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Post by coherentsheaf on Feb 12, 2014 17:27:35 GMT 5
Does that mean the peak Megalodon in that graph is at >70 tonnes and the smallest just under 100 kg (embryonic ?) ? Yes. |
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Post by Grey on Feb 12, 2014 18:36:28 GMT 5
They've (Pimiento and Balk) measured crown height to estimate the total lenght then they applied this in the regression weight estimate by Gottfried. That means they've measured a tooth belonging to a Megalodon in excess of 18 m long. What is interesting is that this specimen is apparently not the 17.9 m TL specimen from Gatun as Gatun is 10 MYO and this peak size in the graph is 12 MYO. This confirms that they did not have all the data at the time this graph was made and the results are not definitive.
Beyong this, we see that Megalodon apparently knew huge variations of size.
Also, this is probably not relevant at all yet, but one can remark that this peak size in Megalodon existence on the graph roughly corresponds with Livyatan coexistence.
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Post by creature386 on Feb 12, 2014 18:50:09 GMT 5
Beyong this, we see that Megalodon apparently knew huge variations of size. Yeah, as I once said on carnivora, Megalodon was probably comparable to oras or crocodiles in this regard (you can see the discussion here, the Livyatan thing appeared there too: carnivoraforum.com/topic/9391850/16/ ). |
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Post by Grey on Feb 12, 2014 19:19:23 GMT 5
Read it. Sam1 and coherentsheaf both forget that isolated teeth only most of the time represent a shark which lost a tooth at one given moment. Also, as debattable Gottfried regression and life stage representiations are, these are the best and most tenable available, potentially tweaked a bit with Pimiento's incoming paper. None of the Megalodon specialists I've consulted strictly rejected Gottfried's weight regression until someone presents something even better.
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Post by theropod on Feb 12, 2014 21:08:20 GMT 5
Grey: If you measured correctly the upper end of the is at 10^4.86=72,444=~72.5t As you pointed out, the potentially different tooth proportions make the method of size estimation problematic (especially Gottfried et al. and Shimada et al. since those both base directly on individual tooth heights of Carcharodon). As for the mass estimates, there is of course the problem that no data on this shark’s built has been published so far (which might change soon), so that it remains hypothetical. Gottfried’s regression’s results are a little higher than average for great white or lamniform lenght- weigh regressions, but it’s a matter of 5.5-8% in C. megalodon’s size range. coherentsheaf: My bad, I didn’t even read the article enough to see the lines can mean so many different things lol So the way as shown here? is the most commonly used?
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Post by creature386 on Feb 12, 2014 21:41:59 GMT 5
By the way, is it possible that the large mean size 13 mya ago came through a smaller sample than in the others (the range was among the smallest) and they simply got many large specimens?
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Post by theropod on Feb 12, 2014 22:02:23 GMT 5
Probably, they gave no sample size and the study isn’t finished yet.
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Post by coherentsheaf on Feb 12, 2014 22:46:35 GMT 5
Grey: If you measured correctly the upper end of the is at 10^4.86=72,444=~72.5t As you pointed out, the potentially different tooth proportions make the method of size estimation problematic (especially Gottfried et al. and Shimada et al. since those both base directly on individual tooth heights of Carcharodon). As for the mass estimates, there is of course the problem that no data on this shark’s built has been published so far (which might change soon), so that it remains hypothetical. Gottfried’s regression’s results are a little higher than average for great white or lamniform lenght- weigh regressions, but it’s a matter of 5.5-8% in C. megalodon’s size range. coherentsheaf: My bad, I didn’t even read the article enough to see the lines can mean so many different things lol So the way as shown here? is the most commonly used? Yes. |
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Post by coherentsheaf on Feb 12, 2014 22:49:03 GMT 5
By the way, is it possible that the large mean size 13 mya ago came through a smaller sample than in the others (the range was among the smallest) and they simply got many large specimens? I agree. Small sample size can skew results to the large and to the small as well. This is known as the law of small numbers. For example the best care can often be found in small hospitals. So can the worst. This is because the probability of having three god doctors at the same time is higher than the one of having 10 good doctors. |
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Post by Grey on Feb 12, 2014 23:29:02 GMT 5
That is why I recall this graph was made when all the data was not collected. And even when it will be done, nothing says that Pimiento and Balk would had access to all collections. But I thin they announced to have already collected datas from more than 400 teeth.
As for the size estimates from Shimada and Gottfried, regarding the weight estimates that's the best available, one can discuss that such results are not all necessary accurate in absolute terms but more interestingly they are indicative. No one says that one Meg was exactly 72.5 tonnes, but one can assume that a Meg 12 MYA approached or tipped the 70 tonnes mark. We have no evidences for Meg's body mass but assumptions with a relatively global acknowledgement. ~50-60 tonnes by study or guess estimate is a relative consensus.
Shimada is based on Carcharodon but it is valuable as it works with any teeth from the jaws. In the purpose of this study, Shimada's is the best to use. Also, it is somewhat more conservative if we compare Gottfried and Shimada's results for the same tooth. (15.9 m vs 15.1 m).
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Post by theropod on Feb 12, 2014 23:54:27 GMT 5
Inserting the tooth sizes in known, assembled megalodon dentitions as a template does too, but it actually bases on the dentition of C. megalodon, which you yourself noted it not proportioned the same way as that of Carcharodon. A method can be convenient, but that doesn’t mean it must be accurate.
How conservative Shimada’s method is depends entirely on the tooth position. It is likely prone to giving underestimates for anterior teeth, and overestimates for posterior ones.
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Post by Grey on Feb 13, 2014 0:09:27 GMT 5
That's what I agree, it's roughly indicative, not necessary accurate.
We have to verify if it is "likely" to prone underestimates or overestimates.
The method was used in the plosone paper about the nursery, Ehret, Hubbell, Pimiento and MacFadden being the authors, and the article itself had reviewers. No one seems to have criticized this method, not even Hubbell, who's perhaps the man having seen the largest quantity of Megalodon teeth in life. Brett neither did not show that kind of criticism regarding Shimada's method applied to Meg.
I don't say that's not the case, but "likely" is too quickly expressed.
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Post by theropod on Feb 13, 2014 2:49:41 GMT 5
It is verifyable:
C. megalodon has much taller posterior teeth if you use toothrow lenght as a proxy:
The larger tooth set has a total perimeter of 1974mm (2.64 times the size of the C. carcharias set). The L8s are 54 and 61mm long, L9s 44 and 47mm.
The smaller tooth set has a total perimeter of 1598mm (2.14 times the size), with L8s at 33 and 36mm and L9s at 24 and 25mm.
By comparison, the 747mm Carcharodon toothrow has 9 and 8mm long L8s, and 6 and 7mm long L9s.
And the anteriors are shorter in C. megalodon (hence also why Gottfried’s method yields such conservative results):
UA1 is 146/138mm in the larger and 112/112mm in the smaller meg dentition, 67/63mm in the great white.
UA2 (this one is important since it’s the one used for Gottfried’s method) is 136/140mm and 109/108mm vs 65/64mm.
Hence, "likely". I would have written "definitely" if I was totally convinced of it, but considering this, I find it very unlikely that scaling based on single teeth would not be considerably error-prone. Their dental proportions are different, especially as far as tooth lenght is concerned.
There are good reasons Kent doesn’t trust those methods in general.
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Post by Grey on Feb 13, 2014 4:52:32 GMT 5
Kent doesn't trust any method at a high degree. He's still using them and considers them as the best available. He simply recalls that one must remain cautious while using them. Mostly, Kent is cautious because of the unknown allometry. He did not express something about the posterior and anterior teeth size in Shimada's, nor any other author or reviewers as I know. But whatever its exactitude, the ability to use it from any tooth makes its purpose.
I know the problematic with methods based on tooth height, as much as I know the issues with methods based on width and perimeter, I had discussed it a few pages earlier. But whatever their absolute accuracy, they are not necessary far off the actual size of Megalodon. As I said, no reviewer has opposed strong position against the use of these methods, because there is nothing better to use as now. To be tenable, this assumption should be verified in various white sharks specimens and Megalodon specimens...
That's why this is not necessary to recall these problems each time a paper or a caculation is made with Shimada's or Gottfried's.
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Post by theropod on Feb 13, 2014 16:40:08 GMT 5
The difference between anterior and posterior teeth is also atttributable to the difference between Carcharodon and Carcharocles. It’s the same type of problem, the difference being that this one can be verified and quantified. And this is certainly the reason why many authors favour the tooth width and jaw perimeter-based methods, which are less or not at all affected by this known and avoidable discrepancy.
The problems with tooth height are considerably more marked than with width. An almost 2-fold difference with some teeth is what I’d classify as "far off".
That does not necessarily apply to all teeth, but it does explain why some posteriors yield such astronomic results, and also why anterior teeth are estimated so conservatively (15.9m for a tooth 16.8cm in perpendicular height, and probably around 18 in slant height for that matter) with Gottfried’s, and even more-so (15.1m) with Shimada’s methods.
You did note the more slight discrepancy there is with tooth widths, so you are certainly aware this is a problem with heights.
Feel free to post measurements of other white sharks. I verified it with both associated megalodon tooth sets, but measurements of Carcharodon dentitions are less spectacular, less rare, and accordingly, fewer people seem to bother to publish them.
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