Body Size

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  • #1596
    Victor Perez
    Keymaster

    This topic has been talked about time and time again, but I still find myself unsatisfied with current methods for estimating body size of what is touted as the largest shark that ever lived. Body size estimates for Megalodon are all based on the living Great White Shark. At the moment, the prevailing method for estimating body length is based a set of linear equations correlating tooth crown height to body length proposed by Shimada (2003). Recently, these equations were tested by middle and high school students on an associated dentition of Megalodon donated by Gordon Hubbell. The students found that the equations resulted in vastly different estimates for the front of the jaw versus the back of the jaw, despite all teeth coming from a single individual. My reason for starting this forum topic is simply to crowd source ideas for improving the current approach to estimating body length. I think with the combined passion and innovation of the paleontology community we can find a solution to a long standing problem.

    For a good summary of estimating body length of Megalodon I suggest checking out: http://www.fossilguy.com and/or Pimiento & Balk (2015)

    #1734
    Lisa Lundgren
    Keymaster

    Are there any more opportunities for students to test body size using different sets of associated dentitions? I remember seeing the associated dentition at the Aurora Fossil Museum. I wonder a couple things — a) are there more sets of associated dentitions that can be scanned and 3D printed so students can measure them? and b) are the associated dentitions that museums/collectors have on display actually associated–how do we know? Although these are just my wonderings, I think it could be a good question to get students thinking about the science of paleontology too. @lcone, do you think that the associated dentition at the Aurora Fossil Museum could be scanned for students to measure body size?

    #1736
    Victor Perez
    Keymaster

    There are certainly more associated dentitions. The challenge is getting them donated to museums and then the time consuming process of scanning each tooth. Most (if not all) Megalodon jaw displays are what we’d call a composite dentition. The artist/exhibitor will take a bunch of beach float teeth and compose a jaw. I don’t think anyone would put a real associated dentition into a plaster jaw as an exhibit, simply because it would be less accessible for research.

    It would be great if we could scan more dentitions and I think we’re on that path, but it will take a long time.

    #1737

    I think first we need to investigate more jaws of the next living relatives, not just the great whites but also other lamniforms – especially the closely related makos. We need to have a look at juvenile and adults, males and females. We also need to check the relations between tooth size and body size depending on different ages and sexes of the sharks. @sgodfrey and @jnance does the Calvert Marine Museum have tooth sets of living lamniform sharks of different individual age and sexes that you can image and measure?

    #1838
    Lee Cone
    Participant

    Hi Lisa @llundgren –  I know that many of the dentitions that are on display are composite dentitions, not associated.  I am thinking of the makos (shortfin, broad-tooth, and one other species as examples).  There are some known associated dentitions for sure – the Parotodus and maybe a great white – I’ll have to check on the latter.  The Paratodus that is displayed is a cast of the original, which was donated to the Smithsonian by George Powell.  The Smithsonian made the cast.  Let me get back to you on your questions when I find out more facts and am not just going off my memory.

    #1839
    Lee Cone
    Participant

    These are certainly interesting questions that are raised.  I do not know if sexual dimorphism is a characteristic in sharks generally, or of particular species within sharks families.  Are females always larger than males?  Victor @vperez is absolutely correct that meg tooth size varies from posterior to anterior in the mouth, as well as the slant shape of lateral vs. anterior.  Further, uppers and lowers have distinct differences in height to width ratios.  To complicate matters, not every meg anterior tooth (from the same position in the mouth) is identical in shape, especially the depth and weight of the tooth.  I have observed this variation even more profoundly in C. angustidens.

    These points were discussed in Nebraska during the Edisto River shark tooth identification lesson plan that I led.  When you have enough teeth to look at, you really begin to see subtle differences that exist within a species, and this complicates the process. And this begs the question, is this simply Darwin’s variation within a species or is it something more?   Clearly, meg teeth are impressive teeth regardless of how you relate them to shark body size, but I do think that a formula that would compare tooth size to shark length does need to specify tooth position.  I have seen composite dentitions that have lateral teeth from smaller sharks used for posterior teeth in dentitions for larger sharks.  True posteriors are quite rare and that is why I love to collect them.

    Still another consideration is related to the questions: (1) Is shark growth uniform throughout an individual’s life, and (2) Is tooth size related uniformly to length throughout its life?  My guess is that the ratio of tooth size to body size (mass and length) of juveniles would be very different from that of a very old shark.   It would be interesting to compare body mass, body length, and tooth size of modern shark species vs. age, especially as one reaches the upper age limits.  Of course, the problem with meg size estimation is that we do not have any extant modern evolutionary descendants to compare data with, and the relationship with the broad tooth mako and modern great white may have no direct genetic relationship or similarity to megalodon, especially if one subscribes to the Otodus lineage.

    #1872

    Lee @lcone,

    The dimorphism of teeth, or the heterodonty, is something that you will find in a lot of sharks families but not in all of them. Sometimes it is stronger, sometimes just weakly developed. For example, you will find very strong sexual dimorphism within the family of the carcharhinids and very strong ontogenetical dimorphism in Mustelus. That is why I have investigated those families intensively. The result of my studies about carcharhinids was that in some cases and with some tooth positions you cannot say anything specific in terms of identification, sex, or jaw position (upper vs. lower). Especially the positions of the upper jaw 3 to 6 are the best for identification, the more you move to the commissure (jaw joint) the harder the identification. The good news is that the heterodonty is not so pronounced in lamniform sharks like it is in most carcharhinids or hexanchids, but we still have to look at it and check the variability (is it interspecific or intraspecific, sexual or ontogenetic) and we need to keep in mind that the commissure teeth of Carcharocles are very big compared to the ones in the Great White. That is why the body length estimates vary so much when you use the model that is based on Great Whites. I recommended checking the teeth of Isurus for body length estimates. We also need to make sure to find more commissure teeth… that is the key. So far you are totally right in being so focused on them Lee 😉

    -Ronny

    #3213
    Teddy Badaut
    Participant

    Hello everyone,

    I think a quite strong correlation between dentition size and body size has been observed in modern lamnids.

    Since we have several available C. megalodon complete or near complete sets of teeth, I’d suggest focusing on the size of the whole dentition (the largest component of the body we have from individuals of this species) rather than individual tooth height and compare it with the closest modern analogue (not only phylogenetically but especially subject to the same ecological constraints influencing body size/body proportions) would suggest a more secure extrapolation of body size.

     

    #3245

    Hello Teddy @teddy-badaut,

    thank you for your interesting comment. Actually that was something scientists have already tried (Masey, Gottfried, Compagno & Bowman etc.) but the problem is that the proportions might be very hard to estimate. The shark can have a proportional very large or very small jaw compared to the body size.

    all the best

    Ronny

    #3246
    Teddy Badaut
    Participant

    Hello Ronny @rleder,

    Thank you for the response.

    Well I think Gottfried et al. simply scaled C. megalodon using isolated upper anterior tooth then inferred some allometry in the proportions derived from the great white shark. But I’m not sure if they decisively implied a larger dentition compared to body size. I think my suggestion was hinted by Bretton Kent in his paper about Parotodus; given the decoupled scaling between upper anterior tooth and dentition size between C. carcharias and C. megalodon, estimates based on sole anterior tooth would substantially underestimate the total lenght of the animal.

     

    I think a quite good correlation between dentition size and body length has already been noted in modern lamnids, at least less variable than tooth size>body size.

    On theoretical grounds, a whole dentition seems a better base for size extrapolation than a single tooth. So without hard data about C. megalodon proportions, I suggest scaling theC. carcharias dentitions with known TL with the reported dentitions of C. megalodon could induce some interesting results.

     

    I’ve tried this with 6 white sharks dentitions from elasmo.com and Peter  Klimley 1996 book and got some interesting estimates for the juvenile and adults C. megalodon associated sets measured in Pimiento 2010, see attached.

    Of course the results are only indicative but I do not a relatively small error bar with this method compared to scaling from upper anterior teeth.

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