Theropod non-locomotory limb function

[theropod]

Infinity Blade: regarding those keels, yes, I think they aid in penetration. for similar reasons the spikes on warhammers, poleaxes etc. (or even my geological hammer for that matter) are generally square in crosssection, not round. I think the mechanism would be that the edges, even if they aren’t really sharp, will tend to focus the force on a small area where a rip through tissue or whatever is being hit/grasped/pierced can originate and propagate through the material more easily than if the force was evenly distributed over a round surface. Also I suppose it does somewhat more damage, because it increases the dimensions of the wound. And finally, it might also have something to do with increasing the rigidity of the claw.

Regarding the tail muscles in use for kicking: I agree, but only for kicking backwards obviously. Also in some theropods, e.g. Dromaeosaurs, where the caudofemoral muscles are already significantly reduced in size and importance, they wouldn’t play a large role.

[Infinity Blade]

Oh. I was actually thinking they'd help in forward kicking; I never thought about backwards kicking (if/when that was ever needed).

Here's my thought process. When the tail muscles and tendons pull back and retract the femur to prepare for a kick, they'd stretch and store elastic energy. When a leg kicks forward, the stretched tail muscles and tendons recoil and that stored energy turns into additional, useful power for the attack.

But that's just what I imagine. I am open to corrections if my understanding of anatomy is skewed here.

[theropod]

Hmm, I think you are thinking the wrong way around here. The caudofemoralis group connects the tail base to the back side of the femur, so it can only retract the femur, whether that’s by active (muscular effort) or passive (stored elastic energy) doesn’t matter as far as the direction is concerned, which is the same for both since they can only pull, not push. The direction of pull is always from the insertion (fourth trochanter of the femur) towards the origin (lateral faces of the anterior caudal centra and the ilium, in the case of Cf. brevis) of a muscle.
So logically the only direction the caudofemoralis will ever be able to move the femur is backwards, whether its by muscle contraction or elastic shortening of a stretched ligament or the like.

It’s certainly not far-fetched to suggest that elastic strain energy played a role in locomotion, perhaps even in kicking, but it can’t move the legs in a way that a muscle in the same position couldn’t, it’s merely a way of conserving energy.

[Infinity Blade]

Alright, I was under the impression that getting the M. caudofemoralis back into a relaxed state would also help move the femur. Not so apparently.

If the tail would only help with backwards kicking, it makes me wonder just how far backward the femur could be displaced. You showed me how being restricted to a 90 degree angle posteriorly actually isn't the case.

Also, I have another idea. If the tail muscles only retract the femur backward, what if a theropod lifted its leg and made some sort of a downward raking motion with its leg instead of a forward kick? The tail muscles would help in such a case, right?

[theropod]

Yup, they would.

Of course mechanical advantage should be considered too; based on Hutchinson et al.’s figures, while it does decrease at more extreme angles of flexion, this drop is nowhere near as pronounced as with extension, and also the optimum is actually at ~15° of flexion. So generally speaking, the muscles seem to be well set-up for performing that raking motion you are suggesting, at least as far as the hip joint is concerned.

Regarding the movement ranges, Hutchinson et al. 2005 is very useful in that regard too (that was the study I cited back then too, wasn’t it?). They are assuming a range of motion from about 45° extension to 65° flexion at the hip joint, i.e. from a vertical posture the femur could rotate 45° backwards and 65° forwards. They also seem to be implying that it can’t be rules out that further motion was possible too. In any case we are talking about a pretty good range of motion here, certainly easily sufficient for kicking. Even a human can (anatomically speaking, of course in most fight situations that would be a stupid thing to do for entirely different reasons) kick backwards quite effectively, and we seem to have got far less range of motion for hip extension.

[Infinity Blade]

I came across this gem while searching for something in the 2016 SVP abstracts.

Poster Session IV (Friday, October 28, 2016, 4:15–6:15 PM)
OSTEOLOGICAL INDICATORS OF PREY SIZE PREFERENCE IN THE
FORELIMBS OF FELIDS AND NONAVIAN THEROPOD DINOSAURS
BURCH, Sara H., SUNY Geneseo, Geneseo, NY, United States of America; GAGE,
Samantha, SUNY Geneseo, Geneseo, NY, United States of America

"As obligate bipeds, nonavian theropod dinosaurs freed their forelimbs for non-locomotory uses, including a likely role in prey capture. Modeling forelimb function has proven difficult due to a lack of extant analogs. Among modern animals, one of the few groups that use their forelimbs in prey capture are cats. Felids of all sizes use their forelimbs to varying degrees in prey subjugation and capture, ranging from quick swipes at small prey to extended grappling with larger prey animals. A previous study showed that aspects of the morphology of forelimb elements can serve as indicators for prey size preference in cats, despite the forelimb’s prominent role as a locomotory structure. To investigate this relationship in theropods and its similarity to that in cats, we performed phylogenetic principle components analyses on a set of forelimb indices that were previously found to be functionally relevant to forelimb use and prey size preferences. The patterns found in cats and theropods show broad congruence. Among cats, most taxa with large prey specialization show strong separation along the first and second principle components (PC) axes from small or mixed prey specialists, which are weakly separated and show a general gradation. Large tyrannosaurids, Torvosaurus, and Acrocanthosaurus occupy a region that is similar to that of large-prey-specialist cats, and are well separated from other theropods along the first two PC axes. Among both taxonomic groups, species occupying this region showed relatively short and robust forearms, as well as long olecranon processes, robust metacarpals, and large radial and metacarpal distal articular surface areas. Large-prey specialist cats were also found to exhibit increased humeral robusticity, and relatively large humeral epicondyles and distal articular surface area. These results suggest that some theropods can be classed as large prey specialists that used their forelimbs in prey capture. The grouping of tyrannosaurids with larger forelimbed taxa such as Acrocanthosaurus and Torvosaurus in the large-prey-specialist region supports previous hypotheses of a functional role for the reduced forelimb in prey capture. Early dinosaur taxa (e.g., Tawa) are grouped in the small-prey-specialist region, whereas coelophysoids show indicators that they may have had a diet of mixed-sized prey. The similarity in the patterns between theropods and cats suggests that theropods may have engaged in similar forelimb-based behaviors in predation, such as grappling with large prey or swiping at small agile prey."

The following is from the SVP abstracts two years earlier.

Romer Prize Session (Thursday, November 6, 2014, 11:15 AM)
OSTEOLOGICAL, MYOLOGICAL, AND PHYLOGENETIC TRENDS OF
FORELIMB REDUCTION IN NONAVIAN THEROPOD DINOSAURS
BURCH, Sara, Ohio University, Athens, OH, United States of America, 45701

"Limb reduction and vestigialization have occurred multiple times in the evolutionary history of Tetrapoda, often related to a change in mode of locomotion. However, little is known about the functional shifts of reduced limbs or the morphological signals of vestigialization. The forelimbs of nonavian theropod dinosaurs diversified into a wide variety of morphologies including extreme reduction relative to body size, but whether these limbs were functional or merely vestigial is a matter of contention. Using phylogenetic inference in a statistical framework, I constructed a complete reconstruction of the pectoral and forelimb musculature of an early theropod to establish the plesiomorphic arrangement of the musculature in Theropoda and trace myological shifts in two theropod lineages exhibiting extreme forelimb reduction. Whereas the ceratosaurian lineage shows some signs of the advanced stages of forelimb reduction preceding limb loss, the forelimb musculature of derived tyrannosaurids was well developed despite reduction of the limb. Many of the myological features that characterize Tyrannosauridae correspond to relative development of some muscle groups and are not consistent with the hypothesis of a functionless limb. Patterns in the myological shifts allowed testing of existing functional hypotheses and show support for the demands of close-quarters grappling with struggling prey or a potential mate. I also investigated evolutionary trends of forelimb reduction using phylogenetic comparative methods to assess the allometric relationships of the forelimb, modeling various scenarios of forelimb evolution as Ornstein-Uhlenbeck processes to test for specific adaptive regimes within clades, and using Bayesian ancestral state reconstruction (ASR) to investigate the patterns of forelimb evolution over time. The phylogenetically informed regressions revealed an overall pattern of isometry of the forelimb across Theropoda. The best-fitting evolutionary model and the results of the ASR both show at least three distinct optima of relative forelimb length and demonstrate that the forelimbs of tyrannosaurs and ceratosaurs were undergoing active selection for their distinctive proportions. The results of these studies indicate that the reduced forelimb proportions and muscular development of some nonavian theropods were the result of selection for a suite of features allowing them to remain functional even at their small size, maintaining roles in prey acquisition, reproduction, or other intraspecific interactions."

[Infinity Blade]

My eyes were correct: modern birds of prey can actually have two cutting edges underneath their claws. I looked back at one of Duane Nash's blog posts and he has this picture of a Buteo sp. claw with two cutting edges, each on one side of the ventrolateral surface of the claw.



These two books seem to be suggesting the same thing.

The claws of raptors are roughly half-oval in cross-section with the flat side down. On the underside, close to the edges of the forward-directed claws, there may be the development of a groove, which makes the edge work like a knife when the bird uses its bill to tear pieces of meat from prey held by the talons.

Raptors of the World

In cross-section most talons are not round but hemispherical, slightly grooved on the underside so they cut like knives when they pierce quarry.

Australian High Country Raptors

I suspect the same may have been true for Mesozoic theropod claws (manual or pedal) whose bone cores don't display a single cutting edge running midway across the ventral surface. As postulated above, this could be beneficial when piercing the skin of prey for a grip (primary function), and perhaps even for cutting soft tissue (secondary function).

[Infinity Blade]

EVIDENCE THAT THE ARMS OF TYRANNOSAURUS REX WERE NOT FUNCTIONLESS BUT ADAPTED FOR VICIOUS SLASHING
STANLEY, Steven M., Geology and Geophysics, University of Hawaii, Post Bldg. 701, 1680 East-West Road, Honolulu, HI 96822, [e-mail]

Abstract: For more than a century, many paleontologists have viewed the small arms of T. rex as having been vestigial. At ~1m long, these arms were not as tiny as often portrayed, and derived traits indicate that they were actually functional. The few previous suggestions of possible functions for the arms are all problematical. Six of the arms’ derived traits indicate that they were adapted for slashing at close quarters: (1) The shortness of the arms would actually have been advantageous for this activity. (2) A large coracoid indicates that the arms were very strong: not only slightly longer than the leg of a six-foot man but also of similar girth. 3) The arm bones were quite robust and would readily have sustained the impact of slashing. (4) The unusual reduction of the number of fingers from three to two would have resulted in 50% more pressure being applied to each claw. (5) The humoral head was part of an unusual quasi-ball-and-socket joint that would have provided considerable mobility for slashing. (6) The huge (8-10cm-long) sickle-shaped claws would have caused deep wounds.
Its short, strong forelimbs and large claws would have permitted T. rex, whether mounted on a victim’s back or grasping it with its jaws, to inflict four gashes a meter or more long and several centimeters deep within a few seconds -- and it could have repeated this multiple times in rapid succession. Infliction of damage by slashing was widespread among other theropod taxa, so in light of its formidable weaponry, why should T. rex not have engaged in this activity?

Tyrannosaur ancestors used long arms primarily for grasping. These atrophied during the evolution that led to the tyrannosaurids because the jaws took over their grasping function. No longer being selected for, the arms were selected against: the expansion of the head deprived them of nutrition in a zero-sum game. Then, as the arms approached their final size, natural selection kicked in opportunistically and put them to good use for slashing at close quarters.

gsa.confex.com/gsa/2017AM/webprogram/Paper297346.html

[Infinity Blade]

Seemingly further support for some sort of a grasping foot in many theropods.

A grasping interpretation for the pes of dromaeosaurids is further supported by analysis of range of motion in Deinonychus, as the pes forms an enclosed fist even without engaging maximum flexion (Figure 8). In contrast to the condition seen in most modern birds, where a fully reversed D-I opposes D-III (anisodactyly), in Deinonychus a medially directed D-I opposes D-IV, with D-II and III moving in parallel (contra Ostrom), enclosing the fist antero-posteriorly across the “palm” of the metatarsus (aided by the low angle with which the metatarsus is carried relative to the substrate). This arrangement is somewhat similar to when owls and ospreys rotate D-IV into a partially zygodactyl arrangement, thought to provide a more complete or even grasp. Similarity of the Deinonychus pes to other dromaeosaurids suggests that the grasping foot is typical of Dromaeosauridae as a whole (independently confirmed by Senter, and comparable phalangeal measurements among dromaeosaurid taxa; Supporting Information Table S1). Although many theropods possess a medially directed D-I and the ability to flex the foot tightly, only paravians possess the extreme digital elongation facilitating greater opposability and hence grasping ability.

journals.plos.org/plosone/article?id=10.1371/journal.pone.0028964

[theropod]

This study suggests that estimates for range of motion based on fossil bones are potentially too conservative: jeb.biologists.org/content/jexbio/early/2012/09/04/jeb.074229.full.pdf

[Infinity Blade]

Sort of relevant to the thread, as this can do with locomotion or non-locomotory activities. I revisited Hutchinson (2005), the paper than looks at the moment arms of Tyrannosaurus' hindlimb muscles, and it seems to suggest that the femur can be completely vertical relative to the tibia (i.e. that it can adopt a columnar posture akin to an elephant's leg). Greg Paul has argued against the ability to completely straighten the knee IIRC. Any problems with such a posture being adopted?

[Infinity Blade]

Hey, sorry for dominating this thread as of late. This post will just be meant as a small data dump on the whole "dromaeosaur vs. big cat" forelimb strength and thickness thing.

So, the one takeaway I got from what blaze stated on Carnivora's cougar vs. Deinonychus was that humeral robusticity and thickness was dependent on the mass of the Deinonychus in question.



So a Deinonychus may have had a humerus rivaling the strength of a similar sized male cougar's. But if the theropod weighed more (say closer to 100 kg), then the cat would indeed have the larger, thicker humerus.

So cats do beat out dromaeosaurs in this regard, right?

Well, blaze doesn't seem to be as convinced of near 100 kg estimates anymore. This is a recent talk I had with him.



A more modest 70 kg estimate would put the Deinonychus back to having a humerus about as relatively thick as a similarly massive cougar (even though it would no longer have that marked weight advantage).

This isn't necessarily to say that humerus thickness is all there is to it to actual forelimb strength. But as of now, I am not convinced of the claim that "cougar or other *insert grappling mammalian carnivore here* has stronger forelimbs than the dromaeosaur at parity because of thicker stronger forelimb bones".

[theropod]

Well, Paul’s volumetric estimates tend to be a bit on the conservative side due to him restoring them in "lean" (some might call it emaciated) condition. His estimate for the only Deinonychus in his data table is actually 60kg, but I’m not sure what specimen you and Blaze were talking about so it might be bigger. But the same table also still estimates Sue at 6.1t, not 8.4, so that wouldn’t be a fair comparison.

See here: gspauldino.com/data.html

If he has revised those estimates since then, he hasn’t published that on his website (the table if dated 2010). For now I think we can confidently add 30-40% to any of his mass estimates to get the masses of animals with more realistic amounts of flesh on their bones.

[Infinity Blade]

30 to 40%? Wow. I understood his estimates tended to be on the conservative side, but that would make Deinonychus at least 78 kilograms (which is still rather close to a 72 kg cougar in size) from 60 kg.

I just checked the discussion blaze and I had again, and we're referring to MCZ 4371. We said something about it having a 41 cm skull, and Greg Paul said the same thing in Predatory Dinosaurs of the World (he also estimated it at 73 kg in that book). Sure enough, it's also the specimen estimated at nearly 100 kg in Campione et al. (2014).

I'm still not quite sold that it would have weighed 100 kg based on femur circumference just yet. I know you noted how this can be misleading too (e.g. T. rex-Acrocanthosaurus), since, you know, different taxa would have different habits/lifestyles and may need more or less robust femora. Dromaeosaurids (the macrophagous ones, at least) I think would have been the ones with really robust femora for their size (I remember how blaze said that Achillobator had femora about as thick as those of a 1.2t bison, 990 kg giraffe, and 1.3t hippo), so intuitively I'd expect some kind of overestimation for the body mass of Deinonychus too. But that's just my opinion.

In that screenshot, blaze said that he himself apparently estimated the specimen at ~71 kg. I'm curious to know more about that, but he doesn't look like he's too active *right this moment*.

[theropod]

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

That percentage was just going by how much lower he estimates Sue compared to Hartman’s estimate that is the most commonly accepted right now. That percentage could very well be lower if, say, the overall density was closer to 0.8 than 0.9. But he definitely does produce major underestimates.