|
|
Post by Grey on Mar 5, 2014 19:17:21 GMT 5
 I guess you refer to Kent's paper, but don't forget it's a not reviewed draft, the 190 is perhaps a confusion itself. It is highly possible that the peruvian one (194 reported) is missing few vertebra. Anyway, 190 to +200 is not of a great difference.  From top (left to right): Otodus obliquus, Carcharodon carcharias, Odontaspis ferox, Carcharocles auriculatus, Carcharias taurus, Isurus paucus, Isurus paucus. I don't see any prominent slenderness in Otodus and Carcharocles compared to the others which are not themselves slender bodied sharks. What Theropod suggests is not impossible in the absence of detailled study of meg column, but less likely than the predictions from Gottfried et al. The weight related problem depends more of the biology of Meg (fat, liver) than the centra IMO. |
|
|
|
Post by Life on Mar 5, 2014 19:32:41 GMT 5
You meant what I was stating about the vertebrae? Obviously, if the centra are proportionally longer, and there were more of them, that implies a more slender vertebral collumn. The estimated vertebral count for C. megalodon is 190-200, not 200+ From Gottfried et al (1996): " Carcharodon megalodon had probably had well over 130 pre-caudal centra 200+ total centra if the Belgian specimen is, in fact, missing a part of the anterior column as well as many of the caudal centra." (Page 64) And vertebral count doesn't implies slenderness. Vertebral morphology can imply slenderness or robustness. What remains are you talking about? Nothing I have seen of O. obliquus puts the great white to shame, other than the size reached in large specimens. Look at these comparisons:  Look at the roots of teeth of O. obliquus:   Roots are very thick, indicating highly robust physiology. This is how O. obliquus would be built in life:  What suggests that C. megalodon was bulkier than predicted by regressions based on extant lamniforms? Especially, what suggests it was both bulkier and longer? Megalodon remains are thick and massive, implying heavily built physiology accordingly. Even at parity, Megalodon remains are thicker then that of great white shark. I think that regression model proposed by Gottfried et al (1996) is good for reference, it covers ontogenetic aspects of shark growth. However, no regression model captures minute details of physiology of megatoothed sharks. |
|
|
|
Post by Grey on Mar 5, 2014 19:41:16 GMT 5
The great white shark has a higher vertebral count than the whale shark, it is not more slender, it's even the other situation which prevails, though the whale shark itself is a robustly built animal. Now Life, does the fact the dention is heavily built necessarily implies that the whole body was robust ? It is possible but after all Gottfried based this on an extrapolation from the white shark itself. If I remember right, the mosasaur Prognathodon has a very robust skull and jaws but a rather body. Megalodon with a somewhat more slender body shape is not impossible, still likely more robust than the basking shark according to Kent. I don't think meg and Cetorhinus centra are all that similar. The large meg vertebrae I've seen are pretty robust. They don't have the more tightly packed septa of the lamnids and odontaspids, but overall they're much more solidly built than those of Cetorhinus. The vertebrae of Cetorhinus are so poorly mineralized that they are almost always badly distorted during fossilization. It's very rare to see this type of distortion in meg vertebrae. Anyway, I note that Kent's revised reconstruction from Gottfried, not yet published, still depicts a heavily built shark. |
|
|
|
Post by Life on Mar 5, 2014 20:07:25 GMT 5
The great white shark has a higher vertebral count than the whale shark, it is not more slender, it's even the other situation which prevails, though the whale shark itself is a robustly built animal. Good point Now Life, does the fact the dention is heavily built necessarily implies that the whole body was robust ? It is possible but after all Gottfried based this on an extrapolation from the white shark itself. If I remember right, the mosasaur Prognathodon has a very robust skull and jaws but a rather body. Mosasaurs are invalid analogues for megatoothed sharks in matters of physiology. However, you can check if prognathodon is heavily built among mosasaurs. Here is an artistic depiction of prognathodon: media.theiapolis.com/d4/hK0/iR3P/k4/lR4G/wW7/prognathodon.jpg--- Size of tooth does not implies heavy built, root thickness does. Megalodon with a somewhat more slender body shape is not impossible, still likely more robust than the whale shark according to Kent. I don't think meg and Cetorhinus centra are all that similar. The large meg vertebrae I've seen are pretty robust. They don't have the more tightly packed septa of the lamnids and odontaspids, but overall they're much more solidly built than those of Cetorhinus. The vertebrae of Cetorhinus are so poorly mineralized that they are almost always badly distorted during fossilization. It's very rare to see this type of distortion in meg vertebrae. Anyway, I note that Kent's revised reconstruction from Gottfried, not yet published, still depicts a heavily built shark. Nothing implies that Megalodon was somewhat slender in its physiology. Its fossils do not give this kind of vibe. Even if Megalodon could become a 100 ton behemoth, this would not have much impact on its performance, constraints imposed by dry land on mobility of animals with respect to their size do not apply to animals in the marine environment. Muscle strength and mass, fin structures, skin morphology and vertebral column morphology influence performance of a shark. Megatooth sharks possibly had additional biological adaptations for high performance at gigantic sizes that are not found in extant sharks. This is good depiction of Megalodon:  |
|
|
|
Post by Grey on Mar 5, 2014 20:21:37 GMT 5
I do believe however in Kent's future published suggestion, in that the largest members of the species may have been primarily huge, virtually almost unattackable scavengers able to intimidate any other predators from kill because of the physiological constraints that implies that a 100 tonnes Megalodon may have been still fast but not manoeuvrable enough, much less than a smaller sized individual. In that it couldn't catch a smaller, slower but more manoeuvrable prey. This is based on the constraints due to the relatively simplistic cardiovascular system in sharks. Even if Megalodon had specific adaptions, it pushed this anatomical system at its apex, and thus may have encountered constraints.
This is not a shocking suggestion to me, this corresponds to what had been suggested too for T. rex, changing lifestyle with time, small game hunter while neaonate, bigger and more generalist while juvenile, big game specialist at adult stage and huge scavenger at later stage, following the pods of whales to get old, sick whales members.
Now, this is not published yet, though it has been reviewed already.
|
|
|
|
Post by Life on Mar 5, 2014 20:25:11 GMT 5
Largest Megalodons would have gone for big game mostly. Their is no reason to believe that they couldn't catch a whale.
|
|
|
|
Post by Grey on Mar 5, 2014 20:28:00 GMT 5
Largest Megalodons would have gone for big game mostly. Their is no reason to believe that they couldn't catch a whale. It is a matter of interpretation. From Brett : Yes, I suspect a gradual change in diet much as you suggest as megs grew larger. This is true for many large carnivores as they get older, so it seems reasonable for meg. I have to admit that a 18+ m meg would certainly be intimidating! As you suggest, carcass availability would be a problem. I could envision really large megs following pods of whales to scavenge carcasses or to possibly attacked injured or diseased individuals.
|
|
|
|
Post by Life on Mar 5, 2014 20:34:11 GMT 5
What Kent have to say about efficiency of Megalodon's muscles?
Very large Megalodons may have possessed extremely powerful (white) muscles to support their active lifestyle. This factor alone would have enabled them to chase even healthy whales and catch them. Another factor is hunting strategy, ambush tactics can be beneficial in deep waters. In addition, even a single bite from an adult Megalodon would be enough for the prey item to falter.
|
|
|
|
Post by theropod on Mar 5, 2014 20:39:58 GMT 5
The robusticity of teeth is not related to the robusticity of the whole animal. Sharks with very robust teeth can be relatively slender-bodied and vice versa. that’s a matter of tooth function and perhaps of size-related necessities, not of overall built.
Otodus and Parotodus have a peculiar tooth morphology for a shark, that’s why their roots also have a peculiar morphology. the Carcharocles lineage derived from these sharks as far as we know, so no wonder they retained some of their features (teeth very thick compared to lamnids), although their morphology became more analogous to Carcharodon.
@life you were the one to argue C. megalodon has proportionally longer centra (and that is easily visible in the pictures you posted, although papers so far do not seem note this difference, perhaps indicating this is a result of the position of the vert).
If its centra are longer, that makes its vertebrae less robust at lenght parity (and, at least if its vertebral count was higher, its whole vertebral collumn), and lenght parity is what we are talking about since the body mass is estimated from total lenght and total lenght is closely related to the lenght of its vertebral collumn.
A more elongate vertebral collumn is not an argument in favour of a more bulky body.
If its total lenght was greater relative to its jaw apparatus, that also does not imply greater bulk at the same lenght. Unless you are envisioning it as having undersized jaws for its body size, which is complete speculation and not consistent with its presumed lifestyle as a macrophagous predator.
As predatory lineages grow bigger, their weaponery almost always displays hypermorphosis, not the opposite. The dentition lenght in C. carcharias relative to body lenght also does not show negative, but slight positive allometry, so why should C. megalodon follow the exact opposite trend (Why should it then have a large, sharp, raptorial dentition)?
I cannot see how there are any Otodus or Carcharocles remains providing evidence that these animals had considerably more robust body shape. The best there is to conclude about it are vertebrae, and those are actually more elongate.
Both a greater bulkiness and a greater total lenght than predicted at once are not compatible with reasonable assumptions about anatomy.
It is possible C. megalodon had a greater total lenght, if, for example, it had an enlarged caudal lobe like Alopias, or simply a more elongated body (or proportionally longer tail). It is also possible it’s built was stockier, if it was shorter and/or had proportionally bigger jaws. But both are speculation without evidence.
Gottfried et al. assume a very bulky, but also a relatively short animal. Someone envisioning a 30m Megalodon would envision an animal with a thresher-shark like tail, but not considerably heavier than a 20m shark with normal proportions.
Artistic reproductions remain artistic reproductions. Dimitry Bodganov has made hundreds of them, focusing on synapsids. He certainly has not conducted extensive studies on each of them, and surely he has not observed subtle evidence for O.obliquus being particularly robust, that was not noted in any publication. That being said the Otodus in the picture also does not put a Great white to shame in terms of its robusticity. I’m not talking size here, but how stocky it was. Obviously if you compare two vertebrae that are different in size the bigger one will be more massive, that does not mean it is also more robust when scaled at the same lenght.
Using the size trends observed in great whites is good enough for now. But it is not realistic for now that C. megalodon was much bulkier than any extant lamnid (lamnids themselves being very bulky animals), nor that it was much longer than suggested by its jaw size.
I would encourage research in other lamniforms to see whether there is a better analogy than Carcharodon. As long as we don’t have one, there is no point in speculating about it.
|
|
|
|
Post by Grey on Mar 5, 2014 20:44:20 GMT 5
LifeI don't know. The problem is that the white muscles present in the white shark are not present in the whale shark wich is the only shark to approach the tremendeous size of Megalodon (including the basking if we count the wide range of possible adult size for Megs). Kent bases this on that the whale shark is the only cartilaginous fish to have pushed it size so far, and that it is a filter feeder. So that it is probably the maximum size for a shark with this lifestyle. Thus, he doubts that a shark as large or larger could have kept a lifestyle just as active as the modern laminds which are at best up to around ~6 m long. It is true that Meg may have had unique adaptations that we're not aware about as it is so much bigger than any other predatory shark, but will these unknown adaptions be enough to get around the potential constraints of such a size ? I guess this is impossible to know. But remember that this is applied to potential sizes above 18 m.
|
|
|
|
Post by Grey on Mar 5, 2014 20:48:11 GMT 5
Regarding meg body shape, Kent's depiction is the most advanced and modern to date. Except for the longer nose and more streamlined chondocranium, it is not much different than Gottfried's. Meg's robustness is wide consensus among specialists as far as I know even if one considers Gottfried's extrapolation to be too bulky.
|
|
|
|
Post by theropod on Mar 5, 2014 21:44:19 GMT 5
I’ve updated my lamnid TL/mass spreadsheet: carcharocleslenghtweightregressions2.xls (10.5 KB) Added the regressions for lamnids from the Kohler et al. paper. Lamna, contrary to previous claims, does not give astronomic weight figures. Quite the contrary actually. But note that the taxon has a very small sample size, so I’d recommend also checking the figure while excluding it. |
|
|
|
Post by Grey on Mar 5, 2014 21:50:13 GMT 5
Skeptical, Lamna is widely recognized as a particularly heavily built lamnid. The small size of the sample might be an eye opener.
|
|
|
|
Post by theropod on Mar 6, 2014 0:22:26 GMT 5
Could you post a source for that claim?
A sample of 13 certainly isn’t amazing, especially compared to the others, mostly in excess of 100. Still, it’s better than direct sizing from single individuals (~13 times better...).
Here are mean estimates for TLs in meg’s size range in 0.5m intervals:
TL(m) Body mass(t)
10 10.3
10.5 12
11 13.9
11.5 16
12 18.2
12.5 20.7
13 23.4
13.5 26.3
14 29.5
14.5 32.9
15 36.6
15.5 40.5
16 44.8
16.5 49.3
17 54.1
17.5 59.2
18 64.7
18.5 70.4
19 76.6
19.5 83
20 89.9
When omitting the regression for Lamna, the results are approximately 2-2.4% greater in this size range, so it does not have what I’d call a significant impact on the whole thing.
|
|
|
|
Post by theropod on Mar 6, 2014 0:43:18 GMT 5
 This seems to be the same regression, all the parameters are identical, although the sample here is claimed to be 15... |
|
