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Post by theropod on Dec 1, 2015 18:26:25 GMT 5
We’ve been over this. What "could very well be" in your opinion isn’t any more relevant than the reverse. You can either take the available data and reach the most likely conclusion, or you can take them and admit that you cannot draw one.
What I wrote, "Most adult megalodons are smaller than the Livyatan holotype" isn’t an opinion, it’s what the published data tell us. And that’s actually all I wrote on the matter, everything else is up to your imagination.
You can use that to suggest the whale is larger. Or you can make the arguments you just made, which suggesting that we cannot say which is larger (which is not the same as suggesting that the shark is larger, mind you). I’m fine with either, and I’ve made that clear every once in a while.
How on the other hand relying only on the maximum size of megalodon for comparison is not comparing oranges to apples eludes me.
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Post by Grey on Dec 1, 2015 23:23:26 GMT 5
In both case we rely on maximum published known size, not assumptions of obligate average-size for the physeteroid and the deduced mean size for the other from a size estimate method notably more conservative.
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Post by elosha11 on Dec 2, 2015 0:04:34 GMT 5
This probable Livytan tooth compares quite well to the holotype. The website says it is 33 cms long, but 38 if measured on the curve. Were the holotype's 36 cm teeth measured in a straight line or on a curve? From the website: www.henskensfossils.nl/CaveBear.htm
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Post by elosha11 on Dec 2, 2015 0:11:05 GMT 5
I also wonder if we could compare this tooth to the holotype's set to determine its relative position, similar to how we estimate for sharks. I'm not sure of the specifics of Livyatan's dentition. I would assume the holotype's front teeth are the longest/largest 36 cm ones. Does this tooth look like one of the holotype's 36 cm teeth or one of the smaller ones?
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blaze
Paleo-artist
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Post by blaze on Dec 2, 2015 2:42:32 GMT 5
About the body shape, it is too my impression that B.K.'s reconstruction looks like a tuna and I'm aware that a finess ratio of 4.5 is said to be the one with the less drag while maximizing volume but then why large migratory whales and sharks do not have such ratio and are instead a lot more elonagated (with values up to 8)? I don't remember the citation right now but I found a paper that found a slight allometric increase of finess ratio with body mass. edit: Ahlborn et al. (2009) is the publication. A finess ratio of 4 (as suggested by B.K.) is still much lower than in the orca in which it is 4.8 (4.6-4.9 range, 3.8-5.6m length, Ahlborn et al. 2009, Fish 1998). I just checked two photos of great white sharks ( 1, 2) and the finess ratio is of ~5 (20%), theropod, you say scaled to 18m it'll only be 7% deeper bodied? that results in a percentage of 21% and a finess ratio of 4.8, your C. megalodon reconstruction has a similar ratio so I do think it was unfairly criticized as it appears that not even taking the allometric trends in the great white shark results in such a deep body as suggested by B.K.
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Post by Grey on Dec 2, 2015 5:04:09 GMT 5
Kent indeed talked me about allometric fineness while scaling animals.
I do not have the impression that he considers his reconstruction as overly bulky. I need to ask him for sure.
About theropod model, other than the body depth, it appeared very small headed, with a very short jaw line (compare with Gottfried 1996) and the caudal region appeared excessively shallow.
We can question the body depth of B.K. model but I still strongly doubt with theropod model which looks like an Alopias-like megalodon except for the tail.
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Post by Grey on Dec 2, 2015 5:08:35 GMT 5
This probable Livytan tooth compares quite well to the holotype. The website says it is 33 cms long, but 38 if measured on the curve. Were the holotype's 36 cm teeth measured in a straight line or on a curve? From the website: www.henskensfossils.nl/CaveBear.htmThe measurements were straight line so this tooth is somewhat smaller. I know a collector who owns a tooth, also 36 cm. There is an unpublished abstract where it is induced that tooth diameter is related to body size in cetaceans but I ve not found the reference and this contradicts what once said Lambert. But I search in this way.
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Post by theropod on Dec 2, 2015 5:58:23 GMT 5
About theropod model, other than the body depth, it appeared very small headed, with a very short jaw line (compare with Gottfried 1996) and the caudal region appeared excessively shallow. We can question the body depth of B.K. model but I still strongly doubt with theropod model which looks like an Alopias-like megalodon except for the tail. Not sure where exactly you see any resemblance to Alopias in my drawing. This is what a thresher shark looks like: www.fishesofaustralia.net.au/images/image/AlopiasPelagKS.jpgThis isn’t the first time I’m writing this; I based it on a great white and increased the bulk, hence why it looks like a bulky great white shark. I just eyeballed it, considering the massive uncertainty, but it would appear the result ended roughly consistent with what you get by doing that calculation. Not that Alopias would necessarily be an unsuitable analogue though (but of course it’s very unlikely it would keep the giant upper fin lobe, already presumed to have been absent in giant fossil alopiids with macrophagous behaviour). We went through that thing about the jaws already, I’m not sure what it is that’s so difficult to grasp about it. Real life sharks have such streamlined heads, they do not look like the one in Gottfried et al.’s skeletal, their jaws do not protrude to form some sort of massive chin, and they certainly aren’t fully visible in lateral view. Look at the pictures blaze posted. It’s not my drawing that doesn’t look like a real shark, it’s drawings always depict it with gaping jaws, and even a short look at real footage proves that. Gottfried et al.’s model has larger jaws than I depicted, and that’s for a good reason. They assumed it had proportionately larger and more robust jaws well beyond what jaw size scaling in the great white implies (which I obviously didn’t, since I based the size estimate on that very jaw size scaling) added to the fact that their reconstruction is also relatively short in terms of total length. And additionally, their’s is a skeletal schematic, not a life reconstruction, it’s meant to showcase the internal anatomy, not how much of it is visible in the living animal (as you may be able to imagine you also wouldn’t be able to see a T. rex’ vertebrae all that clearly in life just because you can on a skeletal). I’m rather sure that gaping-jawed drawing you posted isn’t how Kent envisions C. megalodon swimming around all day long either. It’s also not how I depicted Livyatan, as you will remember, so you’ll understand if I don’t depict one of them with jaws wide open and the other one with its mouth closed, just because you are unhappy with how small shark jaws look while the shark is swimming. blaze: I can get it to ~12% deeper by picking the regression with the biggest differences (Gottfried et al.’s), which would result in a body depth a little over 22% of fork length based on your figures, but no more than that. This is the scope of variation based on the individual regressions: > max [1] 70701.86 69161.38 57088.95 60914.37 68216.90 56992.47 > max/(avg*(18/5)^3) [1] 1.249674 1.145617 1.129359 1.130850 1.211840 1.000000 > max/(avg*(a[2]/a[1])^3) [1] 1.249674 1.145617 1.129359 1.130850 1.211840 1.000000 > sqrt(max/(avg*(a[2]/a[1])^3)) [1] 1.117888 1.070335 1.062713 1.063414 1.100836 1.000000
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blaze
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Post by blaze on Dec 2, 2015 6:58:27 GMT 5
I'll wager that is B.K.'s reconstruction the one that has an excessively thick caudal region, comparing it to the photos I shared before, its caudal region is twice as thick proportionally. His reconstruction also has the pectoral fins in a curious position, overlapping considerably with the dorsal fin. About the size of the head, I mentioned that my reconstruction has a head proportionally too big, well, I based those proportions on Gottfried et al (1996) so the same applies to that reconstruction. Here's a version I made retaining the details of B.K.s reconstruction but with a proportionally smaller (20%) head and a caudal region like a white shark, finess ratio is also 4 as indicated in the notes rather than the 4.2 shown in the drawing. theropodSo it is possible to get it a little closer but not close enough.
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Post by Grey on Dec 2, 2015 8:28:04 GMT 5
The thick caudal peduncle in the model is most likely based on positive allometry>a 15 m megalodon would need a more robust, thick caudal region to move than a 5 m white shark.
Chuck Ciampaglio said similarly he'd expect a thicker peduncle than what we see in most reconstructions.
Regarding the size of the head here again I can ask Brett. But here again are we forgetting positive allometry ? That Kent rejects the pug-nose like shark from Gottfried 1996 does not imply that he rejects the heavy jaws. Or he would have revised this part too.
Personnally, only this 25 % of FL depth intrigues me. And the overlapping fins, I wonder if he has a reason for that.
Theropod I've not talked about gaping jaws but closed something like in the sketch of B.K. reconstruction or balze modified model above.
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blaze
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Post by blaze on Dec 2, 2015 9:49:47 GMT 5
Thicker peduncle but taller or wider? as we all know sharks swim by moving their tails from side to side, a dorsoventrally taller peduncle would offer greater drag to that movement wouldn't it? so a proportionally wider peduncle would make more sense than a taller one.
I did not change the shape of the jaws, just the relative size based on the tooth row. I wish I had access to a an scan of a white shark jaw, that'll be pretty helpful.
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Post by Grey on Dec 2, 2015 9:59:09 GMT 5
Just guessing of course because I should ask him first too but I think thicker in all dimensions. After all we talk about a 50 tonnes cruiser scaled from a 1-3 tonnes model. What about the case in 10-20 tonnes basking/whale sharks ?
Maybe a comparison between the peduncle of a Minke whale and a blue whale could help as well.
Yes I know about the jaws, just thinking that Kent still envisions a very heavy-jawed shark, his sketch does not mention anything wrong about that from Gottfried work.
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Post by theropod on Dec 2, 2015 16:06:02 GMT 5
Neither basking sharks nor whale sharks seem to have particularly deep caudal pedunkles: I mentioned gaping jaws because that's the only way the entire jaw structure would actually be as visible as you wanted me to make it. I think that's easily apparent from the material blaze just posted, as well as from what I gave you back then. Blaze did account for allometry in the jaw size (as did I), that's the formula he based his observation on in the first place ( read again→). It's just that it's relatively slight. Moreover if you estimate size based on jaw size, there's no point in giving it proportionately larger jaws in the reconstruction that you didn't assume for the size estimate. That'd be exactly like showing a Livyatan depiction with head-body proportions corresponding to a 14m animal but scaled to 17m, which I know you’ve criticised in the past. In other words, if you give megalodon as proportionately large jaws as depicted by Gottfried et al., you have to make the size estimate as short as Gottfried et al. did. And do not complain about the jaws looking too small if you aren’t prepared to also complain about the jaws looking too small in photographs like this one→ or this one→.
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Post by Grey on Dec 2, 2015 23:30:02 GMT 5
Not deeper than in smaller white sharks ?
The thicker caudal peduncle in Megalodon is maybe due to the more active, explosive lifestyle expected for it compared to the filter-feeders, plus it is still larger than them.
I want to know why Kent did this before making a conclusion like you do.
Regarding the jaw line, why did Gottfried and Kent use a more apparent figure than what you're proposing ?
Yes I fairly know the differences of proportions that induce jaws perimeter reconstruction and Gottfried et al. allometric scaling. Note however that Siversson too suggests the jaws to have been extremely heavy.
Note in the pics of the white sharks you put, the commissural posterior part is clearly apparent, I don't think you did this in your model.
Anyway the very best before arguing and call it a day is probably to discuss this with the author.
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Post by Grey on Dec 3, 2015 2:41:46 GMT 5
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