A quick word or two regarding the size of everyone’s favourite extinct shark, Carcharocles (also known as Megaselachus, also known as Otodus, also known as Carcharodon–That might be a record for the largest number of genus names in use for a single species in the last two decades’ literature) megalodon.
As we have learned to expect from something really large and with such scrappy remains (isolated teeth, with vertebrae or more complete dentitions being known in a small number of poorly- to undocumented cases), people greatly enjoy speculating about its size. Only in this case, people advising the use of conservative methods are often just drowned out (and trolled and bashed into submission) by the internet movement that understandably enjoys the thought of a species of 60ft shark that killed everything in its path with a single bite, aided by the fact that too many of those people mistake scientistsfic’ enthusiasm (or pretty much any statement they can find anywhere,never mind that it’s often some discovery channel documentary they are using as a source) for some sort of endorsement of their sensationalism.
Scientists have also made enthusiastic statements about 1.7m sauropod footprints, its funny there aren’t thousands of people all over the internet celebrating that.
That isn’t to say that speculating is wrong–we wouldn’t get anywhere without it. And certainly even liberal methodologies deserve their place (in EVERY taxon).
But that doesn’t mean they are the only ones deserving of that, and it doesn’t mean we that place is necessarily as the most commonly cited and reproduced estimation, neither does it mean the only way to get an estimate that doesn’t get labeled as "too low" should be by searching out the individual Great White Shark with the largest body length and proportionately smallest teeth to estimate length, then using the predictive equation for body mass on that which yields the highest results, to come up with such extreme but undeniably popular estimates as the 20m+ and 100t+ figures you can read everywhere.
There is a fundamental difference between nitpicking such a figure to represent this animal’s body size in general, and publishing it as a hypothetical (and, as noted by the authors, unreliable) estimate, as was done by the scientific study that people generally cite when its about its size (see below).
That gets even more comical when it gets to discussing how C. megalodon compares to other giant predators. I can’t help but be amused by their confidence in such biased comparisons when representing C. megalodon as an 18-20m critter even though those are the sizes of a handful of individuals in thousands. To jump ahead a bit, there appears to be quite some sensationalism in claims that it is the biggest predator of all time.
Leaving aside the implication of certainty that is expressed in this statement despite the incompleteness of the fossil record (and its fossil record in particular, with most animals people don’t make confident size estimates from teeth), there is a plain and simple lack of data supporting such claims. Certainly it is among the top 2 or 3 contenders, being comparable in size to its contemporary Livyatan melvillei, to the point where it is warranted to not make any statement about which is larger considering the lack of sufficiently complete fossils, and the lack of a sufficiently large sample of the latter’s population.
I don’t have a problem with the thought that 20m megatooth sharks existed, just like how 1t polar bears exist(ed), but this estimate is useless for all (scientific) intends and purposes because it does not base on solid data, and because it doesn’t relate to a normal individual at all (you know, the kind of megalodon that made all those bite marks, the kind you’d expect and fear to swim into if you were a miocene mysticete, in short, the kind that made up the bulk of the adult population of this species).
Those figures represent hypothetical freak specimens, based on more or less reliable hints, and back when this estimate was published it was conceived as such–deliberately basing on the largest (unreliable, and noted as such by Gottfried and colleagues) and most unusually proportioned report of a great white that was available.
Life reconstruction, based on the anatomy of the extant great white shark (adapted from Compagno 1984) and accounting for allometric increase in robusticity as per the data outlined below.
So for a less biased picture, conservative methods are in order, especially when the whole matter has to be based on a handful of teeth.
Keep in mind the actual meaning of "conservative"; not "the lowest", although it often gets used in that sense, even by myself. Yet in principle I am not an advocate of automatically considering the lowest estimate the most conservative.
In fact, it is not unusual for the lowest estimates to be liberal, when, for example, someone made extra assumptions in order to produce a minimum estimate, under the mistaken impression that it would make it more parsimonious (yet being too small isn’t really any better than being too large).
So here, what I mean is the most parsimonious method, the one that keeps biased (e.g. aimed at producing an extreme estimate in either direction) or poorly assumptions, the one avoiding to sum up such biases and instead leaving them to cancel out each other if they can not be avoided. In short, the best and most objective estimate. Unfortunately that use of the term will probably not gain popular acceptance, so lets get back on topic.
So is there a way of making this more objective and exercising some caution in this sense? Yes, I think there is.
Firstly, the most important size metric of a species isn’t the size of the biggest thing you can find some tiny fragment of (btw teeth in general are actually tiny fragments, and scientists don’t bother estimating body size from them in many cases). It’s the species average size that’s most important, most objective, and least error-prone, so that’s what I’m most interested in here.
The background is that Pimiento & Balk 2015 actually estimated it fairly recently, and came up with an average of ~10m for 544 individuals of all ages.
But this still isn’t a very good means of comparison; depending on an animal’s reproductive strategy, the number of immature specimens in such a sample can vary, and accordingly species whose social and ecological adult stage is larger can end up smaller (or vice versa). To account for that, one can take the mean size of a subsample, namely all those that are above the mean size at maturity.
Gottfried et al. 1996 estimated mean sizes at maturity for both females and males based on the Great White shark, and the average is 11.9m, which is broadly consistent, if not a bit higher, than what would be indicated by the relative size at maturity of C. carcharias (cf. Cailliet et al. 1985, Casey & Pratt 1985, McClain et al. 2015). In Pimiento & Balk’s sample, the average of individuals estimated to be this long or longer is ~14m.
Now, assume we want to know the size of a large megalodon. Not crazy-freaky-outlier-large, but large. One such specimen is known from Denmark, and consists of a huge tooth associated with a number of vertebrae, which means that this specimen is, unlike most megatooth sharks, not a tooth lost by some individual at a random moment of its life, but an actual fossil "skeleton" if that word is appropriate.
Unfortunately there’s no telling whether any of the vertebrae are the largest in the collumn, and there is also vague indication of C. megalodon’s vertebrae being proportioned differently from those of C. carcharodon. So we’re still stuck with a tooth, but at least we have a more substantial specimen that this tooth once belonged to.
The piece of eviscerating awesome in question is 12.6cm wide and 15.8cm tall, with a 11.9cm tall crown. Based on its massive built, slight tilt and size, it is likely to be one of the first three laterals, which include what is often the widest tooth in the dentition.
There are multiple ways of estimating the size from these measurements. One that is currently very popular is based on individual regressions estimating total length from tooth-crown height in Great White sharks (Shimada 2001, cited in Pimiento et al. 2010).
Using these regressions for the first three lateral teeth, the mean estimate is 16.8m.
Another method, and one which I personally prefer because it solves a number of the problems associated with using a significantly smaller relative as the sole analogue, is first extrapolating the length of the entire tooth row from the width of the tooth, and then using a regression designed for estimating total length from the length of the tooth row (Lowry et al. 2009, see also Newbrey et al. 2013/in press, and this list of measurements of two cast megalodon dentitions→. That way differences in proportions within the dentition are reduced to the impact of individual, not interspecific, variation, and the resulting estimate assumes them to follow the same trend in terms of relative jaw size.
Even though the literature seemingly suggests significantly less, I went with an interdental spacing of ~15% in addition to the summed tooth widths, because it corresponds to what one can roughly measure in the most widely spaced pictures of great white shark jaws and it is only fair towards the tested hypothesis to use the most optimistic reasonable regression).
Coincidentally this gives us the same mean size estimate for the three positions, 16.8m. Quite funny actually.
It is an intriguing side note (even though it’s just a coincidence, it pretty much rules out the whole "these figures are different from the official ones" side of criticism, because they aren’t) that this size estimate also coincides with the highest one estimated by Gottfried et al. 1996 that was deemed reliable, as well as with the biggest specimen from the nursery described by Pimiento et al. (2010). This was hence the highest proper size estimate in the literature for almost two decades. Even now, only less than 3% of all megalodon specimens in Pimiento & Balk’s dataset provide a record of the species reaching or exceeding this size, and at most by about 7%, so I find it quite save to assume that this can be considered representative of a very large megalodon.
Length estimates are of course only half of the equation. The most determining factor (though unfortunately highly prone to fluctuations) of the animal’s biology is its body mass. And sure enough, estimating body mass from total length in extant sharks has received almost unparalleled attention as far as size estimates go.
The following is a graph showing 6 [!] different regression equations between total length and body mass for the extant white shark, C. carcharias. In order to avoid biases I’ve applied all of them and marked the mean, maximum and minimum estimates:
As apparent from the exponents, most of these studies found positively allometric growth in terms of body mass, i.e. larger sharks get bulkier. That is also what my reconstruction bases on (unimaginative as you may call it, since it’s the boring bulked-up great white you always see when it’s about meg). I’m certainly open to other possibilities, it’s just that I have not yet seen any convincing arguments to suggest they are more likely than this (e.g. very elongated or compact sharks).
So as you can see, the average adult megalodon is predicted to be ~29t in mass, no more than 32t and no less than 26t, and the large specimen from Denmark is expected to be ~52t (between a minimum of 46t and a maximum of 57t). The average of the entire population would be ~10t, similar to a very large bull orca (which is impressive enough considering that that average corresponds to what is probably a large sub-adult).
And yes, there are some megalodons that realistically reached or exceeded the 18m-mark (at which length they’d be expected to weigh in at ~64t), and some exceptional fossils could indicate lengths of ~20m (and probably ~89t). But their overall relevance is in no proportion to their size. Consider this; of the 544 teeth in Pimiento & Balk’s sample, none produced an estimate higher than 17.9m, so individuals significantly above that size are obviously too rare for much of a demographic impact.
Most fossil organisms are represented by small samples, and it is likely that they are centered around the average and that "maximum size" remains unknown in virtually all of them. That’s because the likelihood of finding exceptionally large (or small) individuals is typically lower than finding normal-sized ones, and the small number of individuals known for most of them is not in favour of finding such exceptions.
So please guys, when you talk about this animal’s size, be a bit more precise, differentiate, and don’t mix average, large, small and maximum-sized specimens up all the time, because it tends to lead to conclusions that are entirely unwarranted.
C. megalodon is not an 18m shark, nor for that matter is there any species of 18m sharks. Only a species (possibly two if one includes Rhincodon) that occasionally gets that big. There is no species of 1t bear either, and sperm whales aren’t 24m long and don’t weigh 130t, occasional outsized specimens notwithstanding.
PS: I do realize this looks like a rant, and honestly, it is. I’m sure we all have issues that annoy us, some more than others. That doesn’t mean I am not appropriately impressed by C. megalodon or anything like that, just that I think some people are overdoing it in that regard.
Bendix-Almgreen, Svend E. (1983): Carcharodon megalodon from the Upper Miocene of Denmark, with comments on elasmobranch tooth enameloid: coronoïn. Bulletin of the geological Society of Denmark, 32 pp. 1-32.
Cailliet, Gregor M.; Natanson, Lisa J.; Welden, Bruce A.; Ebert, David A. (1985) Preliminary studies on the Age and Growth of the White Shark, Carcharodon carcharias, Using Vertebral Bands. Memoirs of the Southern California Academy of Sciences, 9 (Biology of the White Shark, a Symposium.) pp. 49-60.
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.
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Newbrey, Michael G.; Siverson, Mikael; Cook, Todd D.; Fotheringham, Allison M.; Sanchez, Rebecca L. (2013, in press): Vertebral morphology, dentition, age, growth, and ecology of the large lamniform shark Cardabiodon ricki. Acta Palaeontologica Polonica, in press, dx.doi.org/10.4202/app.2012.0047 pp. 1-65.
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