Ancient Jaguars

[Balam]

@tigerluver first of all, great job in the equation for the allometric predictions for Pleistocene jaguars based on dental remains as they compare to extant remains. Unsurprisingly, however, I still have to push against the conclusion for you and KRA's point in regards to relative mass for Pleistocene jaguar forms and their closeness to large extant jaguars.

To begin, the specimen gathered from Mexico (Ramoni et al, 2020) shows c1 value greater in length and width than what's shown for average extant jaguars, despite M1 and c1 length values being in the range of a large extant male:

*This image is copyright of its original author

*This image is copyright of its original author

It's worth pointing out that the specimens for Soyatlan and San Josecito all represent Pleistocene jaguar forms from Mexico, placing it closer to an extant male lion and the lower values were seen for American lions. Now, according to the analysis made for this particular mandible, the proportions of the teeth and their placement in the bone all match the characteristics of P. onca, and not P. atrox. Hence the claim in the article that the values for this particular fragment are much greater than that of any extant jaguar, while simultaneously matching the proportions and shape expected for P. onca, which substantiate the claim that these fossils belong to a large jaguar, largest than what's seen with extant animals from Mexico at least.

The article: The assessment of size in fossil Felidae (Turner & Regan, 2002) offers, in my opinion, great insight into how easy it is to miscalculate the mass of an animal based on dental fragments solely, as their regression analysis performed on jaguars (could not be more perfect for this discussion) shows a direct correlation between mass and skull length, but the correlation becomes random the moment the values are plotted in a correlation between dental, mass, and body length values. The used the data by Hoogesteijn and Mondolfi we've all seen before:

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

When comparing to other species, the authors concluded that it is futile to use interspecific dental fragments without having a clear regression analysis on the correlation between dental remains and overall skull dimensions, to assess if any direct link exists between dental dimension, skull proportions, and overall mass or frame of the species in question:

*This image is copyright of its original author

Now, if we use the data from the mandible form Mexico and we compare it to the data from Hoogesteijn and Mondolfi, what we get are the values from a large jaguar within the range of what is seen in very large extant males, but likewise, in terms of size proportions, the lower canines do overlap in size with what is seen in some extant male lions and the smaller ranges for American lions. This could mean that the specimen from Mexico was indeed much greater in size than what is seen for extant Mexican jaguars, but within the limits of large South American floodplain jaguars, as you pointed out before. 
Here are the skull measurements for extant jaguars in Oaxaca, Mexico, an area of Mexico Hoogesteijn and Mondolfi didn't include in their study, from Cranial measurements of jaguars (Panthera onca) from the State of Oaxaca, Mexico (Lavariega & Salas):

*This image is copyright of its original author

I'm sure the values for the skull of the Pleistocene jaw from Soyatlan would have been much greater than the ranges shown here, but who knows how it would compare with the length and overall size for skulls belonging to Pantanal, Llanos, Chaco, and other jaguars.

One factor to take into account when comparing this data is that we do not know the exact sex of the specimen IGM 9518, or its age. Needless to say that if this represents an adult male, then yes, the values for this specimen of P. o. augusta are in line with large extant jaguars. But if the skull in fact belongs to a female or a sub-adult male, then the size potential for this particular species as it's expressed by this mandible would be much greater than anything seen with extant jaguars.

Now, one last point I'd like to make to substantiate my claim that the data for fossil jaguars is consistently larger than what is seen with modern jaguars, I'd like to go over the data we have for the Eurasian jaguar, P. gombaszogensis. The study The Panthera gombaszogensis story: the contribution of the Château Breccia (Saône-et-Loire, Burgundy, France) by Argant et al. Goes into detail on the craniometric and dental values for this species and how it compares to modern jaguars.
The skull studied also showed values that could be placed within modern jaguars, without knowing the sex and age of the specimen either. The data is very extensive and long and can't be fitted here but is easily accessible in the article for anyone interested. Nonetheless, the authors concluded that the size of the individual in question must have been larger than extant jaguar forms thanks to the measurements of skeletal remains beyond the skull:

*This image is copyright of its original author

I do believe they likely used smaller populations of jaguars to arrive at that conclusion as the largest extant male show larger values for overall skull size than what is seen in the specimen they analyzed.
What I did find interesting are the quoted values and averages predicted for the weight of fossil Old-World jaguars based on what seems unpublished data:

*This image is copyright of its original author

*This image is copyright of its original author

We can compare these values with what is shown for extant males from the Pantanal in recent times, per our table:

*This image is copyright of its original author

My conclusion from all this is that there does seem to be an overlap between the largest extant jaguars and extinct Pliocene and Pleistocene forms, per yours and KRA's point, but it also seems clear to me that in regards to averages and absolute values the ranges for fossil forms were significantly larger than what is seen with living jaguars. This can be explained by the availability of larger prey items during prehistoric times, which allowed the potential in jaguars to be greater in terms of overall mass, while the largest living populations may show similar values with outlier and large males who have access to plenty of prey and a strong genetic background.

Hopefully, in the future we can gain access to a larger sample of complete skulls and skeletons for fossil jaguars to best assess how they directly compare to living jaguars. Reply