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Post by theropod on Jun 7, 2015 19:37:24 GMT 5
The main purpose was to suggest that there were no severe size differences between the various epochs (I remember this was being rumoured some time ago, now we have confirmation that it’s not the case).
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Post by Grey on Jun 7, 2015 19:47:18 GMT 5
But the paper hints at geographical differents size, which I've been suspecting since years. At each period the size is the similar but probably not geographically. The comparison with orcas and great whites is relevant in that.
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Post by theropod on Jun 7, 2015 20:05:28 GMT 5
Geographical variations are not surprising. At least I cannot think of any animals with global distributions that don’t vary depending on the region. Note this tough: On the other hand, it could also be due to sampling and taphonomic biases. For instance, the larger mean size found in the Southern Hemisphere could be the result of a lack of systematic collecting efforts, as most of the southern samples are from the Bahia Formation (Mina Fosforita, Chile, #1 in Fig. 2); these come from illegal confiscations and are biased toward larger teeth (R. Otero personal communication 2013). |
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Post by Grey on Jun 8, 2015 7:38:40 GMT 5
Just asked to Catalina :
There are a few individuals between 18-19 m. That is explained in the tables and supplementary information.
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Post by theropod on Jun 8, 2015 11:43:00 GMT 5
She means in the whole population? And in what table?
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Post by Life on Jun 8, 2015 13:09:15 GMT 5
This thread already contains examples of huge Megalodon teeth that imply sizes in (19 - 20m) TL range.
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Post by coherentsheaf on Jun 8, 2015 13:12:17 GMT 5
Maximum size in the data as available here :http://datadryad.org/resource/doi:10.5061/dryad.6q5t4 is 17.9m.
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Post by theropod on Jun 8, 2015 15:18:59 GMT 5
Ah, good to see it’s available now. The mean size of individuals with TL≥10.5m (n=254) (previously used by Pimiento et al. and Gottfried et al. to distinguish adults) is 13.02m. There are 73 specimens (~29%) ≥ 14m, 45 specimens (~18%) ≥ 15m, 23 specimens (9%) ≥ 16m and 12 specimens (5%) ≥ 17m (finally, 7 specimens or ~3% are bigger than 17.5m). So, at 71%, there are almost 2.5 times as many megalodons between 10.5 and 14m as there are individuals above 14m. @life: Obviously, but for various reasons these were apparently not available for inclusion in the study (of course even 544 meg teeth are only a tiny portion of all the meg teeth that are known, hence why there are a handful of extreme specimens that are probably larger than the largest in their sample). Many of these specimens are in private hands (from what Pimiento & Balk suggested, it even makes sense to find particularly big teeth there). ––References:Gottfried, Michael D.; Compagno, Leonard J.V.; Bowman, S. Curtis (1996): Size and Skeletal Anatomy of the Giant Megatooth Shark Carcharodon megalodon. In: Klimley, Peter A.; Ainley, David G.: Great White Sharks: the biology of Carcharodon carcharias. San Diego pp. 55-66 Pimiento, Catalina; Balk, Meghan A. (2015): Body-size trends of the extinct giant shark Carcharocles megalodon: a deep-time perspective on marine apex predators. Paleobiology, Vol. 41 (3) pp. 479-490 Pimiento, Catalina; Ehret, Dana J.; MacFadden, Bruce J.; Hubbell, Gordon (2010): Ancient Nursery Area for the Extinct Giant Shark Megalodon from the Miocene of Panama. PLoS ONE, Vol. 5 (5) pp. 1-9 + Data Supplement: Dryad Digital Repository. dx.doi.org/10.5061/dryad.6q5t4 |
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Post by creature386 on Jun 8, 2015 16:05:43 GMT 5
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Post by theropod on Jun 8, 2015 16:13:14 GMT 5
That’s actually not that surprising. The nursery study basically contained the only sample of several adult megalodon specimens in the literature at the time, if there was any way of arriving at accurate numbers based on a sample of megs, this was the way. Coherentsheaf remarked below that he also got somewhat similar results, though with a different method.
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Post by creature386 on Jun 8, 2015 18:16:43 GMT 5
Well, I still found it a bit surprising, as my method had problems because the adult sample in a nursery area could be smaller than elsewhere, even though my estimate was even a bit larger (13.1 m vs 13.02 m).
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Post by theropod on Jun 8, 2015 18:38:33 GMT 5
Of course the uncertainty is greater when working with a small sample, that, I think, was the reason of some people to not consider it (though I admit I never really got why a single specimen is somehow supposed to be more representative than a small sample). But by the seem of it, the results from a sample of over 250 specimens pretty much confirm your figure, which indicates that there was no systematic bias affecting that small sample (i.e. possible skew towards one sex, or towards small/large individuals didn’t have a significant effect on the whole). So no, I’m not that surprised you got close (way closer than I did, but of course that is also not unexpected, as the tooth data I was using were from a fossil sales website and probably quite biased towards big specimens). It’s plausible that there was a bias towards females (→larger), but there may also have been more young specimens present than usual (→smaller), and these cancel each other out to a degree. So I guess congratulations are in order  |
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Post by Grey on Jun 8, 2015 19:22:31 GMT 5
Ah, good to see it’s available now. Btw the mean size of individuals with TL≥10.5m (n=254) (previously used by Pimiento et al. and Gottfried et al. to distinguish adults) is 13.02m. There were 73 specimens (~29%) ≥ 14m, 45 specimens (~18%) ≥ 15m, 23 specimens (9%) ≥ 16m and 12 specimens (5%) ≥ 17m. So at 71% there are almost 2.5 times as many adult megalodons below 14m as there are individuals above this size. @life: Obviously, but for various reasons these were apparently not available for inclusion in the study (of course even 544 meg teeth are only a tiny portion of all the meg teeth that are known, hence why there are a handful of extreme specimens that are probably larger than the largest in their sample). Many of these specimens are in private hands (from what Pimiento & Balk suggested, it even makes sense to find particularly big teeth there). ––References:Gottfried, Michael D.; Compagno, Leonard J.V.; Bowman, S. Curtis (1996): Size and Skeletal Anatomy of the Giant Megatooth Shark Carcharodon megalodon. In: Klimley, Peter A.; Ainley, David G.: Great White Sharks: the biology of Carcharodon carcharias. San Diego pp. 55-66 Pimiento, Catalina; Balk, Meghan A. (2015): Body-size trends of the extinct giant shark Carcharocles megalodon: a deep-time perspective on marine apex predators. Paleobiology, Vol. 41 (3) pp. 479-490 Pimiento, Catalina; Ehret, Dana J.; MacFadden, Bruce J.; Hubbell, Gordon (2010): Ancient Nursery Area for the Extinct Giant Shark Megalodon from the Miocene of Panama. PLoS ONE, Vol. 5 (5) pp. 1-9 + Data Supplement: Dryad Digital Repository. dx.doi.org/10.5061/dryad.6q5t4Somehow we can also try it with individuals over 14m because in Gottfried's the largest immature individual was a female expected at 14m, 10.5m representing the smallest mature male. Pimiento used this in her thesis and accounted only adults over 14m. I guess that using a lower adult limit of 10m might allow more individuals to work with (or the first Gatun paper would have had only 1 adult). So I'd also be interested to know the average adult size above 13.9m. |
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Post by theropod on Jun 8, 2015 19:43:13 GMT 5
There are 77 (upper 14% of the population, upper 30% of adults) specimens estimated at 13.9m or more for a mean TL of 15.5m.
I can do the same using "smallest mature female" at 13.3m (upper 18% and 38% respectively), and get an average of 15.1m.
This would suggest mature females were very rare (between 14 and 18% of the population). Is that also the case in Carcharodon?
Overall it’s apparent that estimates like Gottfried et al.’s 15.6-15.9m are too low to be maximum size, but do represent large females (percentage in the adult population < 13%), always assuming similar degrees of sexual dimosphism to great whites of course.
The first nursery study (Pimiento et al. 2010) counted individuals above 10.5m as adults (resulting in 7 individuals being counted as such). The second study was the one that used 10m as the limit: Counting all individuals from that size as adults as done by Pimiento et al. 2013 gives an average length of 12.8m (n=277, upper 51% of the population).
I would prefer to use the "smallest mature" category btw, not the "largest immature", otherwise we’d also have to use the "largest immature male" at 9.1m as the lower bound, resulting in an average length comparable to FMNH PR 2081 and a sample encompassing the larger 60% of the population, which seems too small and too much respectively.
–––References:
Pimiento, Catalina; Gonzàlez-Barba, Gerardo; Ehret, Dana J.; Hendy, Austin J. W.; MacFadden, Bruce J.; Jaramillo, Carlos (2013): Sharks and Rays (Chondrichthyes, Elasmobranchii) from the Late Miocene Gatun Formation of Panama. Journal of Palaeontology, Vol. 87 (5) pp. 755-774
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Post by theropod on Jun 8, 2015 20:08:46 GMT 5
We can try to reduce the impact of the smaller specimens without ignoring all of them, to account for a certain percentage being immature females that should not be counted:
average1=13.02m, n=254,TL >=10.5m specimens as large as, or larger than, the smallest adult male
average2=11.75m, n=157, 13.3m>TL>=10.5m, adult males and immature females below 13.3m
average3=15.08m, n=97, >=13.3m, TL specimens as large as, or larger than, the smallest adult female
If we now just take the average of 2 and 3 (thus giving specimens above 13.3m the same impact overall as individuals below it, even tough their sample size is only 61% that of the specimens in the lower interval) we get an overall average of 13.42m (and a weight of about 26.4t based on C. carcharias).
That’s just playing with numbers of course, not a reliable point estimate. Without knowing which specimens are females we’ll never know for sure, but based on Gottfried et al.’s estimates the average size of adults must lie between 13.0 and 15.5m, and probably closer to the lower end considering that the much greater abundance of smaller specimens in the adult size range.
Another possibility for an intermediate figure is to average the sizes at maturity of 13.3m and 10.5m and then take the mean of specimens that size or larger (>=11.9m), which gives us 14.02m (and ~30.3t). Although that probably ignores a proportionately larger number of mature males than it includes immature females, I can live with that.
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