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![[Plateosaurus engelhardti ] in Triassic morning. by Guindagear](https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/41df78b1-f116-4998-a5e0-1e2885d7fd44/d7xd2ey-ae74799c-0b70-41e9-bf7d-7fa3c438bdd2.jpg/v1/fill/w_500,h_250,q_70,strp/_plateosaurus_engelhardti___in_triassic_morning__by_guindagear_d7xd2ey-250t.jpg?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1cm46YXBwOjdlMGQxODg5ODIyNjQzNzNhNWYwZDQxNWVhMGQyNmUwIiwiaXNzIjoidXJuOmFwcDo3ZTBkMTg4OTgyMjY0MzczYTVmMGQ0MTVlYTBkMjZlMCIsIm9iaiI6W1t7ImhlaWdodCI6Ijw9NjQwIiwicGF0aCI6IlwvZlwvNDFkZjc4YjEtZjExNi00OTk4LWE1ZTAtMWUyODg1ZDdmZDQ0XC9kN3hkMmV5LWFlNzQ3OTljLTBiNzAtNDFlOS1iZjdkLTdmYTNjNDM4YmRkMi5qcGciLCJ3aWR0aCI6Ijw9MTI4MCJ9XV0sImF1ZCI6WyJ1cm46c2VydmljZTppbWFnZS5vcGVyYXRpb25zIl19.IWHhQcx6tgc9G8WFR4hEUhRJ7RR8nBPxNSTx6b5f5xI)
[Bg] Throughout the years, there have been various attempts to reconstruct the extinct giant shark, Carcharocles (Otodus) megalodon. Most of these have based on its modern relative and closest living analogue, the great white shark Carcharodon carcharias.
That is the first thing important to note here, because all the following is exclusively based on the white shark. Of course the very real possibility remains that C. megalodon looked nothing like envisioned here, but that is useless as long as there are no data to aid in coming up with a more likely reconstruction. Most people seem to be quite fine with using the white shark as an analogue, as am I, and it’s primarily those people that this is targeted at.
There is a common trend of portraying C. megalodon as a sort of overgrown great white shark on steroids. As those who have read certain previous posts of mine already know, this is partly based on real data, and partly on conjecture, but so far it has not been possible to largely eliminate that element of conjecture because of the lack of quantitative methodology used in reconstruction.
The real data are two-fold:
> Firstly, the fossil teeth of C. megalodon are thicker and (relative to their height) wider than those of white sharks, at least the larger anterior and anterolateral ones. At a similar overall size (width, length or circumference) of the jaws, the anterior teeth of megalodon would hence be shorter and stouter, i.e. more robust. Most likely, this is an effect of both the gigantic size and the effects of scaling (i.e. allometry), it’s phylogenetic heritage (other otodontid teeth are also very thick labiolingually), and its behaviour (dealing with, in absolute terms, very large prey items). However the effect this might have on jaw morphology is difficult, of not impossible, to estimate.
> Secondly, the allometric scaling of body mass in extant white sharks suggests that larger sharks tend to be more robust than smaller ones, which can be extrapolated, albeit with questionable relevance, to the size of megalodon. There have been six published regression equations for deducing white shark mass from total length (Fig. 1), which provide the data I am relying on here.
Fig. 1: Length-weight relationships of C. carcharias according to various published sources (See references).
These data indeed support that larger sharks should (most likely) be more robust. Which is about where the considerations typically end. But is there a way to approach this question with a bit more rigor than just taking an arbitrary great white shark and bulking it up by an arbitrary amount? It turns out that yes, there is.
[M&M] First, let’s establish what sizes will be used. Changing these slightly will not have major effects, as you are going to see. But for the sake of comparison, I am going to be using a 5m great white shark (good-sized, but still a relatively common size for the species, which is always advantageous) and a 16.8m megalodon (a size suggested in a recent SVP abstract (Perez et al. 2018, SVP abstract volume p. 196) for a specimen with a complete, associated dentition, and also a size I have shown previously). This is not supposed to be a comment on megalodon maximum or average sizes, but on its morphology, so I’ll leave it at that.
The equations suggest that the 16.8m megalodon (46,112-56,797kg) would be between 0 and 23% heavier than an isometrically scaled great white shark of (1,083-1,294kg at 5m). Using the average of all 6, the megalodon is 13.6% heavier than the great white at the same length. Since length is fixed, we can attribute this entire increase in robusticity to the body cross-section, the function of width and depth. Assuming the increase is the same in both dimensions, we get the percentage of increase by taking the cube root, giving us 6.6%. So in other words, we should be drawing megalodon 6.6% wider and deeper-bodied than a great white shark. That’s as much as I’ve written previously. Now, the debatable part is obviously going to be what great white that should be, because most pictures of great whites, especially nice ones in orthogonal views without deformation or distortion, have no weight measurements attached to them.
So to solve this problem, I constructed a digital 3d model of a great white shark in Blender using lateral and top-view pictures of great whites and reference drawings (e.g. ¹) as a basis. I modeled the trunk, pectoral, first dorsal and tail fins separately and then unified them using a boolean modifier. The other fins were omitted from the model to avoid it becoming overly complex and computationally demanding, as they would add very little mass anyway. Then I adjusted its width and depth (isometrically) to make it fit the predicted body masses.
[RESULTS] The resulting reconstruction (Fig. 2) suggests a robust, but streamlined body shape, even for the maximum model scaled to 56.8t.
The bulk increase necessary in terms of scaling of the body cross-section is not extreme. Even the maximum model, based on Gottfried et al.’s results, is still well-within the range of variation seen in extant great whites. The great white reconstruction also lends weight to the drawn representations of white sharks as commonly found in reference books (e.g. Compagno 1984) being good representations of the statistically typical proportions of the species as a whole.
The bulk increase necessary in terms of scaling of the body cross-section is not extreme. Even the maximum model, based on Gottfried et al.’s results, is still well-within the range of variation seen in extant great whites. The great white reconstruction also lends weight to the drawn representations of white sharks as commonly found in reference books (e.g. Compagno 1984) being good representations of the statistically typical proportions of the species as a whole.
Fig. 2: Surface model of C. carcharias and C. megalodon (16.8m) scaled to match predicted volume. Specific gravity is assumed as 1.0. Green model reflects the mass predicted by the allometric equation in Gottfried et al. 1996, which suggests the highest body mass at almost 57t. Blue and red models reflect the mean of masses predicted by 6 regression equations (see references). Scalebars: 1m each. Grid: 2m per cell. Human diver: ~1.8m standing height.
[D&C]One caveat to this is that I did not model the buccal cavity of the shark, as I did not have a reference for it. With the mouth closed, I assumed it would not greatly decrease the volume of the shark for given external dimensions as long as the mouth was closed, as modeled here. If indeed it does, then this might result in an underestimation of the external dimensions of a shark of a given body mass, although I don’t think this is likely. Should you have information to the contrary, please let me know.
There have been well-founded suggestions that megalodon would have had a proportionately larger tail fin to maintain high speeds and activity levels at its large size, and I agree with this, as it is consistent with a possible higher vertebral count and with the scaling effects at its large body size. This would largely have a negligible impact on mass. In the great white model at 1.2t, the tail fin masses less than 30kg. Even doubling the fin size would not greatly increase the size of the model. In the above representation, the fins are scaled up in width and height along with the body largely for practical reasons and because there is no way to precisely estimate their proper size.
Most likely, the tail fin would be somewhat larger in life than it is portrayed here, which is what I am going to use for the drawn reconstruction I plan to follow this up with.
So with that out of the way how does this leave extremely bulky megalodon reconstructions, such as these?
Well, as with great whites, there would have been a large level of variation. These estimates represent the mean. This is, for obvious reasons the most sensible representation for the species. This does not mean that there couldn’t have been some megalodon individuals that were in fact as bulky as portrayed, just like there are some great white sharks that are ridiculously bulky compared to the normal body shape of the species.
It’s just that they get far too much representation, and are ironically often considered to be the most reliable reconstructions. On the other hand, it’s also entirely plausible for some megalodons to not be bulkier than normal great whites at all. One of the aforementioned equations, McClain et al., in fact did not find allometry in their dataset at all.
Bottom line: The most accurate and rigorous data suggest megalodon should be portrayed on average ~7% deeper and wider-bodied than a typical great white shark. The above model (blue reconstruction, Fig. 2) illustrates the resulting body shape. Various scaling equations found slightly different values varying from 0 (i.e. isometry) to 11%, but it does not seem like such differences would have a massive impact on how bulky the animal would look, at least not sufficiently to lend support to the many extremely robust reconstructions out there. These appear to not be supported by any quantitative data and should be considered representations of unusually heavyset, pregnant and or full-stomached individuals, not a typically robust, large megalodon.
References:
Casey, John G.; Pratt, Harold L. 1985. Distribution of the White Shark, Carcharodon carcharias, in the Western North Atlantic. Memoirs of the Southern California Academy of Sciences, 9 (Biology of the White Shark, a Symposium) pp. 2-14.
Compagno, L.J. 1984. Sharks of the world: an annotated and illustrated catalogue of shark species known to date. FAO Fisheries Synopsis Volume 4 (No. 125).
Kohler, Nancy E.; Casey, John G.; Turner, Patricia A. 1995. Length-Length and Length-Weight Relationships for 13 Shark Species from the Western North Atlantic. Fishery Bulletin, 93 pp. 412-418.
McClain, Craig R.; Balk, Meghan A.; Benfield, Mark C.; Branch, Trevor A.; Chen, Catherine; Cosgrove, James; Dove, Alistair D.M.; Gaskins, Lindsay C.; Helm, Rebecca R.; Hochberg, Frederick G.; Lee, Frank B.; Marshall, Andrea; McMurray, Steven E.; Schanche, Caroline; Stone, Shane N.; Thaler, Andrew D. 2015. Sizing ocean giants: patterns of intraspecific size variation in marine megafauna. PeerJ, 3 (715) pp. 1-69.
Mollet, Henry F.; Cailliet, Gregor M. 1996. Using Allometry to Predict Body Mass from Linear Measurements of the White Shark. In: Klimley, Peter A.; Ainley, David G.: Great White Sharks: the biology of Carcharodon carcharias. San Diego, pp. 81-89.
Tricas, Timothy C.; McCosker, John E. 1984. Predatory Behaviour of the White Shark (Carcharodon carcharias) with notes on its biology. Proceedings of the California Academy of Sciences, 43 (14) pp. 221-234.
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