Mosasaur full size estimate from lower jaw

[LordWampa]

Hello, I will tag you directly @pachy-pleuro-whatnot-odon @Praefectus. From this jaw which measurement should one take to estimate the full size of the specimen being this a thalassotitan atrox? I know head to body ratio of thalassotitan ies 1:8 more or less, what I am not sure is here which would be the correct measurement for the head size.

 

From point A to B measures 135cm. Would this be the right measurement? Then full size 135*8/100= 10,8m more or less?

 

Edited June 27, 2023 by LordWampa

[pachy-pleuro-whatnot-odon]

Size, like weight estimations (which are even worse), is always just an educated guess to get at least an impression of the overall size of a specimen. You yourself already agree with this when you state that the body ratio for Thalassotitan atrox "ies [sic] 1:8 more or less", indicating that even the ratio used is more of a guideline than anything else, to which one or two centimeters more will make little of a difference (hence, keep rounding of the resulting size to one decimal maximum - I'd say it's better to round up to ~11m in your case).

 

This brings me to how to estimate the skull length. Unfortunately, I don't quite know how this is officially measured either. However, knowing basic mosasaur skull anatomy, I'd suspect that you'd normally take this measurement from the cranium, which is the least dynamic part of the skull. That is, the lower jaws of mosasaurs, just like those of snakes, have been postulated to have a certain degree of motion that makes it more difficult to specify how to properly measure their length. See the figure below to see what I mean (source):

 

 

 

As you'll also be able to tell from the above diagram, the lower jaw in its most extended position, would be about the same length as the cranium. So it should be possible to get the skull length from that as well. In that case, your points A & B would indeed make the most sense for a size estimate. There are a few caveats with this, however, as the jaw in your specimen seems rather exploded, making it hard to determine the extended size of the jaw. There are also some concerns when it comes to whether the jaw is complete, both anteriorly and posteriorly: both dentary and articular seem incomplete. And it would be the anterior-most point and the dentary and the posterior-most of the articular that you'd probably want to use for your size estimate.

 

That having been said, size-estimates, as already stated, are rather coarse-grained in nature. So if you determine your specimen to be ~11m in size, this is probably a good estimate either way.

[LordWampa]

Hello, first thanks for the reply. Yes, I understand that obviously is an estimate. But some people when they see the jaw the first question is how big was it? So I wanted to have some sort of approximation.

 

You also say that you think that the jaw is incomplete, but even if it has the typical exploded look of most mosasaur stuff, at least trying to compare it with the one from thalassotitan paper and other images online I think the dentary it's complete. The only think I am not that sure it's the articular (posterior part) because I see some bone structure that I can't be 100% sure what is it.

See the picture below:

Do you think my left id hypothesis can be right? The right hole I have no idea. Maybe I will try to do a better post about this in the ID section to see if someone have some ideas.

[pachy-pleuro-whatnot-odon]

On 7/3/2023 at 5:54 PM, LordWampa said:

Hello, first thanks for the reply. Yes, I understand that obviously is an estimate. But some people when they see the jaw the first question is how big was it? So I wanted to have some sort of approximation.

 

In totally understand. I've had similar questions myself when it comes to some of my bigger vertebrae. Makes you wonder how huge the beastie it came from would have been

For these purposes, however, I think the estimate you've managed to arrive at, rounded up to 11m, is probably perfectly fine (and likely as accurate an approximation you'll be able to get from this fossil)

 

On 7/3/2023 at 5:54 PM, LordWampa said:

You also say that you think that the jaw is incomplete, but even if it has the typical exploded look of most mosasaur stuff, at least trying to compare it with the one from thalassotitan paper and other images online I think the dentary it's complete.

Well, the best way to determine this would be to look at the front part of the dentary head-on. If there's enough space there for it to have a natural rounded termination, than it may indeed be complete. However, as the fossil was jacketed so close to the bone, it may also simply have been cut off for a variety of reasons. In that case, you'd expect the cut to either be unnaturally smooth or show tool marks. That's rather the thing I wanted to point out. It wouldn't be the first time I'd have come across a fossil jaw that's cut off at the margin of the jacket.

 

One of the reasons for me to wonder whether this may be the case here is that, in those Thalassotitan atrox skulls I've seen - like the one at the Sauriermuseum Aathal in Switzerland (below) and at fossil fairs like that of Sainte Marie-aux-Mines - typically have longer dentaries compared to the rest of the mandible than appears to be the case here...

 

 

 

On 7/3/2023 at 5:54 PM, LordWampa said:

The only think I am not that sure it's the articular (posterior part) because I see some bone structure that I can't be 100% sure what is it.

 

As to the bone with the hole in it, I suspect that's what's left of the quadrate (so, from the area in between the cranium and mandible). Compare to the specimen below, again the one from Aathal.

 

 

Maybe @LordTrilobite could also share his insights on this...

[Praefectus]

Off the top of my head, I know there was an Everhart paper that says the dentary is 56% of the skull length in tylosaurines (Everhart 2002). I'm not sure how true that rings for mosasaurines, but it probably isn't tooooo different . To get a rough approximation of size, I would try using this measurement (below), doubling it, and multiplying by 7 or 8. That would give you a rough range for total body size.  

 

[LordWampa]

On 7/10/2023 at 10:43 PM, pachy-pleuro-whatnot-odon said:

 

In totally understand. I've had similar questions myself when it comes to some of my bigger vertebrae. Makes you wonder how huge the beastie it came from would have been

For these purposes, however, I think the estimate you've managed to arrive at, rounded up to 11m, is probably perfectly fine (and likely as accurate an approximation you'll be able to get from this fossil)

 

Well, the best way to determine this would be to look at the front part of the dentary head-on. If there's enough space there for it to have a natural rounded termination, than it may indeed be complete. However, as the fossil was jacketed so close to the bone, it may also simply have been cut off for a variety of reasons. In that case, you'd expect the cut to either be unnaturally smooth or show tool marks. That's rather the thing I wanted to point out. It wouldn't be the first time I'd have come across a fossil jaw that's cut off at the margin of the jacket.

 

One of the reasons for me to wonder whether this may be the case here is that, in those Thalassotitan atrox skulls I've seen - like the one at the Sauriermuseum Aathal in Switzerland (below) and at fossil fairs like that of Sainte Marie-aux-Mines - typically have longer dentaries compared to the rest of the mandible than appears to be the case here...

 

 

 

 

As to the bone with the hole in it, I suspect that's what's left of the quadrate (so, from the area in between the cranium and mandible). Compare to the specimen below, again the one from Aathal.

 

 

Maybe @LordTrilobite could also share his insights on this...

 

Thanks again for the answer. Yes as you say with your vertebras, for me, the fossils are 50% looking cool and 50% the story of the life creature behind the fossil. This is why even if it's not a "paper worthy" estimate I like to imagine how was the animal, etc etc.

 

In your dentary / posterior jaw proportion, looking at my jaw in person I think it follows ell the proportion, maybe in the photo as it's taken closer to the posterior part it has some kind of optic effect. While almost all the pictures of jaws/heads on the internet, the photographer is closer to the dentary.I measured the dentary and the posterior part and it's dentary (80cm) + posterior part (55cm) more or less the 135cm in total. So I think the proportion makes sense.

 

In the case of the dentary being complete, I have checked the front part. Here is a photo. 

 

 

At first it seems that the green mark is some kind of tools mark. And maybe that more white part it's sanded down. But in the photo the red marked thing (in real life it's easier to appreciate  trust me) it's one of this typical "socket" (don't know if they have a better scientific name) of the mosasaur dentary and the dentary it's already curving there, so maybe it has been sanded down just a little or it's preparation marks or whatever natural weird thing. 

At the same time, if you take the mandible and count the teeth:

 

 

I counted the 14 of this skull, in the thalassotitan paper it says that the dentary has 14 teeth but in some other dentaries I only see 13 so I am not sure why this couple of posterior smaller teeth sometimes is only one?

At the same time the thalassotitan paper says this: The tip of the dentary is bluntly rounded, but lacks apredental prominence, as in other Prognathodontini. So you should expect the dentary to not continue a lot after the last teeth.

 

And last, for the bone with the hole I started thinking that it could be the quadrate but looking at it for different perspectives I don't really see it. At the same time I don't have a counterargument so who knows. The only other options that I have in my mind is that is a man made hole while preparing/discovering it or this hole that sometimes I see on the back of jaws but not in all of theme.

 

But anyway, even if I can't be sure, I will be able to sleep in the night. 

 

Edited July 13, 2023 by LordWampa

[LordWampa]

11 hours ago, Praefectus said:

Off the top of my head, I know there was an Everhart paper that says the dentary is 56% of the skull length in tylosaurines (Everhart 2002). I'm not sure how true that rings for mosasaurines, but it probably isn't tooooo different . To get a rough approximation of size, I would try using this measurement (below), doubling it, and multiplying by 7 or 8. That would give you a rough range for total body size.  

 

 

Thanks for the answer, I measured it and it's 75/80 cm so the minimum would be 75*2*7= 10,50cm and the maximum 80*2*8=12,80m (that I think would be an anomaly of the thalassotitans seeing that the paper says 9-10m).

 

But anyway, at the end the answer it that was a 10+ probably 11 meters large animal and that answer is enough for my imagination.