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Post by elosha11 on Apr 19, 2016 19:04:42 GMT 5
Here's a plausible scenario in my mind. If Megalodon's larger prey base did begin to seasonally migrate and go to arctic locations Meg did not follow (which in turn could have been influenced by climate change), then Megalodon could have been outcompeted for remaining smaller prey items by raptorial sperm whales, orca ancestors, and smaller sharks like prehistoric great whites. In other words, if Meg's historic larger prey base dried up, and Megalodon gradually had to start relying more on pinnipeds, smaller cetaceans, and perhaps other more difficult to kill predators as its sources of food, it could have been been very vulnerable. I can easily see smaller predators being far more adapted to hunting smaller, more agile prey that would have been difficult for Megalodon to hunt and of much less caloric value. If Megs were facing this pressure, plus if its juveniles were subject to attack/competition from these smaller predators, it may have been a one-two punch. But I still think it likely that environmental factors and/or baleen whales biological adaptations for colder water played the triggering and largest role in Meg's demise. So in a way if Meg's prey developed the ability to escape to colder water, or developed faster swimming mechanisms as some have theorized, Meg could still have been "outcompeted."
However, I doubt smaller predators could have been the primary cause of the extinction. There were likely other more fundamental factors at play which allowed them to fill a niche for which Megalodon was not as well suited.
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Post by theropod on Apr 19, 2016 19:17:19 GMT 5
^Sounds reasonable.
Also I don’t think that’s completely contradicting what the new study proposed, although their focus ended up being more on the competitors. But as they wrote, further studies are needed.
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Post by Grey on Apr 19, 2016 19:36:14 GMT 5
What is needed though is a qualitative study of meg's prey-items range.
The only study on this is Purdy 1996 and is not very precise.
As well of those odontocetes, including the Early orcicines.
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Post by Life on Apr 20, 2016 13:10:21 GMT 5
Here's a plausible scenario in my mind. If Megalodon's larger prey base did begin to seasonally migrate and go to arctic locations Meg did not follow (which in turn could have been influenced by climate change), then Megalodon could have been outcompeted for remaining smaller prey items by raptorial sperm whales, orca ancestors, and smaller sharks like prehistoric great whites. In other words, if Meg's historic larger prey base dried up, and Megalodon gradually had to start relying more on pinnipeds, smaller cetaceans, and perhaps other more difficult to kill predators as its sources of food, it could have been been very vulnerable. I can easily see smaller predators being far more adapted to hunting smaller, more agile prey that would have been difficult for Megalodon to hunt and of much less caloric value. If Megs were facing this pressure, plus if its juveniles were subject to attack/competition from these smaller predators, it may have been a one-two punch. But I still think it likely that environmental factors and/or baleen whales biological adaptations for colder water played the triggering and largest role in Meg's demise. So in a way if Meg's prey developed the ability to escape to colder water, or developed faster swimming mechanisms as some have theorized, Meg could still have been "outcompeted." However, I doubt smaller predators could have been the primary cause of the extinction. There were likely other more fundamental factors at play which allowed them to fill a niche for which Megalodon was not as well suited. My friend, I seriously doubt that other macropredators would be willing to attack, kill and/or disperse neonate and juvenile Megalodon in the feeding grounds. This would have been an extremely risky strategy for any potential competitor. To give you an idea, a juvenile Megalodon was responsible for the death of even a massive (rorqual) whale during Pliocene. Ever heard of something like this in modern times? I seriously doubt that any macro-predatory species of shark would (or could) deny Megalodon access to similar food sources. This isn't how things work out at present. For example, three white pointers and two tiger sharks were documented feasting on the carcass of same dead sperm whale: mashable.com/2015/07/05/hungry-sharks-australia/#7Dc5Qih5Iiq0More importantly, Paleontologist Dana Ehret figured out faster growth rate in Megalodon than the modern great white shark through fossil evidence: Striving to find meals may explain a key difference between C. megalodon and modern great whites. From the same vertebra he used to calculate the shark’s birth size, Ehret calculated that megalodon grew comparatively faster than great whites. “They just wanted to get a big as they could as fast as they could, and they were a big shark to begin with,” Ehret says. Packing on the pounds would have allowed young megalodon to start taking larger prey, and would have prevented them from ending up in the stomachs of their sharp-toothed cousins. “They were trying to get big and get out into more open waters to not have as much competition for resources,” Ehret says.Source: phenomena.nationalgeographic.com/2014/08/13/megalodon-the-monster-sharks-dead/I think that faster growth rate was an ideal anti-predator strategy for Megalodon! It makes sense. One paper (Sylvain et al., 2010) captures the ecological replacement process (among macro-predatory shark) through the fossil records within the Mediterranean region. It reveals that great white shark filled the ecological void created in total absence of Megalodon over time. The paleobiogeographic story of the modern White Shark in the Mediterranean is simpler according to the fossil record. The extant species has probably taken advantage of the rapid restoration of the normal marine conditions at the beginning of the Pliocene to newly invade the place and to gradually occupy the empty niche of high marine predators in the Mediter-ranean (BIANUCCIet al. 2000), especially after the total disappearance of M. megalodon at the end of theEarly Pliocene (MARSILI2006).The paper does not dismisses the possibility of competition between the two species but it was largely inconsequential. Moreover, raptorial delphinids of the era were not large and powerful enough to displace Megalodon either. However, "of much less caloric value" point of yours says it all. It is a good point. --- My assessment is that a major diversity crash (during Late Miocene) set the stage of extinction for C. megalodon. Survivors began to migrate to polar regions in search of food on seasonal basis and C. megalodon could not make inroads into such regions as they became increasingly cooler. Temperatures in polar regions go noticeably below 1 degree Celsius on average. This is coincident with C. megalodon's inability to expand its reach in polar regions on consistent basis during Pliocene. Have a look at the differences in distribution patterns of Megalodon over time:  Look at the presence of Megalodon in waters above much of Europe during Middle Miocene, Late Miocene and Pliocene. You can notice absence of Megalodon in such waters during Pliocene. Now, the food supply that existed in the latitudes suitable for Megalodon had significantly dwindled with passage of time and leftovers were of much less caloric value to sustain this species. This is qualitative observation and it makes more sense than the competition theory. This is a good observation from Pimiento et al. (2016): The coincidental timing of the decline in global abundance of C. megalodon and the drop in diversity of mysticetes and odontocetes during the late Miocene suggests that the evolutionary history of cetaceans played a role in the extinction of C. megalodon.There were additional possibilities such as cannibalism and loss of suitable nursery regions with passage of time. Figuring out extinction drivers of Megalodon isn't easy. This would require a long and painstaking effort to piece together various environmental and biotic developments over time and inclusion of qualitative assessments in the mix. |
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Post by Life on Apr 20, 2016 13:34:47 GMT 5
The teeth resemble those of other big-game hunting stem-physeteroids and show signs of wear, in almost every description of a stem-physeteroid that had these features, an inferred orca-like ecology has been mentioned, so why would they not be big-game hunters? Teeth do not tell you everything. Look at the skeleton of False Killer Whale, its skull in particular:  And False Killer Whale isn't a big-game hunter. If False Killer Whale had been a prehistoric species, some would have assumed it to be a big-game hunter based on the logic you subscribe to. You cannot prove that even the likes of Livyatan were big-game hunters. Even some types of modern Killer Whales are not big-game hunters. |
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Post by creature386 on Apr 20, 2016 13:39:08 GMT 5
LifeActually, I don't believe other macropredators would have needed to risk battles with little Megalodons. Eating away their food before they can reach it would have been competition as well.
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Post by theropod on Apr 20, 2016 15:26:53 GMT 5
Very true. Teeth don’t tell you everything. There are tons of limitations to what teeth can tell us, e.g. about size, anatomy, physiology… But there are also things that teeth are comparatively good at telling us, and using teeth to draw data about their owner’s diet is among the more warranted lines of inference considering that there is a very direct link between both. And correctly so, in a way, since it does have that capability and has at least occasionally done so. As far as large aquatic predators are concerned, it’s reasonable to consider that a big-game-hunter, since even the most macrophagous apex predators still have generalist diets and largely subsist on animals smaller than themselves (the only exception that comes to mind are some orca populations that specialize in baleen whales, but see the next point). All the more so considering Pseudorca’s behaviour is nowhere near that well-studied owing to its generally pelagic habits. As you remark correctly, not even all killer whales are big-game hunters, but the species is usually considered a big-game hunter (even though it has a very varied diet); some of its members somewhat regularly hunt big game, that’s enough. AFAIK the teeth of the various types of orcas aren’t much different from one another, except when it comes to wear patterns, and some of those engage in big-game hunting, which means that the functional anatomy of the species overall is suitable for it. I.e. other’s could do it too (at least as of now, eventually O. orca will have undergone speciation), but they specialize in different prey on a cultural basis. Much like some humans don’t eat meat, but it doesn’t mean they, or humans overall, are incapable of it, or not physically adapted for it (don’t get me wrong, I don’t think there are orcas who have ethical problems with eating other mammals, but orca hunting techniques still include lots of learned behaviour that is segregated between the different ecotypes). Hence why I wrote "potentially macrophagous". You seemed interested in whether there were potential odontocete competitors, and there are. The evidence for it is relatively thin (large, robust teeth with thick enamel caps and pronounced tooth wear, with pronounced similarity to those of relatives for which we have more substantial data, e.g. Lambert et al. 2014), the evidence against it is non-existent. If they were, that would be more consistent with the hypothesis. The irony is that P. crassidens was indeed described from a fossil and considered extinct, though extant individuals were discovered later on. Owen’s description sadly seems to be devoid of inferences about its diet, otherwise that would have been a fun way to test this. But by the same logic, we cannot prove C. megalodon was a big game hunter either. The largest confirmed prey items are what? 7m cetotheres? That’s not any bigger in relation to the shark than a common dolphin or tuna is in proportion to P. crassidens. So obviously in any extinct taxon, more is being considered than trace-fossil evidence when determining its ecology. creature386: …at which a larger number of smaller individuals might be at an advantage. Lambert, Olivier; Bianucci, Giovanni; Beatty, Brian L. (2014): Bony outgrowths on the jaws of an extinct sperm whale support macroraptorial feeding in several stem physeteroids. Naturwissenschaften 101 (6) pp. 517-521. |
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Post by creature386 on Apr 20, 2016 16:09:10 GMT 5
creature386: …at which a larger number of smaller individuals might be at an advantage. This is of course true, but I have to admit that my post about the larger number of smaller individuals got refuted by what Life posted on the fast growth rate of Megalodon, so small Megalodons were maybe not as common as I thought. |
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Post by theropod on Apr 20, 2016 17:24:58 GMT 5
I was referring to the odontocetes and smaller sharks actually. For every adult megalodon there could be a number of those, so if they all competed for small to mid-sized prey with adult megalodon because large whales wheren’t readily available, more often than not one of the smaller predators would catch it first. If prey was comparatively scarce (compared to miocene levels), that could be a crucial difference.
A faster growth rate has nothing to do with how common juveniles are. It’s certainly very plausible (Was there concrete evidence posted though? overlooked the quote, I just didn't find it in the article because I searched for "growth") that C. megalodon had a faster growth rate compared to known sharks, contributing to its giant size. But the only way there would be fewer juveniles because of that would be if maturity was pre-displaced too, i.e. reached at a younger age and a proportionately smaller size.
But hasn’t it been suggested that C. megalodon also had an excessively long lifetime? If so, then that’s very unlikely, more likely is that its size was the result of an acceleration of growth in combination with a longer lifetime during which to grow.
In any case, the statistical data suggest that well over half of all C. megalodon teeth are from juveniles. They may have lost their teeth at a higher rate, but even so, there were lots of them. But even if there weren’t, that wouldn’t make them less important for the species’ survival.
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Post by Grey on Apr 20, 2016 18:19:40 GMT 5
Regarding the size of the prey items, the Kallal et al. bone suggests that an adult meg wasn't probably restricted to preys under 7m TL. Going by great whites standards, whales over 40 tons were probably vulnerable to predation. But the exact occurences of such hypothetical predation events need to be studied further and how many big whales species exusted with the shark and fell in its prey range.
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Post by creature386 on Apr 20, 2016 18:39:53 GMT 5
A faster growth rate has nothing to do with how common juveniles are. It’s certainly very plausible (was there concrete evidence posted though?) that C. megalodon had a faster growth rate compared to known sharks, contributing to its giant size. But the only way there would be fewer juveniles because of that would be if maturity was pre-displaced too, i.e. reached at a younger age and a proportionately smaller size. You can look at it that way: If they grew slower, they would have needed to produce more offspring to ensure that at least some of them reach adulthood than they would have at a fast growth rate. According to the article Life cited, the evidence for the growth rate lies in the vertebrae. |
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Post by theropod on Apr 20, 2016 23:46:34 GMT 5
A faster growth rate has nothing to do with how common juveniles are. It’s certainly very plausible (was there concrete evidence posted though?) that C. megalodon had a faster growth rate compared to known sharks, contributing to its giant size. But the only way there would be fewer juveniles because of that would be if maturity was pre-displaced too, i.e. reached at a younger age and a proportionately smaller size. You can look at it that way: If they grew slower, they would have needed to produce more offspring to ensure that at least some of them reach adulthood than they would have at a fast growth rate. Only given that a faster growth rate results in reaching maturity earlier, which likely isn't the case. |
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Post by creature386 on Apr 20, 2016 23:58:58 GMT 5
Well, Life referred to it as an ideal anti-predation strategy which would maybe make other species survival strategies (like producing as much offspring as possible) less necessary. Though I have to admit that fast growth and r-strategism are not mutually exclusive.
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Post by theropod on Apr 21, 2016 2:50:58 GMT 5
Well, Life referred to it as an ideal anti-predation strategy which would maybe make other species survival strategies (like producing as much offspring as possible) less necessary. Though I have to admit that fast growth and k-strategism are not mutually exclusive. You mean fast growth rate and r-strategism? Indeed it isn't, fast growth is a perfect strategy for r-strategists since there is little parentaal investment in the young, making them vulnerable as long as they are small. I'm not saying it isn't a good anti-predator strategy. It is an excellent one, simply because being large is a great anti-predator strategy aand growing faster means you get large faster. All I'm saying is that it doesn't mean there are fewer juveniles, it's just that juveniles are bigger. Obviously, a juvenile megalodon is the size of an adult great white shark. If you have to grow to 10m before you mature, you're going to have to grow 2.5 times as fast as an animal that matures as 4m just to do it in the same amount of time. |
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Post by creature386 on Apr 21, 2016 4:22:32 GMT 5
Yes, I meant r.
The problem is that I interpreted the article slightly different. It didn't specify if the faster growth rate was in absolute terms or in relative terms (because a faster growth rate in absolute terms as in more cm/per year seems to be a rather trivial information).
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