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Post by theropod on Feb 28, 2014 17:51:57 GMT 5
It´s rather that the shark teeth are so common because a shark sheds a huge number of teeth in its life. Thats why the largest centra seem small compared to the largest teeth, the largest teeth come from a disproportionate sample. But the sample of centra is much more comparable to the sample of other giant marine predators.
So you can assign a megalodon tooth with absolute certainty?
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Post by Grey on Feb 28, 2014 18:32:56 GMT 5
Pliosaurs too shed their teeth their whole life, not as much as in sharks but at a high rate. Still there are much more skeletal parts from the reptiles and not from the shark. Comparing skeletal remains of pliosaurs and Megs and make a day is biased, large skeletal parts of pliosaurs, small or large, are much more common than skeletal remains from Megs, small or large. It's pointless to want to compare both, the sampling bias is still bigger in the shark side with its cartilaginous skeletal remains. Also, in the case of MoA, there are 7 assigned articulated pectoral vertebrae and 9 cervical vertebrae. As the assignement is known, it is possible to deduce the size of the animal compared with more complete related species. Whereas no one has been able to assign the position of the rare Meg centra.
There's a reason why sizes estimates for Meg are still based on teeth/jaws parameters and not isolated centra.
Not absolute certainty, with high probability I can assign an UA or LA.
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Post by theropod on Feb 28, 2014 20:40:59 GMT 5
Pliosaurs don’t replace their teeth every few days, and pliosaur teeth are deeply rooted in bone, not loosely attached by soft tissues.
Comparing tooth fossils of sharks to other vertebrates holds an even greater bias. In addition, there is the problem of differentiating between adult and subadult specimens, making it difficult to derive an average size for mature individuals.
Your point about vertebral assignment is correct, but that’s the problem you have with megalodon, an animal known from teeth and a few vertebrae. I recall not too long ago you considered the material of C. megalodon better than that of the Liassic giant Ichthyosaurs. I think now you see my point.
Dental remains represent a small and variable part of an organism’s anatomy. You were in contact with Dr Kent, you know what I’m talking about. There are methods one can use to try and estimate the shark’s size, but even with the best there remains a big amount of uncertainty (and that is not counting sources of error in the estimate method itself, that are not always accounted for).
I wasn’t talking about "anterior or lateral". That assignment is easy enough to make, and of course certain teeth are more easy to assign than others (e.g. the probable UA from your last pic vs some random lateral tooth). Exact tooth positions are sometimes impossible to give, and certainly not with a high amount of certainty, especially without morphometric data, but they are tremendously important to estimating the size of the shark, i.e. an estimate made based on a tooth position guessed based on a rough line drawing is not rigorous.
There’s a reason teeth are never or only very tentatively used for size estimation in most animals, e.g. with the Pliosaurus tooth in McHenry 2009. We are often forgetting what these estimates really mean.
* As I have shown, with Shimada’s method there is often a 2-3 fold difference in size between the possible positions of a specimen. Although that’s probably overestimating the actual difference since C. carcharias has a more marked disparity between anterior and posterior tooth sizes, the difference remains marked even between immediately adjacent teeth.
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Post by elosha11 on Feb 28, 2014 21:03:23 GMT 5
Grey, I actually don't know if the Pimiento specimen is from the associated Belgium set or not, so I should not really have presumed that is the case. Someone would have to ask her directly. It does not appear to be the largest, and I agree it certainly appears smaller than 23 cms in diameter. I just thought it would be pretty rare to find such centra anywhere, so it very well may be from the associated set. There's a pdf research article somehwhere that also discusses the Belgium teeth and centra, and we should definitely post it in the Megalodon profile page, if it hasn't been already.
Do all marine macro-predatory species, i.e. sperm whales/livyatan, megalodon, pliosaurs, ichthyosaurs, and largest mosasaurs share similar sized vertebral centra?
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Post by Grey on Feb 28, 2014 21:14:13 GMT 5
The same problem for pliosaur vertebra. But even greater problems in sharks. So muche why all the published sizes for Megs are not based on the very few centra but on teeth.
That some pliosaurs have been estimated in size because of their centra is the another point, centra and skeletal structure of pliosaurs is much well known than Meg.
Yes and what's the point ? Megalodon is more documented but it remains that the exact structure of its vertebral assignement is much less known than in these marine reptiles and not used as a basis for sizes estimates.
You know how much I know this myself. But it remains that any data about centra in Meg is poor, the material is rare and is more subject at individual longevity than any size estimate. So dental remains are still a better alternative, no wonder why all the methods are based on this. And if one can determine the jaws perimeter of the shark, it is likely it will yeild potentially better reliable estimates than we would have if we had more substantial datas about Megs centra.
I really don't understand the point of that discussion. What is true for pliosaurs is not necessary true for Meg and vice versa.
No totally true, the associated incomplete tooth found with the 20 associated centra was envisionned at its original shape and size and no one has complained about it so far. That Megalodons dentition is difficult to reconstruct does not mean it is a mystery. The dental formula is known.
I fully know that. However you forget that using teeth for estimate size has been much more studied in sharks than in others species. Which explains why these estimates are the best we have because, and that despite the degree of caution we have to keep, this is not the poor data we have about centra that will revise this a lot. At best, the 23 cm centra indicates that the shark was potentially at the very least 12.6-13.8 m. Then, just like teeth, estimates based on centra are also subject to potential allometric issues.
We have already discussed this, I'm agreed with you but it remains that using the mean estimate between these points is reasonnable and has not received any criticism. Shimada or not, estimates remains estimates, and for any species.
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Post by coherentsheaf on Feb 28, 2014 21:20:25 GMT 5
Do all marine macro-predatory species, i.e. sperm whales/livyatan, megalodon, pliosaurs, ichthyosaurs, and largest mosasaurs share similar sized vertebral centra? Afaik the peruvian Meg verts are larger than the largest ichthyosaur verts. Though we have n verts from the giat triassic temndodotosaur relative which is among the contenders for most massive ichthyosaur. also the centrum width of 23cm is nothing to be scoffed at. Shonisaurus has centra around 20cm wide and Himalayasaurus has 22 cm.
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Post by theropod on Feb 28, 2014 21:22:26 GMT 5
AFAIK the centrum measuring 23cm is part of the specimen from Denmark, the one with the associated partial tooth that would have measured ~16cm (whether that’s slant lenght or perpendicular height would be interesting to know). The biggest centrum in the specimen from Belgium is 15.5cm in diameter.
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Post by Grey on Feb 28, 2014 21:23:51 GMT 5
Grey, I actually don't know if the Pimiento specimen is from the associated Belgium set or not, so I should not really have presumed that is the case. Someone would have to ask her directly. It does not appear to be the largest, and I agree it certainly appears smaller than 23 cms in diameter. I just thought it would be pretty rare to find such centra anywhere, so it very well may be from the associated set. There's a pdf research article somehwhere that also discusses the Belgium teeth and centra, and we should definitely post it in the Megalodon profile page, if it hasn't been already. Do all marine macro-predatory species, i.e. sperm whales/livyatan, megalodon, pliosaurs, ichthyosaurs, and largest mosasaurs share similar sized vertebral centra? Sadly Pimiento rarely responds and seemingly does not have the time to give elaborated responses. My guess is that this vertebra was seen in the Nat Geo doc about Meg in picture and belongs either to MacFadden's lab or to Hubbell collection. That it comes from the associated mount or not I don't know. And indeed centra are very rare and poorly known in their assignement, hence it is lost cause trying to make rigorous estimates based on these. Are you talking about the Gottfried paper in which he talked a bit about the Belgium column ? He was not too much discussing it, only reporting that he expected the column to be inaccurately mounted, to not be complete and that he'd expect in a 16 m Meg to have vertebra about 26 cm. I've not deeply researched in this, but I've always been skeptical at comparing skeletal material from sharks with tetrapods, and even from marine mammals with marine reptiles. I have seen that some membres did not even appreciate the comparison of T. rex vertebra with Spinosaurus vertebra from Andrea Cau....
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Post by Grey on Feb 28, 2014 21:27:38 GMT 5
Do all marine macro-predatory species, i.e. sperm whales/livyatan, megalodon, pliosaurs, ichthyosaurs, and largest mosasaurs share similar sized vertebral centra? Afaik the peruvian Meg verts are larger than the largest ichthyosaur verts. Though we have n verts from the giat triassic temndodotosaur relative which is among the contenders for most massive ichthyosaur. Are you talking about Himalayasaurus ? If not, what is the paper about it ? Klaus sent me a picture of a vertebra about 26 cm but he did not specify if that was the largest in the column. As the discussions with him are old now and were sometimes confusing, I'm not even sure if the vertebra came from that Meg itself or another specimen. For example, the set of numerous teeth on the ground of the desert I've posted in pictures does not come from the reported 18-19 m Meg but from another material. Hence, I'm not sure if that vertebra comes from the large individual which had I thought an articulated column.
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Post by coherentsheaf on Feb 28, 2014 21:31:23 GMT 5
Afaik the peruvian Meg verts are larger than the largest ichthyosaur verts. Though we have n verts from the giat triassic temndodotosaur relative which is among the contenders for most massive ichthyosaur. Are you talking about Himalayasaurus ? If not, what is the paper about it ? Klaus sent me a picture of a vertebra about 26 cm but he did not specify if that was the largest in the column. As the discussions with him are old now and were sometimes confusing, I'm not even sure if the vertebra came from that Meg itself or another specimen. For example, the set of numerous teeth on the ground of the desert I've posted in pictures does not come from the reported 18-19 m Meg but from another material. Hence, I'm not sure if that vertebra comes from the large individual which had I thought an articulated column. Yes I am talking about Himalayasaurus: mygeologypage.ucdavis.edu/motani/pdf/Motanietal1999a.pdfBut Himalayasaurus is probably smaller tha the temnodontosaur relative from MC Gowan's paper. I have seen the fossil remains of complete Temnodontosaurus in the Stuttgard paleontological museum and it has a pretty deep body. If isometric scaling suggests 14-16m for the largest similar remains as MCGowan indicated, these are probably the largest marie reptiles. Awfully little published though.
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Post by Grey on Feb 28, 2014 21:36:13 GMT 5
An interesting point there about comparing vertebrae between very differents species : Shonisaurus has less wide vertebra (but longer) but is larger (at least longer) than would be a Meg in which the widest centra is 23 cm. Hence, the total size comparison based on these pieces is very rough between so distant organisms.
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Post by theropod on Feb 28, 2014 21:36:30 GMT 5
Centra of great white sharks are known... Nothing, just that the estimates for these animals aren’t that bad at all. Because the teeth are common and popular. And Gottfried et al also proposed a method based on centrum diameter. No, it just enhances our sample of specimens. That tooth was preserved in almost its complete lenght, there was only about a centimetre missing in a 16cm tooth... The dental formula being known doesn’t automatically we can identify every tooth. That’s frankly delusional, it’s not even done in papers (by people who probably have access to better data, such as proper photographs, and who can compare the teeth with actual specimens or casts). Not because shark teeth are more reliable (remember the tooth-width regression?) as an indicator, simply because they are so common. In the case of teeth, it is a common approach to make the maximally conservative assumption. One should be a bit more careful when making guesses on a posterior tooth position. The estimates based on vertebrae are also based on such an assumption. McGowan found support for the 15-16m figures for the Temnodontosaurs based on comparing them to the largest vertebrae of a smaller, complete specimen. In a centrum that is reliably reported at a given diameter, we can also estimate the size, at least as well as we can with teeth. Certainly. But as with the teeth, we have no clue how that actually affected the animal’s proportions, and it’s certainly no worse with centra than with teeth. Where do the figures come from? A 23cm centrum would indicates 13.55m±0.01 based on the two available regressions. That was not my point. My point was that it is tremendously important to identify the tooth correctly (again, regardless of how good the method is), otherwise that has a big effect on the results. We should automatically have a healthy degree of scepticism when someone claims to have an L6 tooth giving an estimate of 21m, especially when that supposed L6 doesn’t bear any greater resemblance to an L6 than to an L3. yes, comparing vertebrae (or any single bone for that matter) directly between distantly related organisms is a very bad idea. Estimates derived from this tend to be off by quite a margin. coherentsheaf: Any idea how much those things would have massed?
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Post by Grey on Feb 28, 2014 21:40:36 GMT 5
Are you talking about Himalayasaurus ? If not, what is the paper about it ? Klaus sent me a picture of a vertebra about 26 cm but he did not specify if that was the largest in the column. As the discussions with him are old now and were sometimes confusing, I'm not even sure if the vertebra came from that Meg itself or another specimen. For example, the set of numerous teeth on the ground of the desert I've posted in pictures does not come from the reported 18-19 m Meg but from another material. Hence, I'm not sure if that vertebra comes from the large individual which had I thought an articulated column. Yes I am talking about Himalayasaurus: mygeologypage.ucdavis.edu/motani/pdf/Motanietal1999a.pdfBut Himalayasaurus is probably smaller tha the temnodontosaur relative from MC Gowan's paper. I have seen the fossil remains of complete Temnodontosaurus in the Stuttgard paleontological museum and it has a pretty deep body. If isometric scaling suggests 14-16m for the largest similar remains as MCGowan indicated, these are probably the largest marie reptiles. Awfully little published though. I agree. The only issue to this I see is the mention in the paper about Thalattoarchon, where they consider Himalayosaurus estimate of 15 m as "poorly constrained" and remark that Himalayosaurus dental elements are not much larger than Thalattoarchon's. Now the later estimate of 8.6 m was itself considered as clearly conservative. Now if isometric scaling is good enough, that's potentially the largest, heaviest, marine reptile to date, though I'd like to know how much may have weighed sikanniensis.
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Post by creature386 on Feb 28, 2014 21:49:05 GMT 5
An interesting point there about comparing vertebrae between very differents species : Shonisaurus has less wide vertebra (but longer) but is larger (at least longer) than would be a Meg in which the widest centra is 23 cm. Hence, the total size comparison based on these pieces is very rough between so distant organisms. It can be used for very rough things, but it is already far less reliable than the Tyrannosaurus Spinosaurus vertebra comparison which some have criticized.
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Post by Grey on Feb 28, 2014 22:02:49 GMT 5
The reliability at using them for Meg is not confirmed, even less today than a decade ago when Carcharodon was still suggested as the appropriate genus for Meg. Also, I doubt that the centra assignement of the white shark has been greatly studied itself.
Agreed with that, I just argue that what may be useful for marine reptiles is likely not the case for Meg.
Of course but he specified too that centra are uncommon so far and explained the uncertainties related to the Belgium specimen, so much why most of his research is based on teeth sizes. The teeth are a more stable source.
How's that ?
That's true. It's certainly very difficult to reconstruct megs teeth based on fragments. First, overall tooth morphology is pretty variable in meg and this makes it very difficult to produce reliable estimates. Second, serration size in megs changes very little across a wide range of sizes, so is of little use in assessing size. However, trying to reconstruct based on a tooth only partially uncomplete like the one I had posted does not appear impossible to me.
That's still better than few isolated centra with a poorly known history. There is more experience at assigning a posterior Meg tooth than a vertebra centra, that's the point.
In a centrum which the original position is known, and only in case of vertebral structure of Meg has been as much studied than dentition. Which is not the case yet.
No worse but still the case. I'm more confident for now to use whole Meg dentition than isolated centra for unknown position.
The figure are rough, in lamnids the TL is 55-60 times the width of the largest centra. Not that important...
I am confident in the team of Pimiento et al. (including Gordon Hubbell and 40 years of experience) for this. I've not seen any complaint from others authors so far. But you still can contact them if you're that skeptical...and of course share the results here.
Fair enough. That's just comparing whales vertebras with those or giants sauropods.
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