Post by Grey on Dec 21, 2017 2:37:12 GMT 5
For the record, I'm one of the coauthors of the poster from Leder et al. 2016.
If one wants to use the method based on vertebral diameter on an isolated vertebra, he systematically has to imply that the vertebra is the largest in the entire column.
The Danish 23 cm vertebra was found among only 20 vertebra ( while Gottfried 1996 estimates a total of 130 precaudal centra in this species ), the smallest being 10 cm, some being too badly preserved to have a precise size. It is therefore impossible to assign those vertebra to a precise position and to determine if the 23 cm centra was the largest in the whole column.
Any estimate deduced from this material should thus be treated as conservative and possibly underestimate.
IRSNB 3121 is not an "almost complete vertebral column", it is at best 3/4 complete with a number of its elements being poorly preserved and incomplete. It has since been stored and I've never understood why Gottfried or Leriche never measured the whole column in its state. Pimiento has been working on it recently so hopefully more data will come. Having discussed wih the curator of the Museum where it is stored, I don't expect too much from it about total length questions tough.
An interesting point is that I've read several people who saw the actual specimen when it was exhibited and they reported that in its state it was already 9 m long, so as long as the TL estimate deduced from Gottfried.
It is well possible the species had less wide vertebra for its TL than Carcharodon. Which would corroborate high TL estimates but somewhat lower body mass estimates than proposed in Gottfried et al. 1996.
There is actually only one outlier, adult white shark which gives the lower 15 m result but all the others are more consistant. While all the juvenile/neonates calculations give much lower (but still stable) results.
As said in the conclusion (again), estimates based on lower jaw are too much variable, up to 27 m TL and as low as 16 m even based on an adult GWS, and because of the proportional differences between GWS and meg, are considered inappropriate to use. We focuse on upper dentition for those reasons. This is well explained in our conclusions.
There is an interesting way to suggest that the Danish vertebra may not have been the largest in the column. Using Gottfried calculation on this paricular centra results in a 13.5 m TL estimate.
For the recall, those centra were associated with a large upper anterior tooth. This tooth had an approx. width about 120 mm. This is somewhat larger than the widest upper anterior teeth in the Yorktown dentition (about 108.5mm in crown width). This dentition being approx. 3.5 times larger than the dentition of a 5 m white shark, the mean TL resulted for the Yortown uper dentition being about 18 m TL. The Danish tooth being even larger than the Yorktown dentition would suggest the actual dentition it comes from was even larger, about 1216 mm, against approx. 1100 mm in the Yorktown dentition, which would result in a TL estimate of almost 20 m.
So, if the estimate based on the centra of about 13.5 m TL is accurate, it necessarily implies that this 13.5 m TL shark had an oversized upper dentition that would fit a 18-20 m TL scaled up GWS. Such a much larger dentition would imply a much thicker, wider chondocranium, thorax and overall body and a much heftier, stockier shark than even expected by Gottfried.
This does not sound practical for hydrodynamic and biomechanics purposes and is actually in direct contradiction with the morphology directly suggested by the vertebral centra, consistant with a fusiform, streamlined body. None of the large (>10 m TL) axial, cruisiform swimmers has a body morphology that would be suggested by this ultra-stocky, disporportionately thick-headed megalodon.
The alternative, more likely possibility is that indeed, 23 cm was not the maximum centra diameter in this shark.
Overall, the sheer rarity of vertebral centra and impossibility to place them to a given position in the column is enough to rather rely on dentition parameters, which is what sharks researchers actually do.
However, I believe an interesting way to calculate TL based on centra (again as long as we have an idea of its position) would be to use length/thickness rather than width, which has been tested in pliosaurs TL estimates research (McHenry 2009). Obviously, the length of the centra would be more directly correlated to the length of the column, thus the length of the body.
However, on a basic statistical mean, the largest, preserved and recorded body elements we have from O. megalodon are associated dentitions, so they remain the best material to use readily.
“Vertebrae estimation methods based not on the diameter of random vertebra, but on the maximum diameter of vertebra in all precaudal vertebral column. And in shark's vertebral column at once several vertebrae reach the maximum diameter (see e.g. Cretoxyrhina mantelli by Shimada, 1997).
Complete columns of Otodus megalodon have not been discovered and described? Hmmm... IRSNB 3121 is almost complete vertebral column.
Complete columns of Otodus megalodon have not been discovered and described? Hmmm... IRSNB 3121 is almost complete vertebral column.
If one wants to use the method based on vertebral diameter on an isolated vertebra, he systematically has to imply that the vertebra is the largest in the entire column.
The Danish 23 cm vertebra was found among only 20 vertebra ( while Gottfried 1996 estimates a total of 130 precaudal centra in this species ), the smallest being 10 cm, some being too badly preserved to have a precise size. It is therefore impossible to assign those vertebra to a precise position and to determine if the 23 cm centra was the largest in the whole column.
Any estimate deduced from this material should thus be treated as conservative and possibly underestimate.
IRSNB 3121 is not an "almost complete vertebral column", it is at best 3/4 complete with a number of its elements being poorly preserved and incomplete. It has since been stored and I've never understood why Gottfried or Leriche never measured the whole column in its state. Pimiento has been working on it recently so hopefully more data will come. Having discussed wih the curator of the Museum where it is stored, I don't expect too much from it about total length questions tough.
An interesting point is that I've read several people who saw the actual specimen when it was exhibited and they reported that in its state it was already 9 m long, so as long as the TL estimate deduced from Gottfried.
It is well possible the species had less wide vertebra for its TL than Carcharodon. Which would corroborate high TL estimates but somewhat lower body mass estimates than proposed in Gottfried et al. 1996.
Estimations based on width of Yorktown specimen upper jaw and proportions of large adult great white sharks gives 15.11 m TL as well as 20.59 m TL. If estimation based on width of Yorktown specimen lower jaw gives more stable value in 18.06-23.85 m (not 17-20 m), this even does not mean that this value will be preserved when sample is increased. E.g. O. chubutensis estimations based on width of Caballos specimen lower jaw and proportions of large adult great white sharks gives 10.48 m TL or as much as 16.42 m TL.
And by the way, different results of calculations based on upper jaw and lower jaw of Yorktown specimen already indicate different proportions in comparison with great white shark jaws.
”
And by the way, different results of calculations based on upper jaw and lower jaw of Yorktown specimen already indicate different proportions in comparison with great white shark jaws.
”
There is actually only one outlier, adult white shark which gives the lower 15 m result but all the others are more consistant. While all the juvenile/neonates calculations give much lower (but still stable) results.
As said in the conclusion (again), estimates based on lower jaw are too much variable, up to 27 m TL and as low as 16 m even based on an adult GWS, and because of the proportional differences between GWS and meg, are considered inappropriate to use. We focuse on upper dentition for those reasons. This is well explained in our conclusions.
There is an interesting way to suggest that the Danish vertebra may not have been the largest in the column. Using Gottfried calculation on this paricular centra results in a 13.5 m TL estimate.
For the recall, those centra were associated with a large upper anterior tooth. This tooth had an approx. width about 120 mm. This is somewhat larger than the widest upper anterior teeth in the Yorktown dentition (about 108.5mm in crown width). This dentition being approx. 3.5 times larger than the dentition of a 5 m white shark, the mean TL resulted for the Yortown uper dentition being about 18 m TL. The Danish tooth being even larger than the Yorktown dentition would suggest the actual dentition it comes from was even larger, about 1216 mm, against approx. 1100 mm in the Yorktown dentition, which would result in a TL estimate of almost 20 m.
So, if the estimate based on the centra of about 13.5 m TL is accurate, it necessarily implies that this 13.5 m TL shark had an oversized upper dentition that would fit a 18-20 m TL scaled up GWS. Such a much larger dentition would imply a much thicker, wider chondocranium, thorax and overall body and a much heftier, stockier shark than even expected by Gottfried.
This does not sound practical for hydrodynamic and biomechanics purposes and is actually in direct contradiction with the morphology directly suggested by the vertebral centra, consistant with a fusiform, streamlined body. None of the large (>10 m TL) axial, cruisiform swimmers has a body morphology that would be suggested by this ultra-stocky, disporportionately thick-headed megalodon.
The alternative, more likely possibility is that indeed, 23 cm was not the maximum centra diameter in this shark.
Overall, the sheer rarity of vertebral centra and impossibility to place them to a given position in the column is enough to rather rely on dentition parameters, which is what sharks researchers actually do.
However, I believe an interesting way to calculate TL based on centra (again as long as we have an idea of its position) would be to use length/thickness rather than width, which has been tested in pliosaurs TL estimates research (McHenry 2009). Obviously, the length of the centra would be more directly correlated to the length of the column, thus the length of the body.
However, on a basic statistical mean, the largest, preserved and recorded body elements we have from O. megalodon are associated dentitions, so they remain the best material to use readily.
