same way as guys like Allosaurus had done in eras past.

 

Gorgeous work, thanks for posting it!  Perhaps my little pet theory is more plausible that I thought…?

Edited December 21, 2023 by Sightreader

[BirdsAreDinosaurs]

2 hours ago, Sightreader said:

 

Regarding juveniles, I wouldn’t be surprised if the purpose and use of the forearms change as the animal ages. Perhaps the arms might even be longer relative to the body length when these guys are adolescents, then hit a point where they stop growing while the rest of the animal grows around them.

 

That would make total sense, given the most recent insights on the ontogeny and development of T. rex.