Post by theropod on Dec 13, 2024 22:23:58 GMT 5
On a side note, have you handled and felt the edge of a fossil shark tooth such as Cretoxyrhina and compared it to that of extant ones such as Great white or Bull shark? If yes, could you confirm if the edge sharpness feels similar? I guess if I ever have the opportunity to handle and feel it, it would leave me no doubts, but I have not.
Yes, I have handled both fossil (Otodus obliquus, O. megalodon, Carcharias and Odontaspis sp.) and extant shark teeth (Carcharhinus sp. – possibly bull shark, though our specimen was unlabeled and could be some other species of the genus, Carcharodon carcharias, Isurus spp. and various others) and there is no significant difference in edge sharpness for well-preserved fossils and extant material. E.g. I’ve handled fossil megalodon teeth with edges just as sharp, if not sharper, than those of modern C. carcharias teeth. This is purely looking at how fine the edge is, not other aspects of tooth geometry (obviously in terms of overall cutting performance, O. megalodon would perform significantly worse, simply on account of its teeth being twice as thick and thus having a much more obtuse edge angle).
Of course it depends on preservation. If something is preserved in fine mud, that will have different effects from a rough sandstone or gravel. And a lot of fossilized shark teeth are recovered from loose, sandy or gravely sediments, which obviously causes a lot of abrasion, so those examples will not be as sharp as an extant tooth, but that’s not because of the process of fossilization, it is due to the processes of erosion happening before or after fossilization.
On another side note, the Carcharodon tooth Abler used for comparison with the tyrannosaur teeth was fossilized as well, so given that the study did not unfairly use fossils of heavily different preservational quality, this is an apples to apples comparison. The tyrannosaur teeth were specifically stated to have been selected from a large sample for their excellent preservation, and specifically their serrations are extensively documented on numerous macrophotos and even SEM images showing the denticles in great detail, so I think any significant concern about the quality of their edge preservation is unwarranted.
Well, considering how thick T-rex's tooth is, the fact that it could even cut like dull blade at all means the serrations do their jobs.
The devil is in the details, the "dull blade" Abler tested the tyrannosaur tooth (from the Judith River Fm., I.e. not even a T. rex but some other tyrannosaur with presumably less incrassate teeth) against was not simply a dull knife with a knife’s edge geometry, but had a similar edge angle to the tyrannosaur teeth in question.
These were the results:
I can't even imagine trying to cut flesh with a tooth from an extant Crocodile,
Rather, the standard under examination are various misconceptions that T. rex teeth were "shark as knives" (or similar), which essentially posit they had excellent cutting ability (somehow in spite of their thickened geometry and adaptation to resist strong lateral forces). I don’t think anyone is claiming that tyrannosaur teeth could not cut at all (like a crocodile’s), it’s just that they were poor cutters. A tyrannosaur biting a fleshy area won’t do no damage, in case that’s something that some people seriously claim, it just will do a lot less damage, just like the bite of a more cutting-focused theropod like an allosauroid to a bony area will do less damage than a tyrannosaurid bite.
And yes, given sufficient force, lots of things, even fairly dull ones, can produce effect colloquially described as, and superficially similar to, cutting. Think of highway guardrails or posts, or even train tracks and wheels. But the cost of that is that it only works like that given both huge forces (and in those specific cases, extreme speeds as well). Using forces that our own body can produce, we’d have a fairly hard time cutting ourselves on a guardrail, let alone a train track, let alone so badly we would quickly bleed to death.
A huge factor in a sharp objects effectiveness against soft tissues is that it requires so little force in order to sever the tissues in question that they can’t simply deform to absorb the force without being lacerated, because the force it would require them to move out of the way would be greater than the force it takes to cut them.
Essentially, this is the same reason why an oscillatory cast saw won’t cut your skin (example here: www.youtube.com/watch?v=Bx1AiQdMQro), it moves so little that against an elastic material like human skin, it won’t generate a force sufficient to cut it (while it is fully capable of sawing through more inelastic plaster). Basically for the same reasons, crocodiles need such extremely powerful and violent maneuvers to stand a realistic chance at ripping off pieces of their prey’s flesh.
On a side note, I recall Thomas Holtz mentioned that because Theropod's lower jaw can completely fit inside the upper jaw when closed, it allows the jaw to act like a scissor and make it more efficient at slicing flesh. Do think that would help T-rex with regards to its cutting performance?
This is not properly referred to as cutting, but rather as shearing, but otherwise yes.
That is actually an adaptation for effectively generating shearing forces (indeed the same kind of force a scissor uses–which is why scissor blades don’t have to be super sharp or have super-acute blade angles)
This is particularly useful for biting through bones, because of bone’s specific material properties. Bone is very strong in compression approaching the lower range of steel, reasonably strong in tension, but much weaker (factor 2-4) in shear.
In order to straight-up drive a tooth into bone, you need it to fail in compression, so you need to overcome bone in the type of loading that it is by far the strongest in.
In a thick bone, this quickly becomes prohibitively difficult, because the deeper your tooth penetrates, the more area your bite force gets spread over, and the lower the pressure becomes, until at some point it dips below the compressive strength of bone and you get stuck, even with a massively strong bite force.
If you introduce a shearing force, it makes it easier to potentially bite through a good-sized bone by shearing it off, for which there is only a constant area (the bone’s cross-section in the plane of shearing) that is relevant, and requires a considerably lower amount of stress. So having this kind of shearing jaw layout is quite useful. It’s also by no means unique to theropods, many carnivorans have the same system in their carnassials (arguably even a lot more efficient because puny little mammals sacrificed the ability to replace their teeth in order to maintain precise tooth occlusion).
think in the documentary BBC The Truth about Killer Dinosaurs, they have a mechanical T-rex skull and they experimented its flesh tearing performance against a pig carcass.
Although that was imo a fairly well-done documentary (and the later experiments with the mechanical dromaeosaur claw even made it into a peer-reviewed paper, Manning et al. 2006), there are still a lot of aspects to that experiment that are left unclear, that of how they treated the edges of the teeth not least among them (I do think I recall seeing them grind them with an angle-grinder, which depending on the grinding disk used may well give you something similar to serrations, or otherwise result in an edge sharpness well above what an actual T. rex tooth has, but it’s also equally possible they weren’t sharpening the edges much at all and merely wanted to smooth the surface).
However it is no surprise whatsoever that a 4 ton bite + mechanical tearing force from a steel reproduction of a set of T. rex jaws made short work of a pig, after all a human with a sharp knife and a tiny fraction of the force can do the same.
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Abler, W.L. 1992. The serrated teeth of tyrannosaurid dinosaurs, and biting structures in other animals. Paleobiology 18 (2): 161–183.
Manning, P.L., Payne, D., Pennicott, J., Barrett, P.M. and Ennos, R.A. 2006. Dinosaur killer claws or climbing crampons? Biology Letters 2 (1): 110–112.