Theropod non-locomotory limb function

[Infinity Blade]

Well, quadrupeds are a poor comparison, as obviously a biped will almost always have thicker femora than a quadruped the same mass.

I know, but still. I get that a biped inherently needs relatively more robust limbs than a quadruped (although I don't know if variation among taxa can make this not the case against any two given bipedal-quadrupedal taxa), and that therefore this needs to be taken into account here, but those things outweigh Achillobator at least by a few times, and a couple of them have passed the megaherbivore threshold. It's not like their femora are gracile for their size either.

But there are selective pressures other than body mass that might result in one animal having disproportionately robust limb bones. Since dromaeosaurs almost certainly used their hindlimbs in prey-capture to a degree not seen in other non-avian theropods, we’re talking about some fundamentally different pressures here. Also, perhaps dromaeosaurs had a less vertical femur, and more knee-driven locomotion, more similar to birds. The lack of a prominent fourth trochanter, skinny tail and the general tendency from hip- towards knee-driven locomotion in theropods would support that. If the femur was habitually held more horizontally than in other theropods, it would need to be thicker to accomodate for that. We see the same thing in birds, which afaik have crazy robust femora for their size, because they are held horizontally.
So yeah, femur circumference may not produce the most accurate results in this case.

So you think it's possible my suspicions could hold true?

What do you think of the whole dromaeosaurs supposedly having sub-par forelimbs in terms of strength and robusticity?

Edit: thanks to one of my friends on Discord, I now know of this.



This was a GDI made by randomdinos.

[Infinity Blade]

More on T. rex forelimbs.

www.livescience.com/63858-t-rex-dinosaur-arms.html

And below is an SVP 2017 abstract.

ANALYSIS OF ROBUSTICITY IN THEROPOD FORLIMBS USING GEOMETRIC MORPHOMETRICS TO INDICATE PREY SIZE PREFERENCE GAGE
Samantha, SUNY Geneseo, Geneseo, NY, United States of America; BURCH, Sara H., SUNY Geneseo, Geneseo, NY, United States of America

"There are many hypotheses regarding the function of the forelimbs in nonavian theropods, including a role in prey capture, but a lack of clear extant analogs makes functional inference difficult. Among extant species, felids are one of the few that use their forelimbs in prey capture, and investigating their relationships may provide new insights into forelimb use in nonavian theropods. Previously, we performed phylogenetic principle component analyses (PCA) on a set of functionally relevant forelimb indices and compared the results from theropods to those of extant felids. The cat PC morphospace showed groupings by prey size preference, and the theropod analysis showed promising alignment of morphotypes with the felid model, but was limited by taxon sampling. To expand this study, we used geometric morphometrics to better capture variations in the shape of the bones and allow for the inclusion of more taxa than with linear ratios. Photographs of forelimb elements of theropod taxa representing all major clades were digitized using both stationary and sliding semilandmarks, and a principal component analysis was performed after Procrustes alignment. The analysis showed gradation in morphotypes from more gracile to more robust, with the robust morphotypes typically showing more development in the processes related to muscle attachment sites. A plot of the allometric trends shows that shape scores of the individual elements demonstrated only very weak correlation with the size of the bone itself, indicating that overall robusticity is not a function of element size. This is particularly clear in considering the humerus of Ajancingenia, which is far more robust for its size than other taxa in the analysis. Tyrannosauroids demonstrated significant variation, with Tyrannosaurus being the most robust of the group and considerably more robust than other large tyrannosaurids such as Daspletosaurus. The allometric plot also shows multiple Tyrannosaurus individuals of various sizes all plot at the same relative robusticity regardless of size, whereas Allosaurus shows far more variability in relative robusticity. The diversity of morphotypes within each size class shows that forelimb function is a major driver of the morpology across Theropoda and that robusticity is not merely a function of size. Given our previous work in comparing the types of shape change associated with prey specialization in cats, we hypothesize these functional differences are related to prey specialization."

vertpaleo.org/Annual-Meeting/Annual-Meeting-Home/SVP-2017-program-book.aspx

To be honest, I think the evidence that T. rex used its forelimbs for some kind of predation function, or at least for anything at all, is really piling up.

[Infinity Blade]

So I have been wondering: what evolutionary advantages, if any, would there be to having shorter forelimbs for predatory behavior? Would the shorter length and closer proximity to the body give greater "mechanical advantage" or leverage when holding onto something or even trying to cut/rake deeply with the claws?

[theropod]

Smaller forelimbs are less energetically expensive during development, so if there isn’t sufficient selective pressure to retain long forelimbs, they will get shorter. This doesn’t really improve mechanical advantage, unless the in-levers of the muscle attachments aren’t shortened as well, which they tend to be.

[Infinity Blade]

Wikipedia says that the talons of Haast's eagle were similar in length to those of the harpy eagle. Checking the source verifies this; it turns out that the talons of Haast's eagle really were proportionately shorter than those of the harpy eagle, being similar in length in absolute terms. What Wikipedia doesn't mention, however, was that the talons of Haast's eagle were thicker with deeper flexor tubercles, meaning it could exert even greater force at the tips of its talons than a harpy eagle could. Consistent with such robust claws are the tibiotarsus and tarsometatarsus, both of which, while as long as expected for an eagle this size, were somewhat thicker (source).

[Infinity Blade]

Dilophosaurus forelimb range of motion study.

palaeo-electronica.org/content/2019/2559-dilophosaurus-forelimbs?fbclid=IwAR2fqVwHUTwwA3O8DpeSzTbC9Zh0-A_4WTv2uMCSQAvRHxzxnG-uGImOIaY

[Verdugo]

Does anyone have any data of Theropod lifting strength? Any of them would be appreciated. I only know two studies on T-rex's lifting strength.

One is Carpenter & Smith (2001) (cited in Wiki cause i don't have access to it. If anyone has access, i would be appreciated). The Wiki suggests a lifting strength of 199 kg from the Biceps.

On the other hand, Lipkin & Carpenter 2008 suggested a Biceps lifting strength of only 30-100 kg which is not that strong at all. And then they concluded that the forelimb was strong enough for grasping preys (how? especially since T-rex's preys are supposed to weigh several tonnes). Don't really get it. Without access to Carpenter & Smith (2001) i can't really compare the differences in methodology.

Kenneth Carpenter pointed out in Monster Resurrected 'documentary' in the Acrocanthosaurus episode that he thinks Acro can lift a tonne. But yeah, probably does not have much substance to it.
 
www.youtube.com/watch?v=Aewkm0m3koA
^ At 13:55

EDIT: Also, anyone has data on Theropod humerus robusticity? You know stuffs such as ML or AP diameter or Circumference divided by Length. I'm only aware of data on Acrocanthosaurus (ML diameter) and T-rex (Circumference)

[Verdugo]

Jan 31, 2015 16:34:10 GMT 5 theropod said:

Do you know why Abelisaurid's coracoid and scapula are so robust? I mean they are certainly not used to power the arms for grasping preys, yes they are even more robust than those of T-rex and Acrocanthosaurus. Why do Abelisaurids need such robust muscle origins if what they're all going to do is waving their little arms around?

[theropod]

That is a very good question. It must have been doing something with its forelimbs and shoulder girdle that required them to be so heavily muscled and robust, but what it is I have absolutely no clue. It is noteworthy that this somehow seems to parallel the robust scapulocoracoids of T. rex, even though the latter’s forelimbs also appear poorly suited for raptorial purposes (albeit not as unsuited as those of Abelisaurids).

[Infinity Blade]

I always guessed that the abelisaurid coracoid and scapula would have served as expanded attachment points for neck muscles. But I cannot say with certainty.

[Verdugo]

Oct 25, 2019 13:28:56 GMT 5 theropod said:

That is a very good question. It must have been doing something with its forelimbs and shoulder girdle that required them to be so heavily muscled and robust, but what it is I have absolutely no clue. It is noteworthy that this somehow seems to parallel the robust scapulocoracoids of T. rex, even though the latter’s forelimbs also appear poorly suited for raptorial purposes (albeit not as unsuited as those of Abelisaurids).

Why are T-rex's forelimbs poorly suited for raptorial purposes? Hasn't Infinity Blade already posted several studies on T-rex's forelimbs suggesting that it's adapted for grappling with large preys?

Anyway, regarding the Abelisaurids, the best i could find are these:
Burch 2017

The retention of large scapulocoraoids in nonavian theropod taxa with reduced limbs may be due to a close developmental association between the scapular blade and the axial skeleton (Kuijper et al. 2005; Valasek et al. 2011; Dececchi & Larsson, 2013), but some of the muscles attaching to the scapulocoracoid possibly had important roles in other activities. Those muscles attaching to the neck (e.g. Levator scapulae) could have played a part in feeding, as may be the case in extant crocodylians (Meers, 2003), and those muscles attaching to the ribs (e.g. Serratus muscles) could have had a role in respiration, as they appear to have in extant birds and crocodylians (Codd et al. 2005; Munns et al. 2012). Although several studies have investigated the functional morphology of respiration and craniocervical muscle dynamics in nonavian dinosaurs (e.g. O'Connor & Claessens, 2005; Snively & Russell, 2007; Tsuihiji, 2010; Tickle et al. 2012), these studies focused on the axial musculature, and the role of the muscles attaching to the pectoral girdle has not been explored. Furthermore, these muscles predominantly attach to the perimeter of the scapulocoracoid, so their potential areas of origin would be increased through lengthening of the scapular blade, not by widening it, as is demonstrated in the scapula of Majungasaurus.

Functions of Levator scapulae and Serratus according to Burch (2014)

The origin of Levator scapulae in nonavian theropods is equivocal but would most likely be from the cranial cervical ribs, as in crocodylians (Meers, 2003). With this morphology it would have acted as a rotator of the scapular blade, as well as a lateral flexor of the neck.

This lack of differentiation may indicate the retention of a single, elongate insertion along the posteroventral edge of the distal two-thirds of the scapular blade, as in crocodylians (Meers, 2003) and lepidosaurs (Russell & Bauer, 2008), and this morphology is reconstructed in Tawa (Fig. 2). With this morphology, the Serratus superficialis would have acted to retract and depress the scapula.

It seems like it may add for lateral flexion of the neck which is at least consistent the skull morphologies of T-rex and Majungasaurus which are adapted for such task (sort of like a hyena or pitbull biting and shaking its preys). Serratus muscles probably helps stabilising the scapula and the upper torso for prey grappling.

Still does not quite explain why the coracoid is so robust. Burch 2017 explained that it helps them to wave their arms around fast (high angular acceleration) but not forceful (low torque). Perhaps, intraspecific displays are exceptionally important for Abelisaurids?

There is a source (hyperlink) suggesting that the coracoid and scapula are mainly used for neck muscles attachment instead of forelimbs muscle attachment in these small-armed Theropods. However, it's not published. In published materials, the scapula and the coracoid especially are still reconstructed with muscles connecting to the forelimbs.

While the smaller arms of dinosaurs like Tyrannosaurus and Ceratosaurus might be good for a laugh, Habib notes that there are biomechanical reasons why smaller limbs may have the advantage. “The bones of the chest and shoulder, such as the coracoids and scapula, are anchor points for muscles going into the arm,” Habib says, “but they are also anchor points for neck muscles.” Only so much muscle can attach to any given bone. But by reducing the size of the arms and the muscles needed to move them, evolution may have allowed dinosaurs like Tyrannosaurus to allot more space to the neck muscles that gave them devastating bites.

“Keeping the bones around the chest and shoulder large, while reducing the forelimbs, provided more room for big neck muscles, which actually makes a lot of sense for predators that relied on large heads as their primary weapons,” Habib says. Think less lion, and more hyena or wild dog.

If shorter arms were better for big, knife-toothed dinosaurs, though, this raises the question of why Allosaurus and similar dinosaurs weren’t shaped like Tyrannosaurus. One possibility, Habib says, is that dinosaurs like Allosaurus hunted and fed in such a way that they did not require super-powerful bites. “They could have been jaw slashers or grabbers that focused on small to medium prey,” Habib says, and so there just wasn’t pressure to evolve more powerful neck muscles. It’s also possible that dinosaurs with longer torsos could use their arms for a bit of a push while getting up from a nap, but there’s no definitive answer just yet.

Also do you have access to Carpenter & Smith (2001) (the 199 kg T-rex biceps curl paper)? I would be appreciated if anyone could share it to me...

[theropod]

Why are T-rex's forelimbs poorly suited for raptorial purposes? Hasn't Infinity Blade already posted several studies on T-rex's forelimbs suggesting that it's adapted for grappling with large preys?

I meant compared to other theropods (except abelisaurids).

Good point regarding the neck muscles, that may be part of it, although it doesn’t seem to explain the massive coracoid.

[Infinity Blade]

Verdugo Carpenter & Smith (2001) appears to be published in a book instead of a journal. The book name is Mesozoic Vertebrate Life. We'll need to find a full PDF of the book online (if it exists); what sucks is that Google Books doesn't have any previews of the book.

I'll address Lipkin & Carpenter (2008) later on.

[Verdugo]

Oct 25, 2019 16:31:59 GMT 5 theropod said:

I meant compared to other theropods (except abelisaurids).

Compared to what Theropods? There are many smaller Theropods with forelimbs longer than T-rex, however their forelimbs are nowhere near as robust or as well-muscled as those of T-rex. Also, refer to Infinity's posts above, T-rex's forelimbs would actually put it into the 'big game' category similar to those of 'big game' Felids, separate it from small-game Theropods with weaker and more gracile forelimbs. Also, refer to Carpenter (2002)

Tyrannosaurus has a proportionally large coracoid (textfig.
12 E, 13 E), but this is in combination with an elongated
scapula and shortened forelimb. The enlarged coracoid suggests
that the adductors were large and powerful. In addition,
the short, robust forelimb elements are mechanically stronger
than are the long, slender elements of Deinonychus, making
them ideal for Tyrannosaurus to hold large struggling prey
(CARPENTER & SMITH 2001). Indeed, the great range of motion
at the shoulder, coupled with large, powerful adductors pulling
on mechanically strong forelimbs, implies that the forelimb of
Tyrannosaurus was actively used in predation. The forelimbs
were probably used to clutch large, struggling prey close to the
body until the jaws could be used to kill the prey. Support for
active hunting by Tyrannosaurus is evidenced by partially
healed bite marks on an adult hadrosaur (CARPENTER 1998).

Scaling the forelimbs to the same humeral length
(Figs.12, 13) reveals several trends. The scapula is relatively
longer in Allosaurus, cf. Coelurus and Tyrannosaurus than in
Coelophysis; it is relatively shortest in Deinonychus. The scapula
is the origin of several retractors suggesting that these muscles
were better developed in Allosaurus, cf. Coelurus and
Tyrannosaurus than in Deinonychus.

The humerus of T-rex is robust, especially in ML direction (suggesting it's good for withstanding the force from struggling preys and the force from its adductor muscles). The large coracoid suggests strong adductor and protractor muscles of the forelimbs. On the other hand, the elongated scapula suggests strong retractor muscles to pull the preys back. T-rex forelimbs range of motions is not that bad as well (by Theropods standard). The only thing that T-rex's forelimb really lack is its reach due to its shortness. Even so, i can't see how it's more 'poorly' suited for raptorial purposes when compared to other Theropods.

Good point regarding the neck muscles, that may be part of it, although it doesn’t seem to explain the massive coracoid.

Yeah it does not. The Levator scapulae and Serratus muscle as suggested by Burch 2017 are connected to the scapula blade so it does not have anything to do with the coracoid. In all of the published muscles reconstructions i have seen, the Coracoid muscles are connected to the arms so the Coracoid is related to forelimbs actions. As much as i want to believe that the Coracoid may have something to with neck muscles, i don't have any evidence for that.

---

Oct 25, 2019 16:38:19 GMT 5 Infinity Blade said:

Carpenter & Smith (2001) appears to be published in a book instead of a journal. The book name is Mesozoic Vertebrate Life. We'll need to find a full PDF of the book online (if it exists); what sucks is that Google Books doesn't have any previews of the book.

Yeah, but i don't have access to the book, not even with my Uni library subscription.

[theropod]

Compared to what Theropods? There are many smaller Theropods with forelimbs longer than T-rex, however their forelimbs are nowhere near as robust or as well-muscled as those of T-rex. Also, refer to Infinity's posts above, T-rex's forelimbs would actually put it into the 'big game' category similar to those of 'big game' Felids, separate it from small-game Theropods with weaker and more gracile forelimbs. Also, refer to Carpenter (2002)

Compared to other theropods in a similar weight class. Large Allosaurs and Megalosauroids. Those have larger, more robust forelimbs, larger ranges of motion (at least Allosaurus and Acrocanthosaurus, none of the others have actually been studied), larger muscle insertions (deltopectoral crest, , olecranon), larger claws and fully developed tridactyl manus.

However T. rex has a very large scapulocoracoid despite such a comparatively small, clearly less raptorial forelimb. Similar to how Majungasaurus also has a very small forelimb, but a very robust shoulder girdle. So yes, neck muscle attachment makes sense (other theropods have neck muscles attacking to their scapulae too, of course) as to why they kept a large scapula and only reduced the forelimbs, but the size of the coracoid must be linked to some residual use of the forelimbs. Possibly geting up from the ground?