Megaraptor namunhuaiquii

[DinoFan83]

Here are some of my opinions on certain aspects of Megaraptor's anatomy. I have reason to believe the animal may have markedly differed from the common image of it in many people's minds.

In particular, despite its large claws it's often thought of as having short/small arms, as well as a rather proportionally small head. This is primarily owing to PWNZ3R-Dragon's skeletal, as well as GetAwayTrike's skeletal to a lesser extent.
However, I absolutely cannot replicate these proportions from the information I can find on the animal nor do I know how they were reached. Here is what I can replicate and how I reached it.

Arm size:

In both PWNZ3R-Dragon's skeletal and GetAwayTrike's skeletal of MUCPv-341, the humerus is very short/small. This is rather unprecedented with the data I can find, especially from GetAwayTrike, as his skeletal is the base of my inability to replicate a humerus that small.

Particularly, his skeletal of MUCPv-595 (the only specimen of Megaraptor to preserve a humerus so far) suggests the humerus would have to be far larger (you can even see MUCPv-595's proportionally larger humerus in the above post). MUCPv-341 and MUCPv-595 do not have a whole lot of overlap based on the material listed at the Theropod Database, but one of the only (and yet arguably very important for this purpose) bones we can compare the 2 directly in, the pectoral girdle, is restored as 2.5 times larger in MUCPv-341.

So how large exactly would the humerus, and by extension whole arm, of MUCPv-341 be?

The Theropod Database lists MUCPv-595's humerus as 23.2 cm long, so assuming the humerus/pectoral girdle ratio remains consistent between the 2 (which I see no reason not to assume), we can estimate MUCPv-341's humerus at 58 cm long. I think this estimate is exceptionally well supported because, as can be seen in the post right above this one, the preserved radius and ulna of MUCPv-341 fit quite nicely into the radial and ulnar condyles of a 58 cm Megaraptor humerus.
Some notable measurements for the radius and ulna themselves are 36.9 cm for the radius (as per the Theropod Database) and 44.6 cm for the ulna (no measurement is given, but if Figure 7 of Calvo et al. (2004) is accurate, it ends up that long based on the radius). 

With these forelimbs, Megaraptor would have been anything but short/small-armed. Just to drive that point home better, here is how favorably it stacks up against the top-tier modern mammalian carnivores on a proportional basis (specifically the brown bear, which I will be using here since its forelimbs are quite well developed and stocky for mammalian carnivore standards).

In Sorkin (2006), for instance, a 680 kg Alaskan brown bear (FMNH 63802) has a 44.4 cm humerus, a 34.9 cm radius, and a 35.8 cm ulna. At hypothetical size parity with MUCPv-341 following Greg Paul's 1000 kg estimate (cubic root of (1000/680) times any of its arm bone lengths), it would have a 50.5 cm humerus, a 39.7 cm radius, and a 40.7 cm ulna.
Here are how those bones compare, all to scale assuming size parity. The humerus, radius, and ulna are compared in the written order, with Megaraptor on the left side and the bear on the right side. The humerus/radius of the bear are from here and the ulna is from here, while the Megaraptor humerus is from Porfiri et al. (2014) and the radius/ulna are from Calvo et al. (2004). 

*This image is copyright of its original author

Megaraptor clearly isn't at any disadvantage in terms of proportional forelimb size. Its humerus is 14.9% proportionally longer and its ulna is 9.6% proportionally longer, with both being visibly more robust* than the humerus and ulna of the bear. The only bone which isn't longer is the radius at a mere 7.6% proportionally shorter, and it does not appear to be any less robust than that of the bear, merely shorter.
Last, but certainly not least, its claws were far larger proportionally than those of any grizzly bear (this is obvious in many of the second post's images) and therefore almost certainly significantly more important for prey capture.

*While I am not aware of any published measurements of radius and ulna circumference for either Megaraptor or the brown bear, I know of humerus circumference for both. Campione & Evans (2012) supplementary gives the circumference of a 40.1 cm grizzly bear humerus as 14.6 cm, so we can estimate the humerus circumference of this semi-hypothetical giant bear at 18.4 cm.
On the other hand, the humerus of Megaraptor was more robust than that, with its circumference being approximately 40% of its length. As page 2 of Novas et al. (2016) says:

Quote:Apart from the similarity with some coelurosaurs described for the distal end, the robust construction of the humerus in Megaraptor and Australovenator is closer to Allosaurus (width:length ratio approximately 40; Madsen, 1976; Hocknull et al., 2009; Porfiri et al., 2014) than the elongate and more gracile humeral proportions of Guanlong and Deinonychus (width:length ratio approximately 30; pers. obs.)

From that, we get an estimated 23.2 cm for the circumference of MUCPv-341's humerus. Assuming all the above is roughly accurate, Megaraptor probably had a humerus circumference 26.1% greater than that of a same sized brown bear. Note that the ulna was probably at a similar or greater proportional robusticity advantage, based on the comparison.

Long story short, considering the proportions of its own species, I see no justification whatsoever for Megaraptor to have arms as short/small as existing skeletals depict. Proportionally larger and more robust forelimbs than a brown bear is far more justifiable.

Head size:

In at least PWNZ3R-Dragon's skeletal, the head is clearly quite proportionally small. How this was reached really is beyond me considering how MUCPv-595's proportions apply to MUCPv-341.

MUCPv-595's restored skull in Porfiri et al. (2014) ends up at 39.2 cm long based on the 2 cm long scalebar (5 mm=2 cm). The size difference of the pectoral girdles therefore gives us 98 cm for the estimated length of MUCPv-341's skull, and while that skull wouldn't have been particularly deep or wide, it wouldn't have been small for a 1000 kg animal - the life restoration by sphenaphinae in the above post is probably a fairly accurate depiction of its proportional size.
The skull wouldn't be a bad tool to hunt large prey with either, given the great number of sharp, serrated teeth.

Conclusion/Discussion:

With its long toothrow jammed full of knifelike teeth, gargantuan eagle-like hand claws, and extraordinarily massive forelimbs probably stronger than similarly sized bears, I get the impression Megaraptor would have a very unique hunting strategy that would have aspects akin to the hunting big cats/bears (wrestling the prey down with its forelimbs to gain control), sharks (bleeding the prey to death with those razor sharp teeth), and birds of prey (stabbing and slashing large wounds on the prey with the giant claws).
It would probably be quite the sight to see. Reply